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Capture MySQL Foreign Keys

with one comment

Shantanu asked a follow-up question on my Cleanup a MySQL Schema post from last month. He wanted to know if there was a way to capture foreign keys before removing them. The answer is yes, but how you do it depends on whether the primary key is based on a surrogate key using an auto incrementing sequence of a natural key using descriptive columns.

You can capture foreign keys with a simple query when they’re determined by a single column value. However, this script creates ALTER statements that will fail when a table holds a multiple column foreign key value. The SELECT statement would look like this when capturing all foreign key values in a MySQL Server:

SELECT   CONCAT('ALTER TABLE',' ',tc.table_schema,'.',tc.table_name,' '
               ,'ADD CONSTRAINT',' fk_',tc.constraint_name,' '
               ,'FOREIGN KEY (',kcu.column_name,')',' '
               ,'REFERENCES',' ',kcu.referenced_table_schema,'.',kcu.referenced_table_name,' ' 
               ,'(',kcu.referenced_column_name,');') AS script
FROM     information_schema.table_constraints tc JOIN information_schema.key_column_usage kcu
ON       tc.constraint_name = kcu.constraint_name
AND      tc.constraint_schema = kcu.constraint_schema
WHERE    tc.constraint_type = 'foreign key'
ORDER BY tc.TABLE_NAME
,        kcu.column_name;

You would add a line in the WHERE clause to restrict it to a schema and a second line to restrict it to a table within a schema, like this:

AND      tc.table_schema = 'your_mysql_database'
AND      tc.table_name = 'your_table_name'

Unfortunately, when the primary and foreign keys involve two or more columns you require a procedure and function. The function because you need to read two cursors, and the NOT FOUND can’t be nested in the current deployment of MySQL’s SQL/PSM stored programs. In this example the storedForeignKeys procedure finds the table’s foreign key constraints, and the columnList function adds the column detail. The command_list table stores the commands to restore foreign key constraints.

The command_list table that stores the values is:

CREATE TABLE command_list
( command_list_id  INT UNSIGNED PRIMARY KEY AUTO_INCREMENT
, sql_command      VARCHAR(6)    NOT NULL
, sql_object       VARCHAR(10)   NOT NULL
, sql_constraint   VARCHAR(11)
, sql_statement    VARCHAR(768)  NOT NULL);

This is the storedForeignKeys procedure:

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CREATE PROCEDURE storeForeignKeys
( pv_schema_name  VARCHAR(64)
, pv_table_name   VARCHAR(64))
BEGIN
 
  /* Declare local variables. */
  DECLARE lv_schema_name              VARCHAR(64);
  DECLARE lv_table_name               VARCHAR(64);
  DECLARE lv_constraint_name          VARCHAR(64);
  DECLARE sql_stmt                    VARCHAR(1024);
 
  /* Declare control variable for handler. */
  DECLARE fetched    INT DEFAULT 0;
 
  /* Declare local cursor for foreign key table, it uses null replacement
     because the procedure supports null parameters. When you use null 
     parameters, you get all foreign key values. */
  DECLARE foreign_key_table CURSOR FOR
    SELECT   tc.table_schema
    ,        tc.table_name
    ,        tc.constraint_name
    FROM     information_schema.table_constraints tc
    WHERE    tc.table_schema = IFNULL(lv_schema_name, tc.table_schema)
    AND      tc.table_name = IFNULL(lv_table_name, tc.table_name)
    AND      tc.constraint_type = 'FOREIGN KEY'
    ORDER BY tc.table_name;
 
  /* Declare a not found record handler to close a cursor loop. */
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET fetched = 1;
 
  /* Assign parameter values to local variables. */
  SET lv_schema_name := pv_schema_name;
  SET lv_table_name := pv_table_name;
 
  /* Open a local cursor. */  
  OPEN foreign_key_table;
  cursor_foreign_key_table: LOOP
 
    /* Fetch a row into the local variables. */
    FETCH foreign_key_table
    INTO  lv_schema_name
    ,     lv_table_name
    ,     lv_constraint_name;
 
    /* Catch handler for no more rows found from the fetch operation. */
    IF fetched = 1 THEN LEAVE cursor_foreign_key_table; END IF;
 
    /* The nested calls to the columnList function returns the list of columns
       in the foreign key. Surrogate primary to foreign keys can be resolved 
       with a simply query but natural primary to foreign key relationships
       require the list of columns involved in the primary and foreign key.
       The columnList function returns the list of foreign key columns in 
       the dependent table and the list of referenced columns (or the primary
       key columns) in the independent table. */
    SET sql_stmt := CONCAT('ALTER TABLE ',' ',lv_schema_name,'.',lv_table_name,' '
                          ,'ADD CONSTRAINT ',lv_constraint_name,' '
                          ,'FOREIGN KEY (',columnList(lv_schema_name,lv_table_name,lv_constraint_name));
 
    /* Record the SQL statements. */
    INSERT INTO command_list
    ( sql_command
    , sql_object
    , sql_constraint
    , sql_statement )
    VALUES
    ('ALTER'
    ,'TABLE'
    ,'FOREIGN KEY'
    , sql_stmt );
 
  END LOOP cursor_foreign_key_table;
  CLOSE foreign_key_table;  
 
END;
$$

This is the columnList function:

