How to store select results in mysql? - mysql

Question

In mysql, I have a trigger which checks for collisions, and updates the hp value of rows that have a collision. I also set a boolean value of the projectile to be true to mean that it should be deleted later. But I seem to do something twice that I would rather done once.
DELIMITER //
CREATE TRIGGER checkcollision2 BEFORE UPDATE ON projectiles
FOR EACH ROW
BEGIN
SET #unit_size:=25;
IF (SELECT count(*) FROM players WHERE wolf=1 AND NEW.x>=x AND NEW.x<=x+#unit_size AND NEW.y>=y AND NEW.y<=y+#unit_size)>0 THEN
SET NEW.deleted=1;
UPDATE players SET hp=hp-NEW.dmg WHERE wolf=1 AND NEW.x>=x AND NEW.x<=x+#unit_size AND NEW.y>=y AND NEW.y<=y+#unit_size;
END IF;
END;//
DELIMITER ;
In the above, I do a select which gives me the number of rows that have a collision. Then below I do the same query because I want to update those same rows. I could have just done the update, but I only want to do the SET NEW.deleted=1; if there was at least 1 row affected. Im not sure how to do that if I only keep the update statement.
Does anyone know of a batter way of doing this?
Thanks

Answer 1

If you want to set the NEW.deleted value based on the number of rows affected on update then you can use ROW_COUNT() to check the number of rows affected.
Example:
DELIMITER //
CREATE TRIGGER checkcollision2 BEFORE UPDATE ON projectiles
FOR EACH ROW
BEGIN
SET custom_unit_size := 25;
UPDATE players
SET hp = hp - NEW.dmg
WHERE wolf = 1
AND NEW.x >= x
AND NEW.x <= x + custom_unit_size
AND NEW.y >= y
AND NEW.y <= y + custom_unit_size;
IF ROW_COUNT() > 0 THEN
SET NEW.deleted = 1;
END IF;
END;
//
DELIMITER ;
Refer to:
ROW_COUNT()
returns the number of rows changed, deleted, or inserted by the last statement if it was an UPDATE, DELETE, or INSERT. For other statements, the value may not be meaningful.