經歷過面試的程序員都知道，面試過程中面試官可能對你問出千奇百怪的問題，但萬變不離其宗，終歸也是要問道重點上來，比如像是某一工作的基本操作步驟以及代碼的如何編寫等等，本文講的就是最最經典的數據庫查詢問題。
基本表結構：
teacher(tno,tname) 教師表
student(sno,sname,sage,ssex)學生表
立即學習“Java免費學習筆記（深入）”；
course(cno,cname,tno) 課程表
sc(sno,cno,score) 成績表
NO.1查詢課程1的成績比課程2的成績高的所有學生的學號
select?a.sno?from(select?sno,score?from?sc?where?cno=1)?a,(select?sno,score?from?sc?where?cno=2)?bwhere?a.score>b.score?and?a.sno=b.sno
NO.2查詢平均成績大于60分的同學的學號和平均成績
select?a.sno?as?"學號",?avg(a.score)?as?"平均成績"?from(select?sno,score?from?sc)?a?group?by?sno?having?avg(a.score)>60
NO.2查詢所有同學的學號、姓名、選課數、總成績
select?a.sno?as?學號,?b.sname?as?姓名,count(a.cno)?as?選課數,?sum(a.score)?as?總成績from?sc?a,?student?bwhere?a.sno?=?b.snogroup?by?a.sno,?b.sname
或者：
selectstudent.sno?as?學號,?student.sname?as?姓名,?count(sc.cno)?as?選課數,?sum(score)?as?總成績from?student?left?Outer?join?sc?on?student.sno?=?sc.snogroup?by?student.sno,?sname
NO.3查詢姓“張”的老師的個數
selectcount(distinct(tname))?from?teacher?where?tname?like?'張%‘
或者：
select?tname?as?"姓名",?count(distinct(tname))?as?"人數"?from?teacher?where?tname?like'張%'group?by?tname
NO.4查詢沒學過“張三”老師課的同學的學號、姓名
select?student.sno,student.sname?from?student where?sno?not?in?(select?distinct(sc.sno)?from?sc,course,teacher where?sc.cno=course.cno?and?teacher.tno=course.tno?and?teacher.tname='張三')
NO.5查詢同時學過課程1和課程2的同學的學號、姓名
select?sno,?sname?from?studentwhere?sno?in?(select?sno?from?sc?where?sc.cno?=?1)and?sno?in?(select?sno?from?sc?where?sc.cno?=?2)
或者：
selectc.sno,?c.sname?from(select?sno?from?sc?where?sc.cno?=?1)?a,(select?sno?from?sc?where?sc.cno?=?2)?b,student?cwhere?a.sno?=?b.sno?and?a.sno?=?c.sno
或者：
select?student.sno,student.sname?from?student,sc?where?student.sno=sc.sno?and?sc.cno=1and?exists(?select?*?from?sc?as?sc_2?where?sc_2.sno=sc.sno?and?sc_2.cno=2)
NO.6查詢學過“李四”老師所教所有課程的所有同學的學號、姓名
select?a.sno,?a.sname?from?student?a,?sc?bwhere?a.sno?=?b.sno?and?b.cno?in(select?c.cno?from?course?c,?teacher?d?where?c.tno?=?d.tno?and?d.tname?=?'李四')
?或者：
select?a.sno,?a.sname?from?student?a,?sc?b,(select?c.cno?from?course?c,?teacher?d?where?c.tno?=?d.tno?and?d.tname?=?'李四')?ewhere?a.sno?=?b.sno?and?b.cno?=?e.cno
NO.7查詢課程編號1的成績比課程編號2的成績高的所有同學的學號、姓名
<p style="font-family: " microsoft yahei sans gb helvetica neue tahoma arial sans-serif white-space: normal>select a.sno, a.sname from student a,</p><p style="font-family: " microsoft yahei sans gb helvetica neue tahoma arial sans-serif white-space: normal>(select sno, score from sc where cno = 1) b,</p><p style="font-family: " microsoft yahei sans gb helvetica neue tahoma arial sans-serif white-space: normal>(select sno, score from sc where cno = 2) c</p><p style="font-family: " microsoft yahei sans gb helvetica neue tahoma arial sans-serif white-space: normal>where b.score > c.score and b.sno = c.sno and a.sno = b.sno<br></p>
NO.8查詢所有課程成績小于60分的同學的學號、姓名
select?sno,sname?from?studentwhere?sno?not?in?(select?distinct?sno?from?sc?where?score?>?60)
NO.9查詢至少有一門課程與學號為1的同學所學課程相同的同學的學號和姓名
select?distinct?a.sno,?a.snamefrom?student?a,?sc?bwhere?a.sno??1?and?a.sno=b.sno?andb.cno?in?(select?cno?from?sc?where?sno?=?1)
?或者：
select?s.sno,s.sname?from?student?s,(select?sc.sno?from?scwhere?sc.cno?in?(select?sc1.cno?from?sc?sc1?where?sc1.sno=1)and?sc.sno1group?by?sc.sno)r1where?r1.sno=s.sno
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