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CREATE FUNCTION columnList
( pv_schema_name      VARCHAR(64)
, pv_table_name       VARCHAR(64)
, pv_constraint_name  VARCHAR(64)) RETURNS VARCHAR(512)
BEGIN
 
  /* Declare local variables. */
  DECLARE lv_schema_name              VARCHAR(64);
  DECLARE lv_table_name               VARCHAR(64);
  DECLARE lv_constraint_name          VARCHAR(64);
  DECLARE lv_column_count             INT UNSIGNED;
  DECLARE lv_column_name              VARCHAR(64);
  DECLARE lv_column_list              VARCHAR(512);
  DECLARE lv_column_ref_list          VARCHAR(64);
  DECLARE lv_referenced_table_schema  VARCHAR(64);
  DECLARE lv_referenced_table_name    VARCHAR(64);
  DECLARE lv_referenced_column_name   VARCHAR(64);
  DECLARE lv_return_string            VARCHAR(768);
 
  /* Declare control variable for handler. */
  DECLARE fetched    INT DEFAULT 0;
 
  /* Declare local cursor for foreign key column. */
  DECLARE foreign_key_column CURSOR FOR
    SELECT   kcu.column_name
    ,        kcu.referenced_table_schema
    ,        kcu.referenced_table_name
    ,        kcu.referenced_column_name
    FROM     information_schema.key_column_usage kcu
    WHERE    kcu.referenced_table_schema = lv_schema_name
    AND      kcu.table_name = lv_table_name
    AND      kcu.constraint_name = lv_constraint_name
    ORDER BY kcu.column_name;
 
  /* Declare a not found record handler to close a cursor loop. */
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET fetched = 1;
 
  /* Assign parameter values to local variables. */
  SET lv_schema_name := pv_schema_name;
  SET lv_table_name := pv_table_name;
  SET lv_constraint_name := pv_constraint_name;
 
  /* Set the first column value. */
  SET lv_column_count := 1;
 
  /* Open the nested cursor. */
  OPEN  foreign_key_column;
  cursor_foreign_key_column: LOOP
 
    /* Fetch a row into the local variables. */    
    FETCH foreign_key_column
    INTO  lv_column_name
    ,     lv_referenced_table_schema
    ,     lv_referenced_table_name
    ,     lv_referenced_column_name;
 
    /* Catch handler for no more rows found from the fetch operation. */
    IF fetched = 1 THEN LEAVE cursor_foreign_key_column; END IF;
 
    /* Initialize the column list or add to it. */
    IF lv_column_count = 1 THEN
      SET lv_column_list := lv_column_name;
      SET lv_column_ref_list := lv_referenced_column_name;
 
      /* Increment the counter value. */
      SET lv_column_count := lv_column_count + 1;
    ELSE
      SET lv_column_list := CONCAT(lv_column_list,',',lv_column_name);
      SET lv_column_ref_list := CONCAT(lv_column_ref_list,',',lv_referenced_column_name);
    END IF;
 
  END LOOP cursor_foreign_key_column;
  CLOSE foreign_key_column;
 
  /* Set the return string to a list of columns. */
  SET lv_return_string :=
        CONCAT(lv_column_list,')',' '
              ,'REFERENCES',' ',lv_referenced_table_schema,'.',lv_referenced_table_name,' ' 
              ,'(',lv_column_ref_list,');');
 
  RETURN lv_return_string;
END;
$$

You can call the procedure with a schema and table name, and you’ll get the foreign keys from just that table. You can create the following parent and child tables to test how multiple column foreign keys work in the script (provided because most folks use surrogate keys):

CREATE TABLE parent
( first_name  VARCHAR(20)  NOT NULL DEFAULT ''
, last_name   VARCHAR(20)  NOT NULL DEFAULT ''
, PRIMARY KEY (first_name, last_name)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
CREATE TABLE child
( child_name  VARCHAR(20)  NOT NULL
, first_name  VARCHAR(20)  DEFAULT NULL
, last_name   VARCHAR(20)  DEFAULT NULL
, PRIMARY KEY (child_name)
, KEY fk_parent(first_name, last_name)
, CONSTRAINT fk_parent FOREIGN KEY (first_name, last_name)
  REFERENCES parent (first_name, last_name)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

You call the storeForeignKeys procedure for the child table with this syntax:

CALL storeForeignKeys('studentdb', 'child');

You call the storeForeignKeys procedure for all tables in a schema with this syntax:

CALL storeForeignKeys('studentdb', null);

While unlikely you’ll need this, the following calls the storeForeignKeys procedure for all tables in all schemas:

CALL storeForeignKeys(null, null);

You can export the command sequence with the following command to a script file:

SELECT sql_statement
INTO OUTFILE 'c:/Data/MySQL/apply_foreign_keys.sql'
FIELDS TERMINATED BY ','
OPTIONALLY ENCLOSED BY '"'
LINES TERMINATED BY '\r\n'
FROM command_list;

While preservation of tables and foreign keys is best managed by using a tool, like MySQL Workbench, it’s always handy to have scripts to do specific tasks. I hope this helps those looking for how to preserve foreign keys. You also can find a comprehensive treatment on how to write SQL/PSM code in Chapter 14 of my Oracle Database 11g and MySQL 5.6 Developer Handbook.

