Archive for June, 2012
Derived Table Aliases
In my database class, students write solutions as group exercises against the Oracle 11g XE database and then they port the solution individually to the MySQL 5.5 database. One of the students copied over a query like the one below to MySQL (a query used to track the expected number of row returns).
SELECT COUNT(*) FROM (SELECT DISTINCT k.kingdom_id , kki.kingdom_name , kki.population FROM kingdom_knight_import kki LEFT JOIN kingdom k ON kki.kingdom_name = k.kingdom_name AND kki.population = k.population); |
It got an error they didn’t understand:
ERROR 1248 (42000): Every derived TABLE must have its own alias |
Providing a dt query alias fixes the problem in MySQL for the following query. The fact that it was just an alias was a revelation to the student. That’s because Oracle databases don’t require aliases for inline views (what Oracle calls MySQL derived tables and Microsoft calls a Common Table Expression (CTE)).
SELECT COUNT(*) FROM (SELECT DISTINCT k.kingdom_id , kki.kingdom_name , kki.population FROM kingdom_knight_import kki LEFT JOIN kingdom k ON kki.kingdom_name = k.kingdom_name AND kki.population = k.population) dt; |
MySQL requires that every derived table have its own alias. This make sense when you think about query optimization engines work, but that’s a story for a much longer post.
You can find more about inline views, derived tables, and common table expressions in this older post of mine. This post was promised so that future students can simply google and answer rather than ask me or a class tutor.
Single Wildcard Operator
Somebody wanted to understand why you can backquote a single wildcard operator (that’s the underscore _ character) in MySQL, but can’t in Oracle. The answer is you can in Oracle when you know that you required an additional clause.
While I prefer using regular expression resolution, the LIKE operator is convenient. Here’s an example of backquoting an underscore in MySQL, where it looks for any string with an underscore anywhere in the string:
SELECT common_lookup_type FROM common_lookup WHERE common_lookup_type LIKE '%\_%'; |
You can gain the same behavior in Oracle by appending the ESCAPE '\' clause, like this:
SELECT common_lookup_type FROM common_lookup WHERE common_lookup_type LIKE '%\_%' ESCAPE '\'; |
The ESCAPE '\' clause is one of those Oracle details that often gets lost. It only works when the SQL*Plus ESCAPE parameter is set to OFF.
The SQL*Plus ESCAPE parameter default value is a backslash. and when the ESCAPE parameter is enabled a statement like this raises the following exception:
ERROR at line 3: ORA-01425: escape CHARACTER must be CHARACTER string OF LENGTH 1 |
If you drop the ESCAPE '\' clause with the ESCAPE parameter enabled it will return all rows from the table not just those strings with an underscore in the string. Hope this helps if need to look for an underscore in a table.
Overloading Procedures
A student asked, “Can you successfully overload MySQL stored procedures, like PL/SQL does in stored packages?” I said, “MySQL doesn’t formally support overloading stored procedures, and doesn’t support packages like Oracle 11g. You can, however, mimic (or fake) overloading with prepared statements. It requires a careful combination of stored procedures and session variables.”
Unfortunately, they asked for an example. So I wrote this set of code. It uses queries as the dynamic statements to make it simple to test but you can substitute INSERT, UPDATE, or DELETE statements. I didn’t provide type validation in the example, which would be required for dates or date-timestamps.
It’s presented in steps with test programs at each level. If you see an opportunity to improve on any of the example components, leave a comment. As to whether I’d implement this in production code, the answer is no but I believe all possibilities should at least be explored.
Step #1
The first step requires defining a stored procedure that sets a session variable. It’s fairly straight forward, but remember to change those DELIMITER values when testing. (As a note, you don’t require a semicolon after the END statement of a stored function or procedure in MySQL. You only need the redefined DELIMITER, which is $$ in these examples.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | DELIMITER $$ -- Drop the procedure if it exists. DROP PROCEDURE IF EXISTS set_session_var$$ -- Create the procedure. CREATE PROCEDURE set_session_var ( pv_session_name VARCHAR(32767) , pv_session_value VARCHAR(32767)) CONTAINS SQL BEGIN /* Insert statement with auto commit enabled. */ SET @sql := concat('SET',' ','@',pv_session_name,' := ','?'); SELECT @sql AS "SQL String"; PREPARE stmt FROM @sql; SET @sv_session_value := pv_session_value; EXECUTE stmt using @sv_session_value; DEALLOCATE PREPARE stmt; END; $$ |
The test case for the set_session_var is:
CALL set_session_var('sv_filter1','One'); CALL set_session_var('sv_filter2','Two'); SELECT @sv_filter1, @sv_filter2; |
Important note: If you call this from another stored program you can’t use the stmt variable name in the calling program’s scope.
Step #2
The second step exists because you can’t pass arrays in MySQL (a restriction that also exists for T-SQL in Microsoft SQL Server). You have to pass a varying list of parameters as a serialized string. This is often called flexible parameter passing, which many PHP programmers leverage outside the database (flexible parameter passing is covered in this PHP tutorial I wrote a while back).
The deserialize example does three things:
- Takes a serialized set of parameters and deserializes the parameters into a set of name-value pairs, which are stored by their name as session level variables.
- It records the number of name-value pairs written as session variables in a temporary table. The use of a temporary table is required because a MySQL stored function can’t hold a dynamic statement or a call to a procedure with a dynamic statement.
- The script checks for the existence of the table during execution and truncates it when found. It creates the table when it isn’t found. While it would be ideal to use a temporary table, they’re not disclosed in the the
information_schemaand therefore more tedious to manage. Creating the table in the MEMORY database avoids fragmenting the InnoDB tablespace.
Here’s the working code:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 | CREATE PROCEDURE deserialize ( pv_param_list VARCHAR(32767)) CONTAINS SQL BEGIN DECLARE lv_name VARCHAR(9) DEFAULT 'sv_filter'; DECLARE lv_length INT; DECLARE lv_start INT DEFAULT 1; DECLARE lv_end INT DEFAULT 1; DECLARE lv_counter INT DEFAULT 1; /* Skip when call parameter list is null or empty. */ IF NOT (ISNULL(pv_param_list) OR LENGTH(pv_param_list) = 0) THEN /* Read line by line on a line return character. */ parse: WHILE NOT (lv_end = 0) DO /* Check for line returns. */ SET lv_end := LOCATE(',',pv_param_list,lv_start); /* Check whether line return has been read. */ IF NOT lv_end = 0 THEN /* Reset the ending substring value. */ SET lv_end := LOCATE(',',pv_param_list,lv_start); CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,lv_end - lv_start)); ELSE /* Print the last substring with a semicolon. */ CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,LENGTH(pv_param_list))); END IF; /* Reset the beginning of the string. */ SET lv_start := lv_end + 1; SET lv_counter := lv_counter + 1; END WHILE parse; END IF; /* Check for a temporary table that holds a control variable, create the table if it doesn't exist, and remove rows from the table. */ IF EXISTS (SELECT null FROM information_schema.tables WHERE table_name = 'counter') THEN TRUNCATE TABLE counter; ELSE /* It would be ideal to use a temporary table here but then it's not recorded in the INFORMATION_SCHEMA and cleansing the temporary table is more tedious. */ CREATE TABLE counter ( counter INT ) ENGINE=MEMORY; END IF; /* Insert the counter value for a list of parameters. */ INSERT INTO counter VALUES ( lv_counter - 1 ); END; $$ |
Bill Karwin made a wonderful point about the overhead of looking up a table name and using a table rather than a temporary table. You can replace the logic above with a CONTINUE HANDLER and a temporary table. It should resolve the problems mentioned in the note but as mentioned earlier there are other solutions for production systems.
Bill also referenced a great post by Jay Pipes. It points out that there are serious problems scaling MySQL stored programs. This occurs because MySQL stored procedures are not compiled and stored in a global stored procedure cache, like Microsoft SQL Server or Oracle.
This deficit in MySQL certainly plays to the philosophy of developing everything in the application layer, suggested by Bill. At least, that is true when the application layer is co-resident on the server-tier as a Server Side Includes (SSI) library.
It does beg the question why Microsoft SQL Server and Oracle enable placing an efficient set of stored logic in the database server, effectively creating an application layer that can be split between an SSI library and the database server. After all, the server has been the most scalable component and the application layer can distribute to the middle-tier.
Today’s limitations on MySQL stored programs present a clear and compelling case toward their general avoidance and specific use for delivering a scalable solutions. Will that hold true beyond 2015 when licensing becomes completely proprietary to Oracle? I’d venture (a complete guess or hope on my part) a global stored procedure cache may be the future of MySQL procedures. It would certainly serve to distinguish MySQL from MariaDB and provide an incentive to remain on MySQL or to return from MariaDB to MySQL. Such a change would certainly make MySQL even more competitive against Microsoft SQL Server, which is the only rival purely relational database it competes against.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 | CREATE PROCEDURE deserialize ( pv_param_list VARCHAR(32767)) CONTAINS SQL BEGIN DECLARE lv_name VARCHAR(9) DEFAULT 'sv_filter'; DECLARE lv_length INT; DECLARE lv_start INT DEFAULT 1; DECLARE lv_end INT DEFAULT 1; DECLARE lv_counter INT DEFAULT 1; DECLARE CONTINUE HANDLER FOR 1146 BEGIN /* Create a temporary table. */ CREATE TEMPORARY TABLE counter ( counter INT ) ENGINE=MEMORY; END; /* Skip when call parameter list is null or empty. */ IF NOT (ISNULL(pv_param_list) OR LENGTH(pv_param_list) = 0) THEN /* Read line by line on a line return character. */ parse: WHILE NOT (lv_end = 0) DO /* Check for line returns. */ SET lv_end := LOCATE(',',pv_param_list,lv_start); /* Check whether line return has been read. */ IF NOT lv_end = 0 THEN /* Reset the ending substring value. */ SET lv_end := LOCATE(',',pv_param_list,lv_start); CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,lv_end - lv_start)); ELSE /* Print the last substring with a semicolon. */ CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,LENGTH(pv_param_list))); END IF; /* Reset the beginning of the string. */ SET lv_start := lv_end + 1; SET lv_counter := lv_counter + 1; END WHILE parse; END IF; /* Truncate existing table. */ TRUNCATE TABLE counter; /* Insert the counter value for a list of parameters. */ INSERT INTO counter VALUES ( lv_counter - 1 ); END; $$ |
The test program for deserialize follows below. If you’re curious, those are the Italian version of one through four. 😉 I’ve a partiality for Italian after living in Italy for two years.
CALL deserialize('Uno,Due,Tre,Quattro'); SELECT counter AS "Parameter #" FROM counter; SELECT @sv_filter1, @sv_filter2, @sv_filter3, @sv_filter4; |
You can also eliminate the temporary table completely by replacing the counter with a @sv_counter session variable, as mentioned by Bill in his comment. The following is the version of deserialize uses a session variable.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 | CREATE PROCEDURE deserialize ( pv_param_list VARCHAR(32767)) CONTAINS SQL BEGIN DECLARE lv_name VARCHAR(9) DEFAULT 'sv_filter'; DECLARE lv_length INT; DECLARE lv_start INT DEFAULT 1; DECLARE lv_end INT DEFAULT 1; /* Set a session variable to enable a calling scope to read it without a function return. */ SET @sv_counter := 1; /* Skip when call parameter list is null or empty. */ IF NOT (ISNULL(pv_param_list) OR LENGTH(pv_param_list) = 0) THEN /* Read line by line on a line return character. */ parse: WHILE NOT (lv_end = 0) DO /* Check for line returns. */ SET lv_end := LOCATE(',',pv_param_list,lv_start); /* Check whether line return has been read. */ IF NOT lv_end = 0 THEN /* Reset the ending substring value. */ SET lv_end := LOCATE(',',pv_param_list,lv_start); CALL set_session_var(CONCAT(lv_name,@sv_counter),SUBSTR(pv_param_list,lv_start,lv_end - lv_start)); ELSE /* Print the last substring with a semicolon. */ CALL set_session_var(CONCAT(lv_name,@sv_counter),SUBSTR(pv_param_list,lv_start,LENGTH(pv_param_list))); END IF; /* Reset the beginning of the string. */ SET lv_start := lv_end + 1; SET @sv_counter := @sv_counter + 1; END WHILE parse; END IF; /* Reduce by one for 1-based numbering of name elements. */ SET @sv_counter := @sv_counter - 1; END; $$ |
The test case changes for this version because there isn’t a table involved. You would use the following:
CALL deserialize('Uno,Due,Tre,Quattro'); SELECT @sv_filter1, @sv_filter2, @sv_filter3, @sv_filter4; |
Like the earlier procedure, there’s no exception handling but it wouldn’t be a bad idea to add it. Especially, if you’re using the new SIGNAL feature of MySQL 5.6, which is now backported to MySQL 5.5.
Step #3
The last step creates a store procedure that takes two parameters, a parameterized statement and a serialized list of parameters. This means you can build any statement on the fly as required. The example uses queries simply because they’re the easiest to demonstrate the process.
You should note that the prepared statement is dynamic_stmt not stmt because stmt is used inside the set_session_var procedure. That means using stmt in the calling scope program would impact the called scope program because they use the same identifier (more or less a namespace scope issue).
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 | CREATE PROCEDURE prepared_dml ( pv_query VARCHAR(32767) , pv_filter VARCHAR(32767)) CONTAINS SQL BEGIN /* Declare a local variable for the SQL statement. */ DECLARE dynamic_stmt VARCHAR(32767); DECLARE lv_counter INT DEFAULT 0; /* Cleanup the message passing table when a case is not found. */ DECLARE EXIT HANDLER FOR 1339 BEGIN /* Step #5: */ DEALLOCATE PREPARE dynamic_stmt; /* Cleanup the temporary table that exchanges data between procedures. */ DROP TABLE IF EXISTS counter; END; /* Step #1: ======== Set a session variable with two parameter markers. */ SET @SQL := pv_query; /* Verify query is not empty. */ IF NOT ISNULL(@SQL) THEN /* Step #2: ======== Dynamically allocated and run statement. */ PREPARE dynamic_stmt FROM @SQL; /* Step #3: ======== Assign the formal parameters to session variables because prepared statements require them. */ CALL deserialize(pv_filter); /* Secure the parameter count from a temporary table that exchanges data between procedures. */ SELECT counter INTO lv_counter FROM counter; /* Step #4: ======== Choose the appropriate overloaded prepared statement. */ CASE WHEN lv_counter = 0 THEN EXECUTE dynamic_stmt; WHEN lv_counter = 1 THEN EXECUTE dynamic_stmt USING @sv_filter1; WHEN lv_counter = 2 THEN EXECUTE dynamic_stmt USING @sv_filter1,@sv_filter2; END CASE; /* Step #5: */ DEALLOCATE PREPARE dynamic_stmt; /* Cleanup the temporary table that exchanges data between procedures. */ DROP TABLE IF EXISTS counter; END IF; END; $$ |
If you’re using the deserialize procedure that uses a temporary table, you should make the following changes prepared_dml procedure on line 19 and 62. They modify the DROP TABLE statement to a DROP TEMPORARY TABLE statement. Though the change isn’t technically required because a DROP TABLE works against temporary tables as well as ordinary tables, it generally clearer to those new to MySQL syntax.
17 18 19 | /* Cleanup the temporary table that exchanges data between procedures. */ DROP TEMPORARY TABLE IF EXISTS counter; |
The line 62 change:
60 61 62 | /* Cleanup the temporary table that exchanges data between procedures. */ DROP TEMPORARY TABLE IF EXISTS counter; |
It would be much nicer to avoid the CASE statement or an if-else-if block but prepared statements are limited on what SQL statements they support. For example, you can create or drop stored functions or procedures, and you can’t dispatch a dynamic statement as a dynamic statement. That means you can’t replace the CASE statement with something like this:
43 44 45 46 | SET @dsql := 'EXECUTE dynamic_stmt USING @sv_filter1, @sv_filter2'; PREPARE preparsed_stmt FROM @dsql; EXECUTE preparsed_stmt; DEALLOCATE PREPARE preparsed_stmt; |
Attempt that, and you’ll get the following message. Does that mean there’s hope it’ll become a feature and possibility? Or, that it isn’t recommended?
ERROR 1295 (HY000): This command is not supported in the prepared statement protocol yet |
The session variable solution requires another version of the prepared_dml procedure, as shown below. As you can imagine, removing the table and replacing a session variable simplifies the prepared_dml stored procedure version.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 | CREATE PROCEDURE prepared_dml ( pv_query VARCHAR(32767) , pv_filter VARCHAR(32767)) CONTAINS SQL BEGIN /* Declare a local variable for the SQL statement. */ DECLARE dynamic_stmt VARCHAR(32767); DECLARE lv_counter INT DEFAULT 0; /* Cleanup the message passing table when a case is not found. */ DECLARE EXIT HANDLER FOR 1339 BEGIN /* Step #5: */ DEALLOCATE PREPARE dynamic_stmt; END; /* Step #1: ======== Set a session variable with two parameter markers. */ SET @SQL := pv_query; /* Verify query is not empty. */ IF NOT ISNULL(@SQL) THEN /* Step #2: ======== Dynamically allocated and run statement. */ PREPARE dynamic_stmt FROM @SQL; /* Step #3: ======== Assign the formal parameters to session variables because prepared statements require them. */ CALL deserialize(pv_filter); /* Step #4: ======== Choose the appropriate overloaded prepared statement. */ CASE WHEN @sv_counter = 0 THEN EXECUTE dynamic_stmt; WHEN @sv_counter = 1 THEN EXECUTE dynamic_stmt USING @sv_filter1; WHEN @sv_counter = 2 THEN EXECUTE dynamic_stmt USING @sv_filter1,@sv_filter2; END CASE; /* Step #5: */ DEALLOCATE PREPARE dynamic_stmt; END IF; END; $$ |
The test programs check without a parameter and with one or two parameters, as you can see below.
SELECT 'Test Case #1 ...' AS "Statement"; SET @param1 := 'SELECT "Hello World"'; SET @param2 := ''; CALL prepared_dml(@param1,@param2); SELECT 'Test Case #2 ...' AS "Statement"; SET @param1 := 'SELECT item_title FROM item i WHERE item_title REGEXP ?'; SET @param2 := '^.*war.*$'; CALL prepared_dml(@param1,@param2); SELECT 'Test Case #3 ...' AS "Statement"; SET @param1 := 'SELECT common_lookup_type FROM common_lookup cl WHERE common_lookup_table REGEXP ? AND common_lookup_column REGEXP ?'; SET @param2 := 'item,item_type'; CALL prepared_dml(@param1,@param2); |
As always, I hope this helps those writing MySQL Stored Procedures. If you’re new to stored programs, you can find a chapter on writing them in my Oracle Database 11g & MySQL 5.6 Developer Handbook or you can use Guy Harrison’s MySQL Stored Procedure Programming.
NDS parameters as IN OUT?
A question posed by a student: “Why are Oracle Native Dynamic SQL (NDS) USING clause parameters IN, IN OUT, or OUT when the RETURNING INTO clause manages output values?” It a great question, isn’t it? The followup question was also great, “How do you implement an example of NDS IN OUT parameters?”
The answer is two fold. First, you should use the USING clause for parameter list input values and the RETURNING INTO clause for return values whenever possible. Second, when it’s not possible you’re generally passing parameters into and out of an NDS PL/SQL anonymous block.
