Mysql stored procedure using cursor fetched variable in sql query - mysql

Question

I have defined a following procedure.
create procedure deleteData()
begin
DECLARE no_tbls INT;
DECLARE tbl VARCHAR(64);
DECLARE tbls_cr CURSOR for SELECT DISTINCT table_name FROM INFORMATION_SCHEMA.COLUMNS WHERE table_schema='db';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_tbls=1;
OPEN tbls_cr;
SET no_tbls=0;
while no_tbls=0 do
fetch tbls_cr into tbl;
select tbl.updated_at from tbl limit 1;
end while;
close tbls_cr;
end
After running this procedure i am getting an error db.tbl doesn't exist.
So i was searching if there is a way to use a cursor fetched object in another query. The problem i am doing all this tedious stuff is that i would like to delete data from all tables of a db with a particular where clause.
Note: All tables has a column updated_at with date format.
(I am a newbie to MySQL stored procs).

Answer 1

You cannot have a variable in place of a table name in the from clause in a plain select statement, mysql will look for a table named tbl in the database.
You need to use string concatenation and prepared statements to dynamically create and execute sql statements:
mysql> USE test;
mysql> CREATE TABLE t1 (a INT NOT NULL);
mysql> INSERT INTO t1 VALUES (4), (8), (11), (32), (80);
mysql> SET #table = 't1';
mysql> SET #s = CONCAT('SELECT * FROM ',#table);
mysql> PREPARE stmt3 FROM #s;
mysql> EXECUTE stmt3;
mysql> DEALLOCATE PREPARE stmt3;
Prepared statements also work in stored procedures, the above example demonstrates how to create the sql statement by concatenating string literals with variables, prepare the statement, execute it, and then release the prepared statement from memory.

Answer 2

Using Prepare Statement you can fetch data dynamically
create procedure deleteData()
begin
DECLARE no_tbls INT;
DECLARE tbl VARCHAR(64);
DECLARE tbls_cr CURSOR for
SELECT DISTINCT table_name
FROM INFORMATION_SCHEMA.COLUMNS
WHERE table_schema='db';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_tbls=1;
OPEN tbls_cr;
start_loop : LOOP
fetch tbls_cr into tbl;
set #b = concat('select ', tbl, '.updated_at from ' , tbl, ' limit 1');
prepare stmt3 from #b;
execute stmt3;
DEALLOCATE PREPARE stmt3;
END LOOP start_loop;
close tbls_cr;
end