Written by maclochlainn

March 17th, 2014 at 11:27 pm

SQL Injection Risks

with 10 comments

While I tried to deflect how you perform SQL Injection attacks against a MySQL procedure, my students requested that I post examples of what to do to avoid SQL injection, and what not to do to invite attacks. The best practice to avoid SQL injection attacks is too always bind inputs to data types, and avoid providing completely dynamic WHERE clauses.

Here’s the correct way to dynamically generate a result from a MySQL Stored Procedure:

CREATE PROCEDURE hello (IN pv_input VARCHAR(50))
BEGIN
  SELECT sample_id
  ,      sample_name
  FROM   sample
  WHERE  sample_name = pv_input;
END;
$$

A call to this hello procedure will only return the row or rows where the pv_input value matches the sample_name column value. Any attempt to exploit it like the one below fails.

CALL hello('\'Harriet\' OR 1 = 1');

It fails because there’s no matching 'Harriet' OR 1 = 1 in the table’s sample_name column. However, it works well when we submit 'Harriet' by herself, without the intended SQL inject phrase “OR 1 = 1“, as you can see:

+-----------+-------------+
| sample_id | sample_name |
+-----------+-------------+
|         2 | Harriet     |
+-----------+-------------+
1 row in set (0.00 sec)

There are two approaches that you should never put in your code because they can be readily exploited unless you carefully parse the incoming string parameter. The problem in both cases is causes by concatenation rather than binding. The first example is extremely unlikely as an error but possible.

CREATE PROCEDURE hello (IN pv_input VARCHAR(50))
BEGIN
  SET @sql := CONCAT('SELECT sample_id, sample_name FROM sample WHERE sample_name = ',pv_input);
  PREPARE stmt FROM @sql;
  EXECUTE stmt;
END;
$$

The last hello procedure using concatenation and a prepared statement is vulnerable to SQL injection. A call like the following would return all two rows in the sample table.

CALL hello('\'Juliette\' OR 1 = 1');

It would display:

+-----------+-------------+
| sample_id | sample_name |
+-----------+-------------+
|         1 | Hank        |
|         2 | Harriet     |
+-----------+-------------+
2 rows in set (0.00 sec)

While there’s no sample_name value of 'Juliette', the “OR 1 = 1” is true. Therefore, the SELECT statement filters out nothing and returns all the data from the table. It’s probably clear you’d never do this type of prepared statement inside a stored procedure, but most SQL Injection attacks exploit your scripting language implementation. Unfortunately, bad coding practices can infrequently expose this type of vulnerability; and they typically occur when a junior programmers is following a bad coding example.

A solution with the WHERE clause as part of the parameter would look like this:

CREATE PROCEDURE hello (IN pv_where VARCHAR(50))
BEGIN
  SELECT sample_id
  ,      sample_name
  FROM   sample
  pv_where;
END;
$$

The modified call:

CALL hello('WHERE sample_name = \'Juliette\' OR 1 = 1');

returns all rows from the table.

A solution without the WHERE clause as part of the parameter would look like the following but it fails during runtime and returns no rows [Updated in response to comment]. The failure has nothing to do with the comment’s dialog on the CONCAT function, which also added nothing to the example once I tested it.

CREATE PROCEDURE hello (IN pv_where VARCHAR(50))
BEGIN
  SELECT sample_id
  ,      sample_name
  FROM   sample
  WHERE  pv_where;
END;
$$

It’s simply works only when you provide a “1 = 1″ or other comparison without embedded apostrophes (‘) but fails with embedded apostrophes. That means the following statement fails:

CALL hello('sample_name = \'Juliette\' OR 1 = 1');

but this SQL injection statement works:

CALL hello('1 = 1');

returns all rows from the table.

This example, when you omit the white space also works with embedded strings or numeric operands and an operator:

CREATE PROCEDURE hello (IN pv_where VARCHAR(50))
BEGIN
  SELECT sample_id
  ,      sample_name
  FROM   sample
  WHEREpv_where;
END;
$$

It returns all rows with a call like this:

CALL hello('sample_name = \'Juliette\' OR 1 = 1');

My take initially was that it might be a bug, and I logged one (Bug 68903). That’s was a dumb thing to do because WHEREpv_where simply becomes a table alias in the query.

In conclusion, the first example is a good practice. The other two should never exist! Well, they shouldn’t exist unless you’re parsing the web form inputs vigilantly.

Hope this helps those trying to understand how to avoid SQL injection attacks. Always try to solve dynamic SQL statement problems by binding variables into statements.

Written by maclochlainn

April 5th, 2013 at 8:35 pm

MySQL Database Triggers

with 3 comments

One of the students wanted an equivalent example to an Oracle DML trigger sample that replaces a white space in a last name with a dash for an INSERT statement. Apparently, the MySQL trigger example in the Oracle Database 11g and MySQL 5.6 Developer Handbook was a bit long. I have to agree with that because the MySQL DML trigger demonstrated cursors and loops in the trigger code.

Triggers can be statement- or row-level actions. Although some databases let you define statement-level triggers, MySQL doesn’t support them. MySQL only supports row-level triggers. Row-level triggers support critical or non-critical behaviors. Critical behavior means the trigger observes an insert, update, or delete that must be stopped, which means it raises an error. Non-critical behavior means the trigger observes a DML statement and logs it or implements a change during the context of the DML activity.

