sql server查询SQL备忘(2)- cross join

表A有个字段sid,里面有四门课程1，2，3，4； 表B存放某学生(uid)以及这门课程(sid)是否通过(pass)的对应关系，结果是求没有通过考试的学生以及课程的对应关系， 如果表B没有存放是否考过的关系，则也视为不通过 ，如下：
table A
sid
1
2
3
4

table B
uid  sid  pass
c55 1  y
eb4 2  n
c55 3  n
08d 3  y
08d 2  y

结果集应该如下：
uid  sid
c55  2
c55  3
c55  4
eb4  1
eb4  2
eb4  3
eb4  4
08d  1

08d  4

要实现上面红色的要求，left/right join以及full join试过都不行，得用cross join

SELECT
	T1.uid,T1.sid
FROM
(
SELECT * FROM tableA,(SELECT uid FROM tableB GROUP BY uid)T
)T1
	LEFT JOIN tableB T2 ON T1.uid = T2.uid AND T1.sid = T2.sid
WHERE T2.pass IS NULL OR T2.pass='n'

其中

SELECT * FROM tableA,(SELECT uid FROM tableB GROUP BY uid)T

等同于 SELECT * FROM tableA cross join (SELECT uid FROM tableB GROUP BY uid)T， 所以也可以用:

SELECT
	T1.uid,T1.sid
FROM
	(
SELECT * FROM tableA cross join (SELECT uid FROM tableB GROUP BY uid)T
)T1
	LEFT JOIN tableB T2 ON T1.uid = T2.uid AND T1.sid = T2.sid
WHERE T2.pass IS NULL OR T2.pass='n'

以下是建表数据，大家可以逐步调试玩玩：

CREATE TABLE tableA(sid VARCHAR(36) NOT NULL);
CREATE TABLE tableB(uid VARCHAR(36) NOT NULL,sid VARCHAR(36) NOT NULL,pass VARCHAR(1) NOT NULL);
insert tableA values('1');
insert tableA values('2');
insert tableA values('3');
insert tableA values('4');
insert tableB values('c55','1','y');
insert tableB values('eb4','2','n');
insert tableB values('c55','3','n');
insert tableB values('08d','3','y');
insert tableB values('08d','2','y');