The basic prototype for passing and retrieving values from an NDS statement is:
EXECUTE IMMEDIATE sql_stmt USING { IN | IN OUT | OUT } local_variable [, ...] RETURNING INTO { IN OUT | OUT } local_variable [, ...]; |
A quick and hopefully fun example is this parody on Marvel’s The Avengers. The program creates an anonymous block with a super hero of Thor and super villain of Loki, then it uses a USING clause with IN OUT parameters to an anonymous block statement. That’s basically the trick to how you use IN OUT parameters in NDS statements.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 | -- Enable SERVEROUTPUT. SET SERVEROUTPUT ON SIZE UNLIMITED -- Declare an anonymous testing block. DECLARE -- Declare two local variables. lv_super_hero VARCHAR2(20) := 'Thor'; lv_super_villain VARCHAR2(20) := 'Loki'; -- Declare a null statement variable. lv_stmt VARCHAR2(32767); -- Declare a local procedure to parse the NDS block. PROCEDURE print_code_block (pv_block VARCHAR2) IS -- Declare local parsing variables. lv_length INTEGER := 1; lv_start INTEGER := 1; lv_end INTEGER := 1; BEGIN -- Read line by line on a line return character. WHILE NOT (lv_end = 0) LOOP -- Check for line returns. lv_end := INSTR(lv_stmt,CHR(10),lv_start); -- Check whether line return has been read. IF NOT lv_end = 0 THEN -- Reset the ending substring value and print substring. lv_end := INSTR(lv_stmt,CHR(10),lv_start); dbms_output.put_line('| '||SUBSTR(lv_stmt,lv_start,lv_end - lv_start)); ELSE -- Print the last substring with a semicolon and exit the loop. dbms_output.put_line('| '||SUBSTR(lv_stmt,lv_start,LENGTH(lv_stmt) - lv_start)||';'); END IF; -- Reset the beginning of the string. lv_start := lv_end + 1; END LOOP; END print_code_block; BEGIN -- Demonstrate good triumps over evil. dbms_output.put_line('The good '||lv_super_hero||' beats up the bad '||lv_super_villain||'!'); -- Assign the anonymous block to the local statement variable. lv_stmt := 'DECLARE'||CHR(10) || ' lv_super_hero VARCHAR2(20);'||CHR(10) || ' lv_super_villain VARCHAR2(20);'||CHR(10) || 'BEGIN'||CHR(10) || ' lv_super_hero '||CHR(58)||'= :pv_super_hero;'||CHR(10) || ' lv_super_villain '||CHR(58)||'= :pv_super_villain;'||CHR(10) || ' :pv_super_hero '||CHR(58)||'= lv_super_villain;'||CHR(10) || ' :pv_super_villain '||CHR(58)||'= lv_super_hero;'||CHR(10) || 'END;'; -- Run the NDS program. EXECUTE IMMEDIATE lv_stmt USING IN OUT lv_super_hero , IN OUT lv_super_villain; -- Print the diagnostic code block, that's why it used line returns afterall. dbms_output.put_line('--------------------------------------------------'); print_code_block(lv_stmt); dbms_output.put_line('--------------------------------------------------'); -- Demonstrate the world is upside down without Johnny Depp playing Capt'n Jack. dbms_output.put_line('The good '||lv_super_hero||' beats up the bad '||lv_super_villain||'!'); END; / |
You’ll get the following printed output:
The good Thor beats up the bad Loki! -------------------------------------------------- | DECLARE | lv_super_hero VARCHAR2(20); | lv_super_villain VARCHAR2(20); | BEGIN | lv_super_hero := :pv_super_hero; | lv_super_villain := :pv_super_villain; | :pv_super_hero := lv_super_villain; | :pv_super_villain := lv_super_hero; | END; -------------------------------------------------- The good Loki beats up the bad Thor! |
As always, I hope it helps you understand the concept of the USING clause with IN OUT parameters but I hope there’s always better way.
Implicit Commit Functions?
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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | 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.
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