NOT IN 與 NOT EXISTS 并不能完全等價，只有當子查詢中，select 關鍵字后的字段有not null約束或者有這種暗示時才用NOT IN。
<pre>
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);
<b>Empty set</b>
</pre>
