Msg 512, Level 16, State 1, Line 2
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
We're all familiar with the error when using sub-queries; that lets you know you subquery returns more than 1 value, and so the Query Engine doesn't know which value you intend to compare. But perhaps you don't care which value it compares against. You could be wanting to find out if any value match the given criteria in the sub-query. The follow snippets allows for that:
SELECT * FROM tblVoucherAvailable
WHERE VoucherID > ANY (SELECT VoucherID FROM tblVoucherExpired)