This is a simple, but helpful example of changing a boolean or bit flag in a table to 'True' when the condition in the subSelect is met.  In this context, the SQL statement finds all the cases where a sales agent's email exists in the Metro_SalesForce table and exists in the related table Metro_SalesForceMult. The Metro_SalesForceMult table contains the USA state assignments only for sales agents representing more than a single state.

UPDATE Metro_SalesForce
 SET multipleStates = 'True'
       From Metro_SalesForce x, Metro_SalesForceMult y
        Where =