not in 注意实现

NOT IN 与 NOT EXISTS 并不能完全等价，只有当子查询中，select 关键字后的字段有not null约束或者有这种暗示时才用NOT IN。

 db=# DROP TABLE IF EXISTS test1;

CREATE TABLE test1(id INT, name VARCHAR(32));
DROP TABLE IF EXISTS test2;
CREATE TABLE test2(id INT, name VARCHAR(32));
INSERT INTO test1 VALUES(1,'A'),(2,'B'),(3,'C'),(4,'D');
INSERT INTO test2 VALUES(1,'A'),(2,'B'),(3,NULL),(4,'DD');
 Command OK
Command OK
Command OK
Command OK
Command OK - 4 rows affected
Command OK - 4 rows affected
db=# SELECT
 t1.ID,
 t1.NAME
 FROM
 test1 t1
 WHERE
 t1.NAME not in (SELECT t2.name FROM test2 t2);
Empty set