Using a variable to reference another database in SELECT statement - mysql

Question

I am trying to cycle through all rows in a table that holds the database name for another database. This is what I have which compiles but doesn't work as MySQL takes the 'database' bit as the actual database name rather than the contents.
How can I change the following so that it takes the contents of the variable?
DROP PROCEDURE IF EXISTS CYCLE;
DELIMITER ;;
CREATE PROCEDURE CYCLE()
BEGIN
DECLARE data_name CHAR(255);
DECLARE done INT DEFAULT FALSE;
DECLARE cursor_i CURSOR FOR SELECT database FROM company;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cursor_i;
read_loop: LOOP
FETCH cursor_i INTO database;
IF done THEN
LEAVE read_loop;
END IF;
SELECT * FROM database.table LIMIT 1;
END LOOP;
CLOSE cursor_i;
END;
;;
DELIMITER ;
Update: to be clear I know that 'database' is a reserved word. I have used that here in place of the actual variable name I have used.

Answer 1

You can't evaluate the content on a variable on a table select statement, in that case you must build a query in a string an execute:
DROP PROCEDURE IF EXISTS CYCLE;
DELIMITER ;;
CREATE PROCEDURE CYCLE()
BEGIN
DECLARE data_name CHAR(255); -- unused?
DECLARE done INT DEFAULT FALSE;
DECLARE cursor_i CURSOR FOR SELECT database_name FROM company;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cursor_i;
read_loop: LOOP
FETCH cursor_i INTO data_name;
IF done THEN
LEAVE read_loop;
END IF;
-- now build the query before execute:
set #sql = concat('SELECT * FROM `',data_name,'`.`your_table` LIMIT 1');
-- and execute
prepare stmt from #sql;
execute stmt;
deallocate prepare stmt ;
END LOOP;
CLOSE cursor_i;
END;
;;
DELIMITER ;
*Would better not confusing us using variable names that causes more questions. #JustSaying