How to Truncate All or Some of the Tables in a MySQL Database

Truncating all tables in a database is a common problem which arises a lot during testing or debugging.
One of the most common answers to this question is to drop & recreate the database, most likely utilizing the shell. For example, something like this:

mysqldump --add-drop-table --no-data [dbname] | mysql [dbname]

This dumps the entire schema structure to disk, without dumping any data, and with commands for dropping existing tables. When loading it back into mysql, it essentially truncates all the tables in the database. Basically, this is a decent solution for many uses.

We had a requirement for a solution that needed these additional features:

1. Does not require shell (We work with both Linux and Windows)
2. Resides inside the MySQL Server (To minimize outside dependencies - for example - the mysql command line client)
3. Can truncate only specified tables using a some sort of filter

In other words, the main requirement here is encapsulation and simplicity of use for the developers.
Basically, this means I've got no other choice except writing a stored procedure for this purpose, ugly as it may be. Specifically, this will require using the pseudo-dynamic SQL in MySQL (more about this later) and a server-side cursor.

The code of the Stored Procedure:

DELIMITER $$
CREATE PROCEDURE TruncateTables()
BEGIN
DECLARE done BOOL DEFAULT FALSE;
DECLARE truncate_command VARCHAR(512);
DECLARE truncate_cur
 CURSOR FOR /*This is the query which selects the tables we want to truncate*/
  SELECT CONCAT('TRUNCATE TABLE ',table_name)
  FROM INFORMATION_SCHEMA.TABLES
  WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME LIKE 'prefix_%';
DECLARE
  CONTINUE HANDLER FOR
  SQLSTATE '02000'
   SET done = TRUE;

OPEN truncate_cur;

truncate_loop: LOOP
 FETCH truncate_cur INTO truncate_command;
 SET @truncate_command = truncate_command;

 IF done THEN
  CLOSE truncate_cur;
  LEAVE truncate_loop;
 END IF;

 /*Main part - preparing and executing the statement*/
 PREPARE truncate_command_stmt FROM @truncate_command;
 EXECUTE truncate_command_stmt;

END LOOP;
END$$

DELIMITER ;

This is a generalized version, you can (and should) modify it to suit your needs.

Regarding dynamic SQL. Initially I've tried to make this query a lot simpler, and use something along the lines of the previous blog post. This would look something like:

DELIMITER $$
CREATE PROCEDURE TruncateTables()
BEGIN   /*Build a long user variable containing the varchars*/
 SELECT GROUP_CONCAT('TRUNCATE TABLE ',table_name SEPARATOR ';')
 INTO @truncate_command
 FROM INFORMATION_SCHEMA.TABLES
 WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME LIKE 'prefix_%';

 SET @truncate_command = CONCAT(@truncate_command,';');

 PREPARE truncate_command_stmt FROM @truncate_command;
 EXECUTE truncate_command_stmt;

END$$

DELIMITER ;

The main (and unsolvable) problem with this code is that the PREPARE cannot prepare more than one statement. In other words, while in other databases you could have sent an entire batch (real dynamic SQL), in MySQL the only way to do this is to run statement by statement. On the other hand, there is no GROUP_CONCAT() in other databases (even though it can apparently be created with ease in PostgreSQL).

The second and less worrying problem is the fact that by default, GROUP_CONCAT() result tends to be limited to a certain length. This is easily solved by setting group_concat_max_len to something higher than default.

End Transmission.