SQL：选择表A中不在表B中的所有唯一值

Question

I have table A

Id  | Name      | Department
-----------------------------
0   | Alice     | 1
0   | Alice     | 2
1   | Bob       | 1

and table B

Id  | Name
-------------
0   | Alice     

I want to select all unique Ids in table A which do not exist in table B. how can I do this?

Answer 1

select distinct id 
from TableA a
where not exists (
    select id 
    from TableB 
    where id = a.id
)

Answer 2

Just to provide a different solution than NOT IN :

SELECT DISTINCT A.Id
FROM A
LEFT OUTER JOIN B
    ON A.Id = B.Id
WHERE B.Id IS NULL

The "good" solution is usually MINUS or EXCEPT, but MySQL doesn't support it.

This question was asked a few time ago and someone posted an article comparing NOT IN, NOT EXISTS and LEFT OUTER JOIN ... IS NULL. It would be interesting if someone could find it again!

Answer 3

The most efficient answer is to use a left join, as using "NOT IN" can sometimes prevent a query from using an index, if present.

The answer in this case would be something like

SELECT DISTINCT 
    * 
FROM 
    TableA a 
LEFT JOIN 
    TableB b
ON 
    a.Id = b.Id 
WHERE 
    b.Id IS NULL

Alternatively, this is more readable than a left join, and more efficient than the NOT IN solutions

SELECT * FROM TableA a where NOT EXISTS (SELECT * FROM TableB where Id = a.Id)

Answer 4

I'd use a NOT EXISTS Like this:

SELECT A.Id
FROM TableA A
WHERE NOT EXISTS (SELECT B.Id FROM TableB B WHERE A.Id = B.Id)
GROUP BY A.Id

Answer 5

SELECT DISTINCT Id FROM A WHERE Id NOT IN (SELECT ID FROM B);

Answer 6

SELECT DISTINCT Id FROM A WHERE Id NOT IN(SELECT DISTINCT Id FROM B);

Answer 7

The subquery will get all the IDs in B. The group by...having will get all the unique IDs in A that are also not in B.

select *
from A 
where id not in
(
    select distinct id
    from B
)
group by ID
having count(*) > 1;