The first example shows you non-critical behavior. It observes an attempt to enter a two-part last name, and replaces the white space with a dash (you can find help on MySQL Regular Expressions in this other post). This means the trigger ensures compliance on how names are entered in the database, which should be protected in the web form (through JQuery or JavaScript) and the database.

DELIMITER $$
DROP TRIGGER IF EXISTS contact_insert$$
 
CREATE TRIGGER contact_insert
BEFORE INSERT ON contact
FOR EACH ROW
BEGIN
  IF new.last_name REGEXP '^.* .*$' THEN
    SET new.last_name := REPLACE(new.last_name,' ','-');
  END IF;
END;
$$
 
DELIMITER ;

The problem with implementing a non-critical trigger is that the database performs the work but clerks entering the data don’t learn the business rule. A critical trigger simply disallows non-conforming data entry. The next program shows a critical behavior with an UPDATE statement row-level trigger. After all, won’t a data entry clerk update the entry with a white space after the INSERT statement didn’t?

Yes, that was a rhetorical question. Spelling out the business rule in the UPDATE statement row-level trigger should educate the persistent errant behavior. While letting the INSERT statement row-level trigger manage the behavior probably saves time for most end-users who make a casual mistake.

DELIMITER $$
DROP TRIGGER IF EXISTS contact_update$$
 
CREATE TRIGGER contact_update
BEFORE UPDATE ON contact
FOR EACH ROW
BEGIN
  IF new.last_name REGEXP '^.* .*$' THEN
    SIGNAL SQLSTATE '42000';
  END IF;
END;
$$
 
DELIMITER ;

Somebody wanted to know why I choose SQLSTATE 42000. That’s because it signals an error in the SQL statement, and that’s the closest existing SQLSTATE to the actual behavior that exists. Moreover, the error identified by the critical trigger signals non-compliance with the application’s SQL standards that protects the data.

Hopefully, this helps somebody looking for a MySQL database trigger example that raises an exception. The example works with MySQL 5.5 forward because the critical trigger uses the SIGNAL feature, which was introduced in MySQL 5.5. Another article shows you how to leverage MyISAM tables to create a logging mechanism for critical event triggers, and you click this link to the MySQL Triggers with Logging blog entry.

Written by maclochlainn

November 22nd, 2012 at 11:32 pm

Placement over substance

with 5 comments

I was stunned when a SQL query raised an ERROR 1630 (42000) telling me the SUM function didn’t exist in MySQL 5.5.23. The fix was simple. The opening parenthesis of the SUM function must be on the same line as the SUM keyword without an intervening white space. Alternatively phrased, you can’t have a line return or white space between the SUM function name and the opening parenthesis of the call parameter list. The same rule doesn’t apply to the opening parenthesis of the FORMAT function and it seems to me that this parsing inconsistency is problematic.

Therefore, my surprise, observation, and complaint is that all functions don’t parse the same way, using the same rules. That is, unless you use specialized SQL_MODE settings. This assumption was borne out by Kolbe Kegel’s comment on this post, and there are 30 remaining built in functions that have specialized parsing and resolution markers.

A simplified version of the code that raises the error follows. As you’ll notice the opening parenthesis for the FORMAT and SUM function have intervening white space and a line return.

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SELECT   t.transaction_account AS "Transaction"
,        LPAD(FORMAT
           (SUM
             (CASE
                WHEN EXTRACT(MONTH FROM transaction_date) = 1 AND
                     EXTRACT(YEAR FROM transaction_date) = 2011 THEN
                  CASE
                    WHEN t.transaction_type = cl.common_lookup_type THEN
                      t.transaction_amount
                    ELSE
                      t.transaction_amount * -1
                  END
             END),2),10,' ') AS "JAN"
FROM     TRANSACTION t CROSS JOIN common_lookup cl
WHERE    cl.common_lookup_table = 'TRANSACTION'
AND      cl.common_lookup_column = 'TRANSACTION_TYPE'
AND      cl.common_lookup_type = 'DEBIT'
GROUP BY t.transaction_account;

Based on the comments, the SQL_MODE is:

mysql> SELECT @@version, @@sql_mode;
+-----------+----------------------------------------------------------------+
| @@version | @@sql_mode                                                     |
+-----------+----------------------------------------------------------------+
| 5.5.23    | STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+-----------+----------------------------------------------------------------+
1 ROW IN SET (0.00 sec)

It raises the following error:

ERROR 1630 (42000): FUNCTION studentdb.SUM does NOT exist. CHECK the 'Function Name Parsing and Resolution' SECTION IN the Reference Manual

Moving ONLY the opening parenthesis to the end of the SUM keyword (or removing the line return and white space from between the SUM keyword and opening parenthesis) prevents the error but it would be more convenient if it supported both approaches. It seems odd that an intervening line return and white space for the SUM function raises an exception while the same intervening line return and white space doesn’t raise an exception for the FORMAT function. It strikes me the parser should support both or reject both. Here’s the fixed code that works without enabling the IGNORE_SPACE SQL Mode option.

