MySQL "NOT IN" query

To use IN, you must have a set, use this syntax instead:

SELECT * FROM Table1 WHERE Table1.principal NOT IN (SELECT principal FROM table2)

The subquery option has already been answered, but note that in many cases a LEFT JOIN can be a faster way to do this:

SELECT table1.*
FROM table1 LEFT JOIN table2 ON table2.principal=table1.principal
WHERE table2.principal IS NULL

If you want to check multiple tables to make sure it's not present in any of the tables (like in SRKR's comment), you can use this:

SELECT table1.*
FROM table1
LEFT JOIN table2 ON table2.name=table1.name
LEFT JOIN table3 ON table3.name=table1.name
WHERE table2.name IS NULL AND table3.name IS NULL