How can I find all results with a null foreign key in MySQL?

Question

I have 2 tables... meat and potatoes in MySQL.

meat_id is the primary key for the meat table and there is a meat_id in the potatoes table that links the 2 tables. I want to find all rows in the potatoes table that don't have a valid meat_id . Any ideas?

Answer 1

Using LEFT JOIN/IS NULL:

   SELECT p.*
     FROM POTATOES p
LEFT JOIN MEAT m ON m.meat_id = p.meat_id
    WHERE m.meat_id IS NULL

Using NOT EXISTS:

SELECT p.*
  FROM POTATOES p
 WHERE NOT EXISTS(SELECT NULL
                    FROM MEAT m
                   WHERE m.meat_id = p.meat_id)

Using NOT IN:

SELECT p.*
  FROM POTATOES p
 WHERE p.meat_id NOT IN (SELECT m.meat_id
                           FROM MEAT m)

Summary

LEFT JOIN/IS NULL is the best performing option if the column(s) compared in the join can not be NULL . If those values can be NULL , then NOT EXISTS or NOT IN perform best .

Answer 2

 SELECT *
 FROM potatoes p
 WHERE NOT EXISTS (SELECT 1 from meat m where m.meat_id = p.meat_id)

Answer 3

Do a left join and use having:

SELECT p.potato_id, m.meat_id
FROM potatos p
LEFT JOIN meat m ON p.meat_id = m.meat_id
HAVING m.meat_id IS null