列在SQL Server中的子查詢

Question

已成立這樣

select '1' a, '2' b into #tmp1 
select '3' c, '4' d into #tmp2 

鑑於臨時表爲什麼沒有下面的SQL Server提供一個錯誤？

select * from #tmp1 where a not in (select b from #tmp2) 

Answer 1

沒有錯誤，因爲主查詢中的列可以在子查詢中訪問。因此，在

select b from #tmp2 

列b從#tmp1到來。

Here是解釋它的文章。

If a column is referenced in a subquery that does not exist in the table referenced by the subquery's FROM clause, but exists in a table referenced by the outer query's FROM clause, the query executes without error. SQL Server implicitly qualifies the column in the subquery with the table name in the outer query.