mysql - create dynamic union - mysql

Question

Database is mysql/mariaDB.
I have database designed to store monthly reports about something. and their names are (example): table1, table2, table3...
I want to create function/procedure that will create/recreate view that contains all tables union (union ALL).
Something like:
1. first select all table names from information schema.
SELECT TABLE_NAME from information_schema.`TABLES` where TABLE_NAME like 'table%'
then i would try to set it in some loop to use result set from first query.
But i have problem with first step where i try to merge only one fixed table + one from first query and it returns error to me.
i try:
select * from `table4`
UNION
SELECT * from (SELECT TABLE_NAME from information_schema.`TABLES`
where TABLE_NAME like 'table%' limit 1) as dd
it returns me error: The used SELECT statements have a different number of columns ,
but when i execute sub query i get 1 result with correct name of table, and when i set that name in from clause without sub query, it works.
Any idea why it is happening, and maybe some advice how to accomplish that dynamic union.

Answer 1

I think a little push will help you to the correct way of handling this problem.
First, as Tim Biegeleisen suggests, the way to proceed is to use dynamic SQL, this is your only avenue if the table names cannot be absolutely determined before you try to run the query.
Second, you are correct to think that you need to start by querying the information_schema.TABLE, which you should do using a CURSOR. The results from that query should then be used to build up a query string which you then PREPARE and EXECUTE.
Third, I take it that the error message you included in your post refers specifically to the running of that query and doesn't indicate that the monthly tables differ in any way. You can't do a UNION unless the results from each part return the same number of columns.
Fourth, because we are going to build the query dynamically, this has to done within a stored procedure, it's not possible to do it in a stored function.
There are good tutorials in the mysql docs for using CURSOR and PREPARE/EXECUTE, which you should read. The version I give below will be based on those examples. I'm assuming the only input parameter will be the schema name (in case you happen to have some similarly named tables in another database on the server).
DELIMITER //
DROP PROCEDURE IF EXISTS dyn_union //
CREATE PROCEDURE dyn_union(IN v_sname VARCHAR(64))
READS SQL DATA
BEGIN
-- NB the order of declaration for variables cursor
-- and handler must be strictly observed
DECLARE sname VARCHAR(64); -- variable the schema names
DECLARE tname VARCHAR(64); -- variable the table names
DECLARE done INT DEFAULT FALSE; -- cursor control variable
DECLARE cur1 CURSOR FOR
SELECT table_schema, table_name
FROM information_schema.TABLES
WHERE table_schema = v_sname
AND table_name LIKE 'table%';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
SET #sql = ''; -- build the query string in this var
OPEN cur1;
read_loop: LOOP -- loop over the rows returned by cursor
FETCH cur1 INTO sname, tname; -- fetching the schema and table names
IF done THEN
LEAVE read_loop;
END IF;
IF #sql = '' THEN -- build the select statement
SET #sql := CONCAT('SELECT * FROM `', sname, '`.`', tname, '`');
ELSE
SET #sql := CONCAT(#sql, ' UNION ALL SELECT * FROM `', sname, '`.`', tname, '`');
END IF;
END LOOP;
CLOSE cur1;
select #sql;
PREPARE stmt FROM #sql; -- prepare and execute the dynamically
EXECUTE stmt; -- created query.
DEALLOCATE PREPARE stmt;
END //
DELIMITER ;
-- call the procedure
CALL dyn_union('your_db_name');