If you are using mysql > 4.2 you can use a select statement in the where clause. Check mysql.com documentation on this, I have always been using mysql 4.0 so I am not sure about the syntax.
ALternatively you might join the two or (more) tables and do the combination in the join statement.
Suppose you have table1 and table2. Table1 contains your field which can have the value '2'. Table1 also has the column outcome, which value you want to use in table2 to find other values. Say Table2 contains a column 'othervalue'.
Then the SQL statement would be:
SELECT field, outcome, othervalue FROM table1 JOIN table2 ON table1.outcome=table2.othervalue WHERE table1.field= '2'
All the fields I mention behind the SELECT are for illustration purposes, you can show whatever you need.