Mysql calling procedure failed when dynamically alter table in it

Question

I want to alter my tables dynamically based on whether the table has specific column.

My database name is summer_cms, and there are over 50 tables in it.

Answer 1

There a a few alterations you can make to your procedure to make it more streamlined as well as getting round a few problems.

First using a cursor to select the table names rather than using the two selects your using. Secondly to use a prepared statement to allow you to dynamically set the table name...

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `ALTER_SUMMER_TABLE`()
BEGIN
    DECLARE done INT DEFAULT 0;
    DECLARE tableName VARCHAR(64);
    declare cur cursor for SELECT TABLE_NAME
                             FROM information_schema.COLUMNS
                            WHERE TABLE_SCHEMA = 'summer_cms'
                            AND COLUMN_NAME = 'add_time';
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

    open cur;

    start_loop: loop
        fetch cur into tableName;
        if (done = 1 )THEN
             LEAVE start_loop;
        END IF;
        SET @sql = CONCAT('ALTER TABLE ', tableName,' ADD COLUMN `add_user_id` INT NOT NULL DEFAULT 0 ');
        PREPARE stmt FROM @sql;
        EXECUTE stmt;

    end loop;

    close cur;
END$$
DELIMITER ;

You could do a few tweaks - only fetch table names where the column doesn't already exist for example.

Answer 2

Here's an example of dynamic sql

drop procedure if exists alter_table;
delimiter //

CREATE DEFINER=`root`@`localhost`  procedure alter_table()

begin
declare tablename varchar(20);
set tablename = 'u';

set @sqlstmt =  concat('ALTER TABLE ', tableName, ' ADD COLUMN ', char(96), 'add_user_id', char(96), ' INT NOT NULL DEFAULT 0 COMMENT', char(39), 'add user id', char(39),';');
prepare stmt from @sqlstmt;
execute stmt;
deallocate prepare stmt;

end //

delimiter ;

Note I have used ascii backticks and single quotes.