mysql dynamic queries execution - mysql

Question

I'm trying to write MySQL script dropping some tables selected by pattern but my procedure doesn't compile. Could anybody please advice what is wrong with it please?
delimiter #
drop procedure if exists drop_audit_tables #
create procedure drop_audit_tables()
begin
declare done int default false;
declare cmd varchar(4000);
declare cmds cursor for select 'drop table [' + table_name + ']' from information_schema.tables where table_name like '%_audit';
declare continue handler for not found set done = true;
open cmds;
tLoop: loop
fetch cmds into cmd;
if done then
leave tLoop;
end if;
PREPARE STMT FROM cmd;
EXECUTE STMT;
DEALLOCATE PREPARE STMT;
end loop tLoop;
close cmds;
end #
the error message:
[42000][1064] You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'cmd; EXECUTE STMT; DEALLOCATE PREPARE STMT; end loop tLoop; close cm' at line 13

Answer 1

You can avoid the cursor:
mysql> DROP TABLE IF EXISTS `one_audit`;
Query OK, 0 rows affected (0.00 sec)
mysql> DROP TABLE IF EXISTS `two_audit`;
Query OK, 0 rows affected (0.01 sec)
mysql> DROP TABLE IF EXISTS `three_audit`;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE `one_audit`(`a` INT);
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE `two_audit`(`a` INT);
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE `three_audit`(`a` INT);
Query OK, 0 rows affected (0.00 sec)
mysql> SET #`drop_tables` := (
-> SELECT
-> CONCAT('DROP TABLE IF EXISTS ',
-> GROUP_CONCAT(CONCAT('`', `TABLE_NAME`, '`') SEPARATOR ', '))
-> FROM
-> `information_schema`.`TABLES`
-> WHERE
-> `TABLE_SCHEMA` = DATABASE() AND
-> `TABLE_TYPE` = 'BASE TABLE' AND
-> `TABLE_NAME` LIKE '%_audit'
-> );
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT #`drop_tables`;
+--------------------------------------------------------------+
| #`drop_tables` |
+--------------------------------------------------------------+
| DROP TABLE IF EXISTS `one_audit`, `three_audit`, `two_audit` |
+--------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> PREPARE `exec` FROM #`drop_tables`;
Query OK, 0 rows affected (0.00 sec)
Statement prepared
mysql> EXECUTE `exec`;
Query OK, 0 rows affected (0.00 sec)
mysql> DEALLOCATE PREPARE `exec`;
Query OK, 0 rows affected (0.00 sec)
You must be careful with the system variable group_concat_max_len.
UPDATE
Using cursor:
DELIMITER #
DROP PROCEDURE IF EXISTS `drop_audit_tables`#
CREATE PROCEDURE `drop_audit_tables`()
BEGIN
DECLARE `done` BOOL DEFAULT 0;
DECLARE `cmd` VARCHAR(4000);
DECLARE `cmds` CURSOR FOR
SELECT
CONCAT('DROP TABLE IF EXISTS `', `TABLE_NAME`, '`')
FROM
`information_schema`.`TABLES`
WHERE
`TABLE_SCHEMA` = DATABASE() AND
`TABLE_TYPE` = 'BASE TABLE' AND
`TABLE_NAME` LIKE '%_audit';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET `done` := 1;
OPEN `cmds`;
`tLoop`: LOOP
FETCH `cmds` INTO `cmd`;
IF `done` THEN
CLOSE `cmds`;
LEAVE `tLoop`;
END IF;
SET #`cmd` := `cmd`;
PREPARE `STMT` FROM #`cmd`;
EXECUTE `STMT`;
DEALLOCATE PREPARE `STMT`;
END LOOP `tLoop`;
SET #`cmd` := NULL;
END#
CALL `drop_audit_tables`#
DELIMITER ;
14.5.1 PREPARE
Syntax
PREPARE stmt_name FROM preparable_stmt
...
preparable_stmt is either a string literal or a user variable that
contains the text of the SQL statement.
...

Answer 2

Your line:
declare cmds cursor for select 'drop table [' + table_name + ']' from information_schema.tables where table_name like '%_audit';
.. uses table_name without defining it first.
Try defining it first with something like:
create procedure drop_audit_tables(IN table_name VARCHAR(64))
You may want to consider the security implications of taking a variable directly from the stored procedure and placing it into your ad-hoc query.
Still, define table_name somewhere. In this case table_name would be supplied as an argument to your stored procedure. Your task then is to gather an array of table names and run this code in a for/foreach loop.
Basic (non-robust) PHP (PDO)
/* Get the audit tables. */
$stmt = $pdo->query(`CALL get_audit_tables()`)
$tables = $stmt->fetch();
$stmt->close()
$stmt = $pdo->prepare('CALL drop_audit_tables(:table)')
/* Drop each audit table. */
foreach($tables as $table)
{
$stmt->bindParam(:table, $table, PDO::PARAM_STR)
$stmt->execute();
}
$stmt->close();
Something like that, anyway.
MySQL: CREATE PROCEDURE
Specifying a parameter as IN, OUT, or INOUT is valid only for a PROCEDURE. For a FUNCTION, parameters are always regarded as IN parameters.
PHP Manual: PDO::prepare
Prepares an SQL statement to be executed by the PDOStatement::execute() method. The SQL statement can contain zero or more named (:name) or question mark (?) parameter markers for which real values will be substituted when the statement is executed.
A solution like this would make your life easier. You only need to define a basic query that finds your audit tables. Less code. Simpler.