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SELECT   t.transaction_account AS "Transaction"
,        LPAD(FORMAT
           (SUM(
              CASE
                WHEN EXTRACT(MONTH FROM transaction_date) = 1 AND
                     EXTRACT(YEAR FROM transaction_date) = 2011 THEN
                  CASE
                    WHEN t.transaction_type = cl.common_lookup_type THEN
                      t.transaction_amount
                    ELSE
                      t.transaction_amount * -1
                  END
             END),2),10,' ') AS "JAN"
FROM     TRANSACTION t CROSS JOIN common_lookup cl
WHERE    cl.common_lookup_table = 'TRANSACTION'
AND      cl.common_lookup_column = 'TRANSACTION_TYPE'
AND      cl.common_lookup_type = 'DEBIT'
GROUP BY t.transaction_account;

As noted by the comments, adding the IGNORE_SPACE to the SQL_MODE lets both queries work without moving the open parenthesis. You can do that in a session with the following syntax (which is covered in an older post):

SET SQL_MODE=(SELECT CONCAT(@@sql_mode,',IGNORE_SPACE'));

Hope this helps folks…

Written by maclochlainn

July 3rd, 2012 at 4:10 pm

Implicit Commit Functions?

without comments

Somebody asked about the possibility of putting DML statements inside MySQL stored functions. DML statements like the INSERT, UPDATE, and DELETE. When I said, “Yes, you can put DML statements inside functions.” They showed me the error they encountered, which is only raised at compilation when you put an explicit COMMIT statement or a Data Definition Language (DDL) statement (CREATE, ALTER, DROP, or RENAME) inside a MySQL function. The actual error message displayed is:

ERROR 1422 (HY000): Explicit OR implicit commit IS NOT allowed IN stored FUNCTION OR TRIGGER.

While an explicit COMMIT is obvious when placed inside a function, the implicit COMMIT statement isn’t obvious unless you know a DDL statement generates one. This means you can’t include any DDL statement inside a stored function.

The following example shows how a DDL statement creates an immediate implicit COMMIT. It requires two sessions and Transaction Control Language (TCL) statements.

In session one, create a table, start a transaction scope, and insert one row into the table:

-- Create a table.
CREATE TABLE message
( message_id    INT UNSIGNED PRIMARY KEY AUTO_INCREMENT
, message_text  VARCHAR(20));
 
-- Start a transaction context.
BEGIN WORK;
 
-- Insert a row into the MESSAGE table.
INSERT INTO message (message_text) VALUES ('Implicit Commit?');

In session two, the empty set is displayed when you query the MESSAGE table. Returning to session one, add a new column to the MESSAGE table with this statement:

ALTER TABLE message ADD (sent DATETIME);

The ALTER statement automatically makes current any pending data changes, effectively committing the record from session one. You can return to the second session and query the table you get the following results:

+------------+------------------+------+
| message_id | message_text     | sent |
+------------+------------------+------+
|          1 | Implicit Commit? | NULL |
+------------+------------------+------+

When you embed DML statements inside functions, they rely on automatic commit behaviors or an external transaction scope. This function compiles and returns 1 when successful and 0 when unsuccessful, which effectively controls all error conditions within the function:

CREATE FUNCTION modifies_data ( pv_message  CHAR(20) ) RETURNS INT
MODIFIES SQL DATA
BEGIN
  /* Declare Boolean-like variables as FALSE. */
  DECLARE lv_return_value INT DEFAULT FALSE;
  DECLARE lv_error_value INT DEFAULT FALSE;
 
  /* Declare a generic exit handler to reset error control variable to true. */
  DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET lv_error_value := TRUE;   
 
  /* Insert statement with auto commit enabled. */
  INSERT INTO message (message_text) VALUES (pv_message);
 
  /* True unless the CONTINUE HANDLER disables the error control variable. */ 
  IF lv_error_value = FALSE THEN
    SET lv_return_value := TRUE;
  END IF;
 
  /* Return local variable. */
  RETURN lv_return_value;
END;
$$

With auto commit enabled, any call to the function in a query writes a row to the table. If you start a transaction and in the scope of the transaction test the function in one session and query the table in another you’ll see that transaction control can be managed outside the function. You can also manage the transaction control inside a stored procedure, which holds all the TCL commands. An example follows:

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CREATE PROCEDURE test_modifies_data ( pv_message  CHAR(20) )
MODIFIES SQL DATA
BEGIN
  /* Declare a control variable that manages transaction
     success or failure. */
  DECLARE lv_success_value  INT DEFAULT FALSE;
 
  /* Start a transaction context. */
  START TRANSACTION;
 
  /* Set a SAVEPOINT in the transaction context. */
  SAVEPOINT before_transaction;
 
  /* Call the function. */
  SET lv_success_value := modifies_data(pv_message);
 
  /* Check the status of the control variable, and commit
     or rollback the transaction. */
  IF lv_success_value = TRUE THEN
    COMMIT;
  ELSE
    ROLLBACK TO before_transaction;
  END IF;
 
END;
$$

Test the two with the following call to the procedure:

CALL test_modifies_data('Agent Coulson');

Answers my question and I hope it helps others.

Written by maclochlainn

June 5th, 2012 at 1:24 am

Value or Reference?

without comments

In class today, we reviewed pass-by-value (IN-only mode) parameters and pass-by-reference (INOUT and OUT mode) parameters for stored procedures. The analogy that finally seemed to hit home for the students was linking the modes to the story of Alice in Wonderland.

