Question

tableA

id    a_id     b_id
1     100      200
2     101      201
3     100      202


tableB

id    a_id     b_id
2     100      200
3     201      101
4     203      100

所以我希望得到a_id=100的计数，但除了tableB

中也存在一个

所以

in tableA i got 2:
    1     100      200
    3     100      202

但是{B}中存在100/200，因此结果为1

如何编写sql？

Answer 1

您可以使用NOT EXISTS：

SELECT      COUNT(1)
FROM        TableA AS A
WHERE       NOT EXISTS (
                            SELECT    NULL
                            FROM      TableB AS B
                            WHERE     B.a_id = A.a_id
                                  AND B.b_id = A.b_id
                       )
       AND A.a_id = 100

如果您想使用left join：

进行此操作

SELECT      COUNT(1) 
FROM        TableA AS A
LEFT JOIN   TableB AS B
        ON  B.a_id = A.a_id
        AND B.b_id = A.b_id
WHERE       B.a_id IS NULL
        AND A.a_id = 100

Answer 2

SELECT count(*)
FROM tableA a
WHERE a_id = 100 AND NOT EXISTS
(
  SELECT *
  FROM tableB b
  WHERE b.a_id = a.a_id AND b.b_id = a.b_id
)

如何选择表中存在的记录数也存在于表b中？

2 个答案: