当in子查询的结果集中包含null时，如： in('a','b',null)，父查询只返回非NULL结果集，因为null和null无法比较;
当子查询 not in的结果集中包含null时，父查询永远为空结果集;
当子查询中查询字段不存在时,SQL不会报错
select * from t1 where id in (select kk from t2) --t2中不存在kk字段,该语句不会报错,结果集为t1中所有行
如果是delete from t1 where id in (select kk from t2),呵呵,那就哭吧~~~
示例脚本：
CREATE TABLE T1
(
id INT,
name VARCHAR(10)
)
CREATE TABLE T2
(
id INT,
address VARCHAR(10)
)
INSERT INTO T1
SELECT 1, 'Tom'
UNION ALL
SELECT 2, 'Jack'
UNION ALL
SELECT 3, 'Lily'
UNION ALL
SELECT 4, 'Tony'
INSERT INTO T2
SELECT 1, 'Beijing'
UNION ALL
SELECT 2, 'Shanghai'
UNION ALL
SELECT NULL,'Tianjin'
SELECT * FROM T1
WHERE id IN (SELECT id FROM T2)
id name
1 Tom
2 Jack
*********************************************************************************
SELECT * FROM T1
WHERE id NOT IN (SELECT id FROM T2)
id name
*********************************************************************************
SELECT * FROM T1
WHERE id NOT IN (SELECT id FROM T2 WHERE id IS NOT NULL)
id name
3 Lily
4 Tony
*********************************************************************************
SELECT * FROM T1
WHERE name IN (SELECT name FROM T2)
id name
1 Tom
2 Jack
3 Lily
4 Tony
*********************************************************************************
DELETE FROM T1
WHERE name IN (SELECT name FROM T2)
(4 行受影响)