Here’s the analogy and below is the code to support it:

“A pass-by-value parameter in a procedure is like sending an immutable copy of Alice into the rabbit hole, which means she can’t shrink, grow, or learn throughout the story; whereas, a pass-by-reference parameter in a procedure is like sending Alice into the rabbit hole where she can shrink, grow, fight the Jabberwocky, and learn things that make her life better when she exits the rabbit hole – consistent with the storyline of Alice’s revisit to Wonderland.”

The example code creates a stored procedure that accepts two parameters – one pass-by-value and one pass-by-reference. Inside the procedure there’s a local variable and a reassignment of value to the pass-by-reference parameter. It’s in this wonderland procedure (by the way don’t forget to manage the DELIMITER value when you test it):

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CREATE PROCEDURE wonderland
( IN     pv_value_param  VARCHAR(20)
, INOUT  pv_ref_param    VARCHAR(20))
BEGIN
 
  /* Declare a variable. */
  DECLARE lv_value_param  VARCHAR(20);
 
  /* Query the local variable and reference parameter before changing values. */ 
  SELECT   'On Entry' AS "Where"
  ,        pv_value_param AS "Value Parameter"
  ,        pv_ref_param AS "Reference Parameter"
  ,        IFNULL(lv_value_param,'     ') AS "Local Variable";
 
  /* Assign a lowercase value parameter to a local variable. */
  SET lv_value_param := LOWER(pv_value_param);  
 
  /* Assign a uppercase reference parameter value to the reference parameter. */
  SET pv_ref_param := UPPER(pv_ref_param);
 
  /* Query the local variable and reference parameter after changing values. */ 
  SELECT   'On Exit ' AS "Where"
  ,        pv_value_param AS "Value Parameter"
  ,        pv_ref_param AS "Reference Parameter"
  ,        IFNULL(lv_value_param,'     ') AS "Local Variable";
 
END;
$$

A tester procedure than tests how the pass-by-value and pass-by-reference modes of operation differ. It’s here:

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CREATE PROCEDURE tester
( IN  pv_value_param  VARCHAR(20)
, IN  pv_ref_param    VARCHAR(20))
BEGIN
 
  /* Query the local and reference parameters. */ 
  SELECT   'Before  ' AS "Where"
  ,        pv_value_param AS "Value Parameter"
  ,        pv_ref_param AS "Reference Parameter";
 
  /* Call the wonderland procedure that changes the pass-by-reference parameter. */
  CALL wonderland(pv_value_param, pv_ref_param);
 
  /* Query the local and reference parameters. */ 
  SELECT   'After   ' AS "Where"
  ,        pv_value_param AS "Value Parameter"
  ,        pv_ref_param AS "Reference Parameter";
 
END;
$$

You call the tester program with this syntax:

CALL tester('Alice','Alice');

The test case returns the following values:

+----------+-----------------+---------------------+
| Where    | Value Parameter | Reference Parameter |
+----------+-----------------+---------------------+
| Before   | Alice           | Alice               |
+----------+-----------------+---------------------+
1 row in set (0.00 sec)
 
+----------+-----------------+---------------------+----------------+
| Where    | Value Parameter | Reference Parameter | Local Variable |
+----------+-----------------+---------------------+----------------+
| On Entry | Alice           | Alice               |                |
+----------+-----------------+---------------------+----------------+
1 row in set (0.02 sec)
 
+----------+-----------------+---------------------+----------------+
| Where    | Value Parameter | Reference Parameter | Local Variable |
+----------+-----------------+---------------------+----------------+
| On Exit  | Alice           | ALICE               | alice          |
+----------+-----------------+---------------------+----------------+
1 row in set (0.02 sec)
 
+----------+-----------------+---------------------+
| Where    | Value Parameter | Reference Parameter |
+----------+-----------------+---------------------+
| After    | Alice           | ALICE               |
+----------+-----------------+---------------------+
1 row in set (0.03 sec)

Basically, Alice inside the pv_ref_param parameter grows to uppercase during the trip through the wonderland procedure, while Alice inside the pv_value_param remains unchanged. If it didn’t help you learn a principle, maybe it gave you a laugh on how to view the travels of IN-only and INOUT parameters. ;-)

NOTE: Line 12 in the alice procedure is impossible with an immutable variable because the value of a call parameter to an immutable IN-only formal parameter shouldn’t allow the call parameter value to change during the execution of the program. This means that MySQL IN-only mode parameter values actually hold a mutable copy of the call parameter and the call parameter can be either a variable or literal value. The parameter value is discarded at the completion of procedure. This is more easily demonstrated with this assignment procedure:

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CREATE PROCEDURE assignment
( IN  pv_value_param  VARCHAR(20))
BEGIN
 
  /* Query the local and reference parameters. */ 
  SELECT   'Before  ' AS "Where"
  ,        pv_value_param AS "Value Parameter";
 
  /* Call the wonderland procedure that changes the pass-by-reference parameter. */
  SET pv_value_param := UPPER(pv_value_param);
 
  /* Query the local and reference parameters. */ 
  SELECT   'After   ' AS "Where"
  ,        pv_value_param AS "Value Parameter";
 
END;
$$

Therefore a call like this prints an uppercase INBOUND string inside the program but can return nothing to the calling scope since the call parameter is a string literal.

SET @sv_session = 'inbound';
CALL assignment(@sv_session);
SELECT @sv_test AS "Current Value";

The query displays:

+---------------+
| Current Value |
+---------------+
| inbound       |
+---------------+

I hope this helps.

