Mysql calling procedure failed when dynamically alter table in it - mysql

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.
What I want are below:
If a table has a column named add_time, then I would like to add a column add_user_id in it.
Similarly, I would like to add update_user_id in the table if update_time is found.
I know I should get it down in the process of creating the database schemas, but my database has been built and I have to alter it by need.
So I create a procedure to do it:
CREATE PROCEDURE ALTER_SUMMER_TABLE()
BEGIN
DECLARE tableName VARCHAR(64);
DECLARE exitence VARCHAR(64);
DECLARE ntable INT; # number of tables
DECLARE i INT; # index
SET i = 0;
# get the count of table
SELECT COUNT(DISTINCT(TABLE_NAME)) INTO ntable FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = 'summer_cms';
WHILE i < ntable DO
# select the specific table name into the variable of `tableName`.
SELECT TABLE_NAME INTO tableName
FROM information_schema.COLUMNS
WHERE TABLE_SCHEMA = 'summer_cms'
AND COLUMN_NAME = 'add_time'
LIMIT 1 OFFSET i;
# alter table, but I get error in this clause.
ALTER TABLE tableName ADD COLUMN `add_user_id` INT NOT NULL DEFAULT 0 COMMENT 'add user id';
# check if the table has `update_time`
SELECT TABLE_NAME INTO exitence
FROM information_schema.COLUMNS
WHERE TABLE_SCHEMA = 'summer_cms'
AND TABLE_NAME = tableName
AND COLUMN_NAME = 'update_time';
# add `update_user_id` if `update_time` be found.
IF exitence THEN
ALTER TABLE tableName ADD COLUMN `update_user_id` INT NOT NULL DEFAULT 0 COMMENT 'update user id';
END IF;
SET i = i + 1;
END WHILE;
END
But I got an error when I call this procedure.
Procedure execution failed
1146 - Table 'summer_cms.tableName' doesn't exist
Dose anyone could tell me what I was missing or wrong? Any help will be appreciated.

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.