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Dynamic Drop Table

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I always get interesting feedback on some posts. On my test case for discovering the STR_TO_DATE function’s behavior, the comment was tragically valid. I failed to cleanup after my test case. That was correct, and I should have dropped param table and the two procedures.

While appending the drop statements is the easiest, I thought it was an opportunity to have a bit of fun and write another procedure that will cleanup test case tables within the test_month_name procedure. Here’s sample dynamic drop_table procedure that you can use in other MySQL stored procedures:

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CREATE PROCEDURE drop_table
( table_name  VARCHAR(64))
BEGIN
 
  /* Declare a local variable for the SQL statement. */
  DECLARE stmt VARCHAR(1024);
 
  /* Set a session variable with two parameter markers. */
  SET @SQL := CONCAT('DROP TABLE ',table_name);
 
  /* Check if the constraint exists. */    
  IF EXISTS (SELECT NULL
             FROM   information_schema.tables t
             WHERE  t.table_schema = database()
             AND    t.table_name = table_name)
  THEN
 
    /* Dynamically allocated and run statement. */
    PREPARE stmt FROM @SQL;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
  END IF;
 
END;
$$

You can now put a call to the drop_table procedure in the test_month_name procedure from the earlier post. For convenience, here’s the modified test_month_name procedure with the call on line #33 right before you leave the loop and procedure:

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CREATE PROCEDURE test_month_name()
BEGIN
 
  /* Declare a handler variable. */
  DECLARE month_name  VARCHAR(9);
 
  /* Declare a handler variable. */
  DECLARE fetched  INT DEFAULT 0;
 
  /* Cursors must come after variables and before event handlers. */
  DECLARE month_cursor CURSOR FOR
    SELECT m.month_name
    FROM   month m;
 
  /* Declare a not found record handler to close a cursor loop. */
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET fetched = 1;
 
  /* Open cursor and start simple loop. */
  OPEN month_cursor;
  cursor_loop:LOOP
 
    /* Fetch a record from the cursor. */
    FETCH month_cursor
    INTO  month_name;
 
    /* Place the catch handler for no more rows found
       immediately after the fetch operations. */
    IF fetched = 1 THEN 
      /* Fetch the partial strings that fail to find a month. */
      SELECT * FROM param;
 
      /* Conditionally drop the param table. */
      CALL drop_table('param');
 
      /* Leave the loop. */
      LEAVE cursor_loop;
    END IF;
 
    /* Call the subfunction because stored procedures do not
       support nested loops. */
    CALL read_string(month_name);
  END LOOP;
END;
$$

As always, I hope sample code examples help others solve problems.

Written by maclochlainn

February 12th, 2022 at 12:33 pm

Posted in MySQL,MySQL 8,sql

Tagged with ,

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