六、子查询的用法
子查询是一个 SELECT 查询，它嵌套在 SELECT、INSERT、UPDATE、DELETE 语句或其它子查询中。任何允许使用表达式的地方都可以使用子查询，子查询可以使我们的编程灵活多样，可以用来实现一些特殊的功能。但是在性能上，往往一个不合适的子查询用法会形成一个性能瓶颈。如果子查询的条件中使用了其外层的表的字段，这种子查询就叫作相关子查询。
相关子查询可以用IN、NOT IN、EXISTS、NOT EXISTS引入。 关于相关子查询，应该注意：
A、NOT IN、NOT EXISTS的相关子查询可以改用LEFT JOIN代替写法。比如：
SELECT PUB_NAME
FROM PUBLISHERS
WHERE PUB_ID NOT IN
(SELECT PUB_ID FROM TITLES WHERE TYPE= 'BUSINESS')
可以改写成：
SELECT A.PUB_NAME
FROM PUBLISHERS A LEFT JOIN TITLES B ON B.TYPE = 'BUSINESS' AND A.PUB_ID=B. PUB_ID WHERE B.PUB_ID IS NULL
FROM PUBLISHERS
WHERE PUB_ID NOT IN
(SELECT PUB_ID FROM TITLES WHERE TYPE= 'BUSINESS')
可以改写成：
SELECT A.PUB_NAME
FROM PUBLISHERS A LEFT JOIN TITLES B ON B.TYPE = 'BUSINESS' AND A.PUB_ID=B. PUB_ID WHERE B.PUB_ID IS NULL
B、 如果保证子查询没有重复 ，IN、EXISTS的相关子查询可以用INNER JOIN 代替。比如：
SELECT TITLE FROM TITLES
WHERE NOT EXISTS (SELECT TITLE_ID FROM SALES WHERE TITLE_ID = TITLES.TITLE_ID)
可以改写成：
SELECT TITLE FROM TITLES LEFT JOIN SALES ON SALES.TITLE_ID = TITLES.TITLE_ID
WHERE SALES.TITLE_ID IS NULL
WHERE NOT EXISTS (SELECT TITLE_ID FROM SALES WHERE TITLE_ID = TITLES.TITLE_ID)
可以改写成：
SELECT TITLE FROM TITLES LEFT JOIN SALES ON SALES.TITLE_ID = TITLES.TITLE_ID
WHERE SALES.TITLE_ID IS NULL
SELECT PUB_NAME
FROM PUBLISHERS
WHERE PUB_ID IN (SELECT PUB_ID FROM TITLES WHERE TYPE = 'BUSINESS')
可以改写成：
SELECT A.PUB_NAME --SELECT DISTINCT A.PUB_NAME
FROM PUBLISHERS A INNER JOIN TITLES B ON B.TYPE = 'BUSINESS' AND A.PUB_ID=B. PUB_ID
FROM PUBLISHERS
WHERE PUB_ID IN (SELECT PUB_ID FROM TITLES WHERE TYPE = 'BUSINESS')
可以改写成：
SELECT A.PUB_NAME --SELECT DISTINCT A.PUB_NAME
FROM PUBLISHERS A INNER JOIN TITLES B ON B.TYPE = 'BUSINESS' AND A.PUB_ID=B. PUB_ID
C、 IN的相关子查询用EXISTS代替。比如：
SELECT PUB_NAME FROM PUBLISHERS
WHERE PUB_ID IN
(SELECT PUB_ID FROM TITLES WHERE TYPE = 'BUSINESS')
可以用下面语句代替：
SELECT PUB_NAME FROM PUBLISHERS WHERE EXISTS
(SELECT 1 FROM TITLES WHERE TYPE = 'BUSINESS' AND PUB_ID= PUBLISHERS.PUB_ID)
WHERE PUB_ID IN
(SELECT PUB_ID FROM TITLES WHERE TYPE = 'BUSINESS')
可以用下面语句代替：
SELECT PUB_NAME FROM PUBLISHERS WHERE EXISTS
(SELECT 1 FROM TITLES WHERE TYPE = 'BUSINESS' AND PUB_ID= PUBLISHERS.PUB_ID)
D、不要用COUNT(*)的子查询判断是否存在记录，最好用LEFT JOIN或者EXISTS。比如有人写这样的语句：
SELECT JOB_DESC FROM JOBS
WHERE (SELECT COUNT(*) FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)=0
应该改成：
SELECT JOBS.JOB_DESC FROM JOBS LEFT JOIN EMPLOYEE
ON EMPLOYEE.JOB_ID=JOBS.JOB_ID
WHERE EMPLOYEE.EMP_ID IS NULL
SELECT JOB_DESC FROM JOBS
WHERE (SELECT COUNT(*) FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)<>0
应该改成：
SELECT JOB_DESC FROM JOBS
WHERE EXISTS (SELECT 1 FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)
WHERE (SELECT COUNT(*) FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)=0
应该改成：
SELECT JOBS.JOB_DESC FROM JOBS LEFT JOIN EMPLOYEE
ON EMPLOYEE.JOB_ID=JOBS.JOB_ID
WHERE EMPLOYEE.EMP_ID IS NULL
SELECT JOB_DESC FROM JOBS
WHERE (SELECT COUNT(*) FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)<>0
应该改成：
SELECT JOB_DESC FROM JOBS
WHERE EXISTS (SELECT 1 FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)
