title: Check if key exists in other table
body: <p>I want to check if an entry in a column is valid by running it by and checking it other entries in a different table e.g A user enters <code>"AB"</code> in column <code>T1.C1</code>, I then want to check if <code>"AB"</code> exists in column <code>T2.C2.</code> Any ideas?</p>