The EXISTS condition is an operator whose right operand is a subquery. The result of an EXISTS condition is true if the subquery resolves to at least one row. The result of a NOT EXISTS condition is ...

For example, Microsoft's SQL Server has an "Exists" keyword allowing you to easily check to see if a record exists; Oracle's SQL does not have this feature, but you can work around the shortcoming ...

Null means "undefined" in SQL, so numerous online services don't think Christopher Null exists.