如何在SQL Server中进行多次插入之前进行一次插入

I'm doing a multiple insert in SQL Server using UNION ALL between the inserts. In the last part of the query I have a WHERE clause. Now it seems that that the WHERE clause is executed before every statement, but I only want the WHERE to be executed one time. If the WHERE clause has a result then none of the inserts should be executed.I'm doing a multiple insert in SQL Server using