MySQL Trigger - Storing a SELECT in a variable

Question

You can declare local variables in MySQL triggers, with the DECLARE syntax.

Here's an example:

DROP TABLE IF EXISTS foo;
CREATE TABLE FOO (
  i SERIAL PRIMARY KEY
);

DELIMITER //
DROP TRIGGER IF EXISTS bar //

CREATE TRIGGER bar AFTER INSERT ON foo
FOR EACH ROW BEGIN
  DECLARE x INT;
  SET x = NEW.i;
  SET @a = x; -- set user variable outside trigger
END//

DELIMITER ;

SET @a = 0;

SELECT @a; -- returns 0

INSERT INTO foo () VALUES ();

SELECT @a; -- returns 1, the value it got during the trigger

When you assign a value to a variable, you must ensure that the query returns only a single value, not a set of rows or a set of columns. For instance, if your query returns a single value in practice, it's okay but as soon as it returns more than one row, you get "ERROR 1242: Subquery returns more than 1 row".

You can use LIMIT or MAX() to make sure that the local variable is set to a single value.

CREATE TRIGGER bar AFTER INSERT ON foo
FOR EACH ROW BEGIN
  DECLARE x INT;
  SET x = (SELECT age FROM users WHERE name = 'Bill'); 
  -- ERROR 1242 if more than one row with 'Bill'
END//

CREATE TRIGGER bar AFTER INSERT ON foo
FOR EACH ROW BEGIN
  DECLARE x INT;
  SET x = (SELECT MAX(age) FROM users WHERE name = 'Bill');
  -- OK even when more than one row with 'Bill'
END//

`CREATE TRIGGER `category_before_ins_tr` BEFORE INSERT ON `category`
  FOR EACH ROW
BEGIN
    **SET @tableId= (SELECT id FROM dummy LIMIT 1);**

END;`;

CREATE TRIGGER clearcamcdr AFTER INSERT ON `asteriskcdrdb`.`cdr` 
FOR EACH ROW
BEGIN
  SET @INC = (SELECT sip_inc FROM trunks LIMIT 1);
  IF NEW.billsec >1 AND NEW.channel LIKE @INC 
    AND NEW.dstchannel NOT LIKE "" 
  THEN
    insert into `asteriskcdrdb`.`filtre` (id_appel,date_appel,source,destinataire,duree,sens,commentaire,suivi) 
      values (NEW.id,NEW.calldate,NEW.src,NEW.dstchannel,NEW.billsec,"entrant","",""); 
  END IF;
END$$

Dont try this @ home

I'm posting this solution because I had a hard time finding what I needed. This post got me close enough (+1 for that thank you), and here is the final solution for rearranging column data before insert if the data matches a test.

Note: this is from a legacy project I inherited where:

1. The Unique Key is a composite of rridprefix + rrid
2. Before I took over there was no constraint preventing duplicate unique keys
3. We needed to combine two tables (one full of duplicates) into the main table which now has the constraint on the composite key (so merging fails because the gaining table won't allow the duplicates from the unclean table)
4. on duplicate key is less than ideal because the columns are too numerous and may change

Anyway, here is the trigger that puts any duplicate keys into a legacy column while allowing us to store the legacy, bad data (and not trigger the gaining tables composite, unique key).

BEGIN
  -- prevent duplicate composite keys when merging in archive to main
  SET @EXIST_COMPOSITE_KEY = (SELECT count(*) FROM patientrecords where rridprefix = NEW.rridprefix and rrid = NEW.rrid);

  -- if the composite key to be introduced during merge exists, rearrange the data for insert
  IF @EXIST_COMPOSITE_KEY > 0
  THEN

    -- set the incoming column data this way (if composite key exists)

    -- the legacy duplicate rrid field will help us keep the bad data
    SET NEW.legacyduperrid = NEW.rrid;

    -- allow the following block to set the new rrid appropriately
    SET NEW.rrid = null;

  END IF;

  -- legacy code tried set the rrid (race condition), now the db does it
  SET NEW.rrid = (
    SELECT if(NEW.rrid is null and NEW.legacyduperrid is null, IFNULL(MAX(rrid), 0) + 1, NEW.rrid)
    FROM patientrecords
    WHERE rridprefix  = NEW.rridprefix
  );
END