Sub-Queries are a powerful tool for doing complex targeting. A sub-query allows you to select more than one field at a time from the same table, thus simultaneously placing multiple conditions on a single set of search results.
Enter the first condition of your query by selecting the Table, Field, Relational Operator and, if necessary, Value. To place a second condition, select a Sub-Query Join from the drop down menu. This join ("and" or "or") will apply to all of the sub-query conditions you enter. The second condition must be a field from the same table as the first, so you do not have the option of selecting the Table.