1。什么是å查询？
ž®†ä¸€ä¸ªæŸ¥è¯¢åŒ…å«åˆ°å¦ä¸€ä¸ªæŸ¥è¯¢ä¸ã€?BR>2。工作原ç†ï¼Ÿ
å查询是牵套在å¦ä¸€ä¸ªselectè¯å¥ä¸çš„selectè¯å¥ã€‚内部selectè¯å¥˜q”回用作于外部selectè¯å¥çš„æ¡ä»¶çš„列。首先从最内部的select开始执行ã€?BR>
3。在大多数查询ä¸åQŒå查询òq¶ä¸å¼•ç”¨çˆ¶æŸ¥è¯¢ã€‚å› æ¤å查询ä¸çš„å€ég¿æŒä¸å˜ã€‚例如：
//MSSQL
//通过IN OR NOT IN引入的å查询¾l“æžœ
//是一列零值或更多å€?BR>//把（titlesåQ‰å查询里结果作ä¸?BR>//(publicshers)父查询的æ¡äšg˜q›è¡ŒæŸ¥è¯¢
USE PUBS
SELECT PUB_NAME TITLE
FROM PUBLICSHERS
WHERE PUB_ID IN(
SELECT PUB_ID FROM TITLES
WHERE TYPE = 'BUSSINESS')
//通过IN OR NOT IN引入的å查询¾l“æžœ
//是一列零值或更多å€?BR>//把（titlesåQ‰å查询里结果作ä¸?BR>//(publicshers)父查询的æ¡äšg˜q›è¡ŒæŸ¥è¯¢
USE PUBS
SELECT PUB_NAME TITLE
FROM PUBLICSHERS
WHERE PUB_ID IN(
SELECT PUB_ID FROM TITLES
WHERE TYPE = 'BUSSINESS')
如果å查询引用了父查询，ž®±éœ€è¦äؓ父查询ä¸çš„毋ơè„P代é‡æ–°è®¡½Ž—å查询的倹{€?BR>
//å查询ä¸ç”¨åˆ°äº†çˆ¶æŸ¥è¯¢çš„å€?BR>//在有大é‡æ•°æ®çš„时候尽é‡é¿å…ä‹É用椾cÀLŸ¥è¯?BR>//它查询数æ®å¯æ˜¯æˆæŒ‡æ•°å¢žé•¿çš?BR>USE PUBS
SELECT A.CITY FROM SUPPLIERS A
WHERE A.SUPPLIERID IN (
SELECT B.SUPPLIERID FROM PRODUCTS B
WHERE A.SUPPLIERID = B.SUPPLIERID)
SELECT A.CITY FROM SUPPLIERS A
WHERE A.SUPPLIERID IN (
SELECT B.SUPPLIERID FROM PRODUCTS B
WHERE A.SUPPLIERID = B.SUPPLIERID)
4。我è¦è¯´ä¸€ä¸‹è”接和å查询的区别
è”接与å查询处熾cÖM¼¼é—®é¢˜ä¸åŒä¹‹å¤„在于åQŒè”接å¯ä»¥åœ¨¾l“果集ä¸æ˜„¡¤ºå¤šä¸ªè¡¨çš„列ã€?BR> 匙”接就是查询包å«äº†åŒ™¾¹è¡¨çš„全部记录åQŒå³ä½¿å·¦è¾¹è¡¨ä¸æ²¡æœ‰åŒ¹é…的倹{€?BR> å·¦è”å’Œå³è”æ£å¥½ç›¸å?BR>
//内è”æŽ?BR>USE PUBS
SELECT A.PUB_NAME, A.TITLE
FROM PUBLISHERS A INNER JOIN TITLES B
ON A.PUB_ID = B.PUB_ID
AND TYPE = 'BUSSINESS')
SELECT A.PUB_NAME, A.TITLE
FROM PUBLISHERS A INNER JOIN TITLES B
ON A.PUB_ID = B.PUB_ID
AND TYPE = 'BUSSINESS')
5。关于ä‹É用EXISTS AND NOT EXISTSçš„å查询
使用EXISTS引入一个å查询åQŒå°±ç›¸å½“于进行一‹Æ¡å˜åœ¨çš„‹¹‹è¯•åQŒå¤–部查询的WHEREå奋¹‹è¯•å查è¯?BR>˜q”回的行是å¦å˜åœ¨ã€‚å查询ä¸äñ”生ä“Q何结果；它媘q”回TRUE或FALSE倹{€?BR>
//查找å˜åœ¨å•†ä¸šä¹¦ç±çš„出版商的åå?BR>//和内è”对比下有什么ä¸å?BR>USE PUBS
SELECT A.PUB_NAME
FROM PUBLISHERS A
WHERE EXISTS (
SELECT *FROM TITLES
WHERE PUB_ID = A.PUB_ID
AND TYPE = 'BUSSINESS')
//å查è¯?å†æ¬¡å¯ÒŽ¯”ä¸?BR>USE PUBS
SELECT PUB_NAME
FROM PUBLISHERS
WHERE PUB_ID IN (
SELECT PUB_ID FROM TITLES
WHERE TYPE = 'BUSSINESS')
SELECT A.PUB_NAME
FROM PUBLISHERS A
WHERE EXISTS (
SELECT *FROM TITLES
WHERE PUB_ID = A.PUB_ID
AND TYPE = 'BUSSINESS')
//å查è¯?å†æ¬¡å¯ÒŽ¯”ä¸?BR>USE PUBS
SELECT PUB_NAME
FROM PUBLISHERS
WHERE PUB_ID IN (
SELECT PUB_ID FROM TITLES
WHERE TYPE = 'BUSSINESS')
通过‹¹‹è¯•æˆ‘猜想EXISTS关键å—它的工作原ç†æ˜¯åQšé历所有的记录åQŒæ¯é历一æ¡è®°å½•ï¼Œå查询就判æ–是å¦å˜åœ¨åQŒå˜åœ¨çš„è¯æ»¡‘Ïx¡ä»¶æ˜¾½Cø™¿™æ¡è®°å½•ï¼Œç„¶åŽä¸€ç›´æ‰¾åˆ°æœ€åŽæ¡è®°å½•ã€‚（如果有ä¸å¯¹çš„地方或者需è¦æ”¹˜q›çš„地方åQŒè¯·å¤§å®¶è¦å‘Šè¯‰æˆ‘啊）
今天ž®±åˆ°˜q™é‡ŒæŠŠï¼Œè±¡æˆ‘˜q™ä¹ˆç‰›çš„íw«ä½“也ç»ä¸è“v¾l常熬夜+_+!ã€?BR>
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