Written by maclochlainn

May 10th, 2012 at 1:48 am

Oracle & MySQL Handbook

with 3 comments

My new Oracle Database 11g & MySQL 5.6 Developer Handbook will be available at Oracle Open World 2011 (OOW2011). It’s a great book to compare and contrast approaches in Oracle and MySQL. It covers Oracle SQL & PL/SQL and MySQL SQL & SQL/PSM (Persistent Stored Modules – functions and procedures). Unfortunately, it seems like the book won’t be available on amazon.com until much later in the month, and not available from amazon.de until November. You can read about it at McGraw Hill Professional’s web site. They’ve made a special effort to get copies to OOW2011. Here’s the source code for the book because I don’t know when it’ll be on the publisher’s web site.

I’ll also be at OOW2011. They’ve scheduled me in the bookstore (probably 2nd floor of Moscone North, as usual) from 10 to 10:30 A.M. on Monday and Wednesday for a book signing. If you’re at OOW2011 and you like to stop by and say hello, I look forward to meeting you. Many folks leave comments on the posts but only a few suggest what they’d like me to write on when I’ve got a chance, and you can do that if you stop by to chat.

It’s also interesting to know how many folks use both Oracle and MySQL (any updates on that are always appreciated). Last year at the Bioinformatics and Computation Biology (ACM-BCB 2010) Conference in Niagara Falls, I found it interesting to discover how many pharmaceutical companies and national labs were using both Oracle and MySQL. They appeared consistent about using Oracle for their systems governed by legal compliance rules and MySQL for actual research.

The pharmaceutical companies also had clear barriers between the researchers and professional IT staff, specifically the DBAs. It seems that the DBAs don’t want to cede any control over installed Oracle instances, and they place barriers to research by denying additional Oracle instances when their site licenses would allow them to do so at no incremental cost. On the other hand, the DBAs are fine with letting researchers host and pilot with the MySQL Community Edition databases. This book supports those trying to figure out how to write portable SQL and how to port solutions from MySQL to Oracle and vice versa.

Hope to meet a few new folks at OOW2011. The Kindle version of the book became available 11/25/2011.

As an addendum to this original post, some folks asked for the summary of content for the new book, and the location of the errata (the errors of omission and commission in the book). Below is a summary of the book from page XVIII of the Introduction, and the errata is in the second comment to this post:

Part I: Development Components

  • Chapter 1, “Architectures,” explains the Oracle 11g and MySQL 5.6 development architectures and highlights the comparative aspects of both client and server environments.
  • Chapter 2, “Client Interfaces,” explains and demonstrates the basics of how you use SQL*Plus and MySQL Monitor client software.
  • Chapter 3, “Security,” explains the security barriers for database servers and Data Control Language (DCL) commands that let you manage user and account privileges in the database servers.
  • Chapter 4, “Transactions,” explains the nature of ACID-compliant transactions and the Two-phase Commit (2PC) process demonstrated by INSERT, UPDATE, and DELETE statements.
  • Chapter 5, “Constraints,” explains the five primary database-level constraints and covers the check, not null, unique, primary key, and foreign key constraints.

Part II: SQL Development

  • Chapter 6, “Creating Users and Structures,” explains how you can create users, databases, tables, sequences, and indexes.
  • Chapter 7, “Modifying Users and Structures,” explains how you modify users, databases, tables, sequences, and indexes.
  • Chapter 8, “Inserting Data,” explains how you insert data into tables.
  • Chapter 9, “Updating Data,” explains how you update data in tables.
  • Chapter 10, “Deleting Data,” explains how you delete data from tables.
  • Chapter 11, “Querying Data,” explains how you query data from a single table, from a join of two or more tables, and from a join of two or more queries through set operators.
  • Chapter 12, “Merging Data,” explains how you import denormalized data from external tables or source files and insert or update records in normalized tables.

Part III: Stored Program Development

  • Chapter 13, “PL/SQL Basics,” explains the basics of using PL/SQL to write transactional blocks of code.
  • Chapter 14, “SQL/PSM Basics,” explains the basics of using SQL/PSM to write transactional blocks of code.
  • Chapter 15, “Triggers,” explains how to write database triggers in Oracle and MySQL databases.

Part IV: Appendix

  • Appendix, Covers the answers to the mastery questions at the end of the chapters.

Written by maclochlainn

September 14th, 2011 at 11:44 pm

MySQL Timestamp Columns

with 4 comments

Somebody asked how to work around an error message they got after converting one of their who-audit columns to a TIMESTAMP column. A TIMESTAMP column has a DEFAULT or ON UPDATE current timestamp. They wanted to have two TIMESTAMP columns in the same table, with the intention of:

  1. Having the created column assign a current timestamp value on insert
  2. Having the updated column assign a current timestamp value on insert and update

This is a sample table with two of the traditional four who-audit columns:

CREATE TABLE sample
( sample_id  INT UNSIGNED AUTO_INCREMENT PRIMARY KEY
, created    TIMESTAMP NOT NULL 
, updated    TIMESTAMP NOT NULL
                       DEFAULT CURRENT_TIMESTAMP
                       ON UPDATE CURRENT_TIMESTAMP);

When they tried to create the table they got the following error:

