數(shù)據(jù)庫(kù)左連接與等值連接

左連接和等值連接的差別

distinct實(shí)際應(yīng)用1、等值與非等值連接查詢（P99）等值連接：連接運(yùn)算符為=[例3.49]查詢每個(gè)學(xué)生及其選修課程的情況

SELECTStudent.*，SC.*FROMStudent，SCWHEREStudent.Sno=SC.Sno； 學(xué)生情況放在Student表里，選課情況放在SC表里；兩個(gè)表的聯(lián)系通過共同的屬性Sno實(shí)現(xiàn)；

等值連接（P99）學(xué)號(hào)Sno

課程號(hào)

Cno

成績(jī)

Grade200215121200215121200215121200215122200215122123239285889080選課SC學(xué)號(hào)Sno姓名Sname性別

Ssex年齡

Sage所在系

Sdept200215121200215122200215123200515125李勇劉晨王敏張立男女女男20191819CSCSMAISStudentStudent.SnoSnameSsexSageSdeptSC.SnoCnoGrade200215121李勇男20CS200215121192200215121李勇男20CS200215121285200215121李勇男20CS200215121388200215122劉晨女19CS200215122290200215122劉晨女19CS200215122380等值連接：嵌套循環(huán)連接學(xué)號(hào)Sno

課程號(hào)

Cno

成績(jī)

Grade200215121200215121200215121200215122200215122123239285889080選課SC學(xué)號(hào)Sno姓名Sname性別

Ssex年齡

Sage所在系

Sdept200215121200215122200215123200515125李勇劉晨王敏張立男女女男20191819CSCSMAISStudent左外連接：Student.SnoSnameSsexSageSdeptCnoGrade200215121李勇男20CS192200215121李勇男20CS285200215121李勇男20CS388200215122劉晨女19CS290200215122劉晨女19CS380200215123王敏女18MANULLNULL200215125張立男19ISNULLNULL外連接（P102）[例3.53]SELECTStudent.Sno，Sname，Ssex，Sage，Sdept，Cno，GradeFROMStudentLEFTJOINSCON(Student.Sno=SC.Sno)；左連接和等值連接的差別tech_record_student同學(xué)們填寫獲獎(jiǎng)信息的表tech_user_record添加成員表tech_record_studenttech_user_recordusername是項(xiàng)目負(fù)責(zé)人的學(xué)號(hào)username是成員的學(xué)號(hào)通過等值連接獲得項(xiàng)目負(fù)責(zé)人username=20150001的tech_record_student信息等值連接selects.*fromtech_record_students,tech_user_recordur

whereur.record_id=s.idands.username=20150001orderbys.iddesc;tech_record_studenttech_user_recordusername是項(xiàng)目負(fù)責(zé)人的學(xué)號(hào)username是成員的學(xué)號(hào)通過左連接獲得項(xiàng)目負(fù)責(zé)人username=20150001的tech_record_student信息左連接selects.*fromtech_record_studentsleftjointech_user_recorduronur.record_id=s.idwheres.username=20150001orderbys.iddesc;對(duì)比等值連接和左連接tech_record_studenttech_user_recordusername是項(xiàng)目負(fù)責(zé)人的學(xué)號(hào)username是成員的學(xué)號(hào)項(xiàng)目負(fù)責(zé)人username=20150001也是該項(xiàng)目負(fù)責(zé)人的獲獎(jiǎng)信息，但是因?yàn)闆]有成員而不能獲得應(yīng)用“左連接”顯示的是某項(xiàng)目負(fù)責(zé)人的獲獎(jiǎng)信息：即：wheres.username=20150001如果其他項(xiàng)目負(fù)責(zé)人添加你為本項(xiàng)目的獲獎(jiǎng)成員：你也能看到。即：賬戶為username=20150001能看到所有自己的獲獎(jiǎng)記錄。應(yīng)用“左連接”tech_record_studenttech_user_recordusername是項(xiàng)目負(fù)責(zé)人的學(xué)號(hào)username是成員的學(xué)號(hào)顯示username=20150001的信息：包括20150001是項(xiàng)目負(fù)責(zé)人的信息和是成員的信息selects.*fromtech_record_studentsleftjointech_user_recorduronur.record_id=s.idwheres.username=20150001orderbys.iddesc;左連接selectdistincts.*fromtech_record_studentsleftjointech_user_recordur

onur.record_id=s.idwheres.username=20150001orur.username=20150001orderbys.iddesc;selectdistincts.*fromtech_record_students,tech_user_recordur

whereur.record_id=s.idand(s.username=20150001orur.username=20150001)orderbys.iddesc;等值連接distinct帖子：/s/blog_631c20b201011uia.htmlselectdistinctnamefromtableselectdistinctname,idfromtableDistinct作用于整個(gè)查詢域tech_record_studenttech_user_recordusername是項(xiàng)目負(fù)責(zé)人的學(xué)號(hào)username是成員的學(xué)號(hào)通過左連接獲得項(xiàng)目負(fù)責(zé)人username=20150001的tech_record_student信息左連接selects.*fromtech_record_studentsleftjointech_user_recorduronur.record_id=s.idwheres.username=20150001orderbys.iddesc;對(duì)比等值連接和左連接tech_record_studenttech_user_recordusername是項(xiàng)目負(fù)責(zé)人的學(xué)號(hào)username是成員的學(xué)號(hào)項(xiàng)目負(fù)責(zé)人username=20150001該獲獎(jiǎng)項(xiàng)目添加了兩個(gè)成員左連接selects.*fromtech_record_studentsleftjointech_user_recorduronur.record_id=s.idwheres.username=20150001orderbys.