Declaring and using MySQL varchar variables

Question

I'm trying to do some simple manipulations with variables in MySQL 5.0 but I can't quite get it to work. I've seen many (very!) different syntaxen for DECLARE/SET, I'm not sure why... in any case I'm presumably confusing them/picking the wrong one/mixing them.

Here's a minimal fragment that fails:

DECLARE FOO varchar(7);
DECLARE oldFOO varchar(7);
SET FOO = '138';
SET oldFOO = CONCAT('0', FOO);

update mypermits 
   set person = FOO 
 where person = oldFOO;

I've also tried wrapping it with BEGIN... END; and as a PROCEDURE. In this case MySQL Workbench helpfully tells me: "SQL syntax error near ')'" on the first line and "SQL syntax error near 'DECLARE oldFOO varchar(7)'" on the second. Otherwise it gives both lines as errors in full, with "SQL syntax error near ..." on both.

Edit: I forgot to mention that I've tried it with and without @s on the variables. Some resources had it with, others without.

What dumb mistake am I making?

Solution 1

This works fine for me using MySQL 5.1.35:

DELIMITER $$

DROP PROCEDURE IF EXISTS `example`.`test` $$
CREATE PROCEDURE `example`.`test` ()
BEGIN

  DECLARE FOO varchar(7);
  DECLARE oldFOO varchar(7);
  SET FOO = '138';
  SET oldFOO = CONCAT('0', FOO);

  update mypermits
     set person = FOO
   where person = oldFOO;

END $$

DELIMITER ;

Table:

DROP TABLE IF EXISTS `example`.`mypermits`;
CREATE TABLE  `example`.`mypermits` (
  `person` varchar(7) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO mypermits VALUES ('0138');

CALL test()

Solution 2

I ran into the same problem using MySQL Workbench. According to the MySQL documentation, the DECLARE "statement declares local variables within stored programs." That apparently means it is only guaranteed to work with stored procedures/functions.

The solution for me was to simply remove the DECLARE statement, and introduce the variable in the SET statement. For your code that would mean:

-- DECLARE FOO varchar(7); 
-- DECLARE oldFOO varchar(7);

-- the @ symbol is required
SET @FOO = '138'; 
SET @oldFOO = CONCAT('0', FOO);

UPDATE mypermits SET person = FOO WHERE person = oldFOO;