MySQL: How creating a function that queries with variables columns names - mysql

Question

I am trying to make a function witch create a new ID from any table given as parameter.
DROP FUNCTION IF EXISTS create_id;
DELIMITER $$
CREATE FUNCTION create_id(db_table TEXT,pkey TEXT,strlen INT,joinner TEXT)
RETURNS TEXT
BEGIN
DECLARE max_id TEXT;
DECLARE new_id TEXT;
SET max_id = (SELECT MAX(pkey) FROM db_table);
SET new_id = max_id;
RETURN new_id;
END;
$$
DELIMITER ;
Thank you for your answers

Answer 1

You can't use variables like you want to; basically, if you want a variable identifier (table name, column name or the-like), you need to use dynamic SQL. But MySQL functions do not support dynamic SQL. So, instead, you need to use a procedure with an OUT paramter.
Consider:
drop procedure if exists create_id;
delimiter $$
create procedure create_id(in _db_table text, in _pkey text, out _max_id int)
begin
set #max_id = null;
set #sql = concat('select max(`', _pkey, '`) into #max_id from `', _db_table, '`');
prepare stmt from #sql;
execute stmt;
deallocate prepare stmt;
set _max_id = #max_id;
end;
$$
delimiter ;
Then, you invoke the procedure and recover the out value like so:
call create_id('mytable', 'id', #max_id);
select #max_id;
Note: I couldn't see the point for the last two arguments to your original function, so I removed them.