insert tab (a,b)
select a=4,b=1001 where not exists (select 1 from tab where a=4)

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sql server帮助文档。

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探讨

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SQL code
sql面试题(xiaoku)

http://topic.csdn.net/u/20081020/15/1abf54d0-f401-42ab-a75e-df90027ceba0.html

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一个题目涉及到的50个Sql语句

Student(S#,Sname,Sage,Ssex) 学生表  
Course(C#,Cname,T#) 课程表  
SC(S#,C#,score) 成绩表  
Teacher(T#,Tname) 教师表  

问题：  
1、查询“001”课程比“002”课程成绩高的所有学生的学号；  
 select a.S# from (select s#,score from SC where C#='001') a,(select s#,score  
 from SC where C#='002') b  
 where a.score>b.score and a.s#=b.s#;  
2、查询平均成绩大于60分的同学的学号和平均成绩；  
   select S#,avg(score)  
   from sc  
   group by S# having avg(score) >60;  
3、查询所有同学的学号、姓名、选课数、总成绩；  
 select Student.S#,Student.Sname,count(SC.C#),sum(score)  
 from Student left Outer join SC on Student.S#=SC.S#  
 group by Student.S#,Sname  
4、查询姓“李”的老师的个数；  
 select count(distinct(Tname))  
 from Teacher  
 where Tname like '李%';  
5、查询没学过“叶平”老师课的同学的学号、姓名；  
   select Student.S#,Student.Sname  
   from Student 
   where S# not in (select distinct( SC.S#) from SC,Course,Teacher where  SC.C#=Course.C# and Teacher.T#=Course.T# and Teacher.Tname='叶平');  
6、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；  
 select Student.S#,Student.Sname from Student,SC where Student.S#=SC.S# and SC.C#='001'and exists( Select * from SC as SC_2 where SC_2.S#=SC.S# and SC_2.C#='002');  
7、查询学过“叶平”老师所教的所有课的同学的学号、姓名；  
 select S#,Sname  
 from Student  
 where S# in (select S# from SC ,Course ,Teacher where SC.C#=Course.C# and Teacher.T#=Course.T# and Teacher.Tname='叶平' group by S# having count(SC.C#)=(select count(C#) from Course,Teacher  where Teacher.T#=Course.T# and Tname='叶平'));  
8、查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；  
 Select S#,Sname from (select Student.S#,Student.Sname,score ,(select score from SC SC_2 where SC_2.S#=Student.S# and SC_2.C#='002') score2  
 from Student,SC where Student.S#=SC.S# and C#='001') S_2 where score2 <score;  
9、查询所有课程成绩小于60分的同学的学号、姓名；  
 select S#,Sname  
 from Student  
 where S# not in (select Student.S# from Student,SC where S.S#=SC.S# and score>60);  
10、查询没有学全所有课的同学的学号、姓名；  
   select Student.S#,Student.Sname  
   from Student,SC  
   where Student.S#=SC.S# group by  Student.S#,Student.Sname having count(C#) <(select count(C#) from Course);  

11、查询至少有一门课与学号为“1001”的同学所学相同的同学的学号和姓名；  
   select S#,Sname from Student,SC where Student.S#=SC.S# and C# in select C# from SC where S#='1001';  
12、查询至少学过学号为“001”同学所有一门课的其他同学学号和姓名；  
   select distinct SC.S#,Sname  
   from Student,SC  
   where Student.S#=SC.S# and C# in (select C# from SC where S#='001');  
13、把“SC”表中“叶平”老师教的课的成绩都更改为此课程的平均成绩；  
   update SC set score=(select avg(SC_2.score)  
   from SC SC_2  
   where SC_2.C#=SC.C# ) from Course,Teacher where Course.C#=SC.C# and Course.T#=Teacher.T# and Teacher.Tname='叶平');