Question

How do I select results from table T1 where

T1.itemID = T2.itemID

and no corresponding record found in T2, i.e. found only in T1?

Answer 1

Like this:

SELECT …
  FROM t1 LEFT JOIN t2 ON (t1.itemid = t2.itemid)
  WHERE t2.itemid IS NULL

The more obvious query using NOT EXISTS runs slower older versions of MySQL. Thanks @OMG Ponies for benchmarks in a newer (unspecified) version.

Here is the NOT EXISTS version:

SELECT …
  FROM t1
  WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t2.itemid = t1.itemid)

(Explanation, to answer comments: LEFT JOIN differs from INNER JOIN in that if there is no row in the right table, a row is still returned in the results but with all the columns=NULL.)