sql - SQL Navigator 中的“with...as”

Question

以下查询有效：

select count(*) from everything where num not in (select num from sometable)

以下查询应该与上面的查询等效，但会导致“无效标识符”错误：

with unwanted as (select num from sometable)
select count(*) from everything where num not in unwanted

第二个查询有什么问题？

Answer 1

语法是这样的：

with unwanted as (select num from sometable)
select count(*) from everything where num not in (select * from unwanted)

select num from sometable显然，这只有在零件更复杂或以后多次使用时才有意义……

Answer 2

您还可以加入表以获得更快的性能

WITH unwanted
AS
(
    SELECT  num 
    FROM    sometable
)
SELECT  COUNT(*)
FROM    everything a
        LEFT JOIN unwanted b
            ON a.num = b.num
WHERE   b.num IS NULL