ERROR 1293 (HY000): Incorrect TABLE definition; there can be ONLY one TIMESTAMP COLUMN WITH CURRENT_TIMESTAMP IN DEFAULT OR ON UPDATE clause

While you can define a table with two columns that have a TIMESTAMP data type, you can’t define a table with two TIMESTAMP columns when one holds a DEFAULT or ON UPDATE CURRENT_TIMESTAMP value. However, you can use a DATETIME data type for the created column provided it’s null allowed, like:

CREATE TABLE sample
( sample_id  INT UNSIGNED AUTO_INCREMENT PRIMARY KEY
, created    DATETIME 
, updated    TIMESTAMP NOT NULL
                       DEFAULT CURRENT_TIMESTAMP
                       ON UPDATE CURRENT_TIMESTAMP);

You can then define an on insert database trigger that fires when you create a new row, like this:

CREATE TRIGGER sample_t
BEFORE INSERT ON sample
FOR EACH ROW
BEGIN
  SET NEW.created = CURRENT_TIMESTAMP();
END;
$$

An INSERT statement would look like this:

INSERT INTO sample VALUES (NULL, NULL, NULL);

You would see the following if you query the table:

+-----------+---------------------+---------------------+
| sample_id | created             | updated             |
+-----------+---------------------+---------------------+
|         1 | 2011-08-11 00:26:21 | 2011-08-11 00:26:21 |
+-----------+---------------------+---------------------+

Hope this helps other too.

Written by maclochlainn

August 11th, 2011 at 1:31 am

Why SELECT-INTO, eh?

without comments

Somebody raised the question about writing stored functions while we were discussing scalar subqueries against COMMON_LOOKUP tables. Common look up tables store collections of possible <OPTION> elements. They’re basically generalized tables that contain a set of smaller tables, where the row sets make up a list of unique values.

When you write a function that must return one and only one row, the SELECT-INTO syntax is ideal in Oracle because it automatically raises an exception when the query returns no row or two or more rows. MySQL doesn’t raise an automatic exception when a SELECT-INTO fails to return a row but it does raise an ERROR 1722 when two or more rows are found.

Here are sample implementations written in Oracle’s PL/SQL and MySQL’s SQL/PSM languages. If you’re an Oracle developer and new to MySQL, the biggest oddity may be the delimiter, or it may be the sizing of formal parameters. Hopefully, its not the lack of formal declaration and exception blocks in SQL/PSM. For those coming from MySQL, the OR REPLACE command or dynamic sizing of formal parameter list values are the big changes.

Oracle PL/SQL Function

The Oracle PL/SQL function is very straightforward and helped by pre-defined exceptions for both the no data found and too many rows found errors. Formal parameters inherit their physical size at run time from the calling scope program.

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-- Create or replace existing function with new implementation.
CREATE OR REPLACE FUNCTION get_common_lookup_id
( pv_table   VARCHAR2
, pv_column  VARCHAR2
, pv_type    VARCHAR2 )
RETURN NUMBER IS
 
  -- Declare a return variable.
  lv_return  NUMBER;
 
BEGIN
 
  -- Query data and assign it to a local variable.
  SELECT   common_lookup_id
  INTO     lv_return
  FROM     common_lookup
  WHERE    common_lookup_table = pv_table
  AND      common_lookup_column = pv_column
  AND      common_lookup_type = pv_type;
 
  -- Return the value found.
  RETURN lv_return;
 
EXCEPTION
 
  -- Handle errors.
  WHEN NO_DATA_FOUND THEN
    RAISE;
  WHEN TOO_MANY_ROWS THEN
    RAISE;
 
END;
/

MySQL SQL/PSM Function

The lack of an implicitly raised exception when a SELECT-INTO structure fails to return a row adds several steps to a SQL/PSM function. You must define a custom exception and an if-block to raise the exception, which is trigger by returning a null value into the lv_return local variable.

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/* Conditionally drop the function before trying to create it. */
DROP FUNCTION IF EXISTS get_common_lookup_id;
 
/* Change the delimiter to write a stored procedure into the database. */
DELIMITER $$
 
/* Create a function that replaces a scalar subquery. */
CREATE FUNCTION get_common_lookup_id
( pv_table   VARCHAR(30)
, pv_column  VARCHAR(30)
, pv_type    VARCHAR(30))
RETURNS INT UNSIGNED
BEGIN
 
  /* Declare a return variable. */
  DECLARE lv_return  INT UNSIGNED;
 
  /* Declare a local variable for a subsequent handler. */
  DECLARE no_data_found CONDITION FOR SQLSTATE '99001';
 
  /* Query data and assign it to a local variable. */
  SELECT   common_lookup_id
  INTO     lv_return
  FROM     common_lookup
  WHERE    common_lookup_table = pv_table
  AND      common_lookup_column = pv_column
  AND      common_lookup_type = pv_type;
 
  /* The local variable is only null when no rows are returned by the implicit cursor. */
  IF lv_return IS NULL THEN
    SIGNAL no_data_found SET MESSAGE_TEXT = 'Result was no rows found.';
  END IF;  
 
  /* Return the value found. */
  RETURN lv_return;
 
END;
$$
 
/* Reset the delimiter to the default to run other programs. */
DELIMITER ;

Hope this help solve your problems.

Written by maclochlainn

February 16th, 2011 at 11:03 pm