SQL NOT IN Clause

问题:

I have a query which is not working as expected

Q1:
SELECT id, name 
FROM vw_x 
WHERE id NOT IN (select pid from table_x)
GROUP BY id, name
Having max(c_date) > GETDATE()

Q2:
SELECT id, name 
FROM vw_x 
GROUP BY id, name
Having max(c_date) > GETDATE()

Q1 is not returning anything even though i know those ids are not in table_x Q2 runs correctly without NOT IN

What could be wrong with my query?

回答1:

you have a NULL value in the table

try this

SELECT id, name 
FROM vw_x 
WHERE id NOT IN (select pid from table_x where pid is not null)
GROUP BY id, name
Having max(c_date) > GETDATE()

or this

SELECT id, name 
FROM vw_x 
WHERE  NOT EXISTS (select 1 from table_x  where pid = vw_x.id  )
GROUP BY id, name
Having max(c_date) > GETDATE()

See also Select all rows from one table that don't exist in another table

回答2:

what about using a left join?

SELECT id, name 
FROM vw_x 
LEFT JOIN table_x on id = pid
WHERE pid IS NULL
GROUP BY id, name
Having max(c_date) > GETDATE()

回答3:

There is another situation: the subquery may return nothing. SQL Server does not work as expected if the NOT IN clause returns a null list. I have a query like the following:

select * from table where id not in (select id from tableB where somecondition(x))

When the subquery contains a list of ids, the query will return the data as expected. But when the subquery returns nothing, the query will still return data, but then becomes stuck.

I changed the query to the following and solved the problem:

select * from table where id not in (select id from tableB where somecondition(x) **union all select 0**)

which makes sure the subquery will contain at least one number.