六、子查詢的用法
子查詢是一個 SELECT 查詢,它嵌套在 SELECT、INSERT、UPDATE、DELETE 語句或其它子查詢中。
任何允許使用表達式的地方都可以使用子查詢,子查詢可以使我們的編程靈活多樣,可以用來實現一些特殊的功能。但是在性能上,
往往一個不合適的子查詢用法會形成一個性能瓶頸。如果子查詢的條件中使用了其外層的表的字段,這種子查詢就叫作相關子查詢。
相關子查詢可以用IN、NOT IN、EXISTS、NOT EXISTS引入。 關於相關子查詢,應該注意:
(1)
A、NOT IN、NOT EXISTS的相關子查詢可以改用LEFT JOIN代替寫法。
比如: SELECT PUB_NAME FROM PUBLISHERS WHERE PUB_ID NOT IN (SELECT PUB_ID FROM TITLES WHERE TYPE = 'BUSINESS') 可以改寫成: 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
(2)
- SELECT TITLE FROM TITLES
- WHERE NOT EXISTS
- (SELECT TITLE_ID FROM SALES
- WHERE TITLE_ID = TITLES.TITLE_ID)
可以改寫成:
- SELECT TITLE
- FROM TITLES LEFT JOIN SALES
- ON SALES.TITLE_ID = TITLES.TITLE_ID
- WHERE SALES.TITLE_ID IS NULL
B、 如果保證子查詢沒有重複 ,IN、EXISTS的相關子查詢可以用INNER JOIN 代替。比如:
- SELECT PUB_NAME
- FROM PUBLISHERS
- WHERE PUB_ID IN
- (SELECT PUB_ID
- FROM TITLES
- WHERE TYPE = 'BUSINESS')
可以改寫成:
- SELECT A.PUB_NAME
- FROM PUBLISHERS A INNER JOIN TITLES B
- ON B.TYPE = 'BUSINESS' AND
- A.PUB_ID=B. PUB_ID
(3)
C、 IN的相關子查詢用EXISTS代替,比如
- SELECT PUB_NAME FROM PUBLISHERS
- WHERE PUB_ID IN
- (SELECT PUB_ID FROM TITLES WHERE TYPE = 'BUSINESS')
可以用下面語句代替:
- 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(*)的子查詢判斷是否存在記錄,最好用LEFT JOIN或者EXISTS,比如有人寫這樣的語句:
- SELECT JOB_DESC FROM JOBS
- WHERE (SELECT COUNT(*) FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)=0
應該改成:
- 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
應該改成:
- SELECT JOB_DESC FROM JOBS
- WHERE EXISTS (SELECT 1 FROM EMPLOYEE WHERE JOB_ID=JOBS.JOB_ID)
-