mysql SELECT NOT IN () — disjoint set?

Question

I\'m having a problem getting a query to work, which I think should work. It\'s in the form

SELECT DISTINCT a, b, c FROM t1 WHERE NOT IN ( SELECT DISTINCT a,b,c          


        

Answer 1

You should use not exists:

SELECT DISTINCT a, b, c FROM t1 WHERE NOT EXISTS (SELECT NULL FROM t2 WHERE t1.a = t2.a AND t1.b = t2.b AND t1.c = t2.c)

Using NOT IN is not the best method to do this, even if you check only one key. The reason is that if you use NOT EXISTS the DBMS will only have to check indices if indices exist for the needed columns, where as for NOT IN it will have to read the actual data and create a full result set that subsequently needs to be checked.

Using a LEFT JOIN and then checking for NULL is also a bad idea, it will be painfully slow when the tables are big since the query needs to make the whole join, reading both tables fully and subsequently throw away a lot of it. Also, if the columns allow for NULL values checking for NULL will report false positives.