嵌套查询
SELECT Sname FROM Student WHERE Sdept IN (“Computer”, ”Management”); //括号里的斜体表是一个小的集合，如果把括号里面的集合换成一个查询语句就变成嵌套查询了
SELECT Sname FROM Student WHERE Sno IN (SELECT Sno FROM SC WHERE Cno = ‘1024’); //变成嵌套集合了
嵌套操作符(=, <, <=, >, >=, <>)：
| x IN S | 如果x是S的成员，则返回真，否则返回假 |
| x > S | 如果x与S中的唯一元素满足>条件，则返回真，否则返回假 |
| x < ALL S | 如果x与S中的所有元素满足>条件，则返回真，否则返回假 |
| x >= SOME S | 如果x与S中的某些(某个)元素满足>条件，则返回真，否则返回假 |
| EXISTS S | 集合S为非空集，则返回真，否则返回假 |
| NOT EXISTS S | 集合S为空集，则返回真，否则返回假 |
IN
SELECT Sno, Sname FROM Student WHERE Sno IN // 3. 最后在Student关系中取出Sno和Sname
SELECT Sno FROM SC WHERE Cno IN // 2. 然后在SC关系中找出选修了111课程号的学生们的学号
SELECT Cno FROM Course WHERE Cname=’Management’; // 1. 先找出名称为Management的课程号，假设为111
以上查询出可以用连接查询实现
SELECT Sno, Sname FROM Student, SC, Course WHERE Student.Sno = SC.Sno AND SC.Cno = Course.Cno AND Course.Cname = ‘Management’;
比较运算符
SELECT Sdept FROM Student WHERE Sno = ‘200323’;
SELECT Sno, Sname, Sdept FROM Student WHERE Sdept = (
SELECT Sdept FROM Student WHERE Sname = ‘Jeff’);
SOME, ALL, EXISTS
SELECT Sname, Sage FROM Student WHERE Sge < SOME (
SELECT Sage FROM Student WHERE Sdept = ‘Management’) AND Sdept <> ‘Management’;
SELECT Sname, Sage FROM Student WHERE Sge < ALL (
SELECT Sage FROM Student WHERE Sdept = ‘Management’) AND Sdept <> ‘Management’;
SELECT Sname FROM Student S WHERE EXISTS (
SELECT * FROM SC WHERE Cno = ‘1003’ AND Sno = S.Sno);
集合操作 - 并操作(UNION), 交操作(INTERSECT), 差操作(EXCEPT)
SELECT Sno FROM Student WHERE Sdept = ‘Computer’
UNION
SELECT Sno FROM SC, Course C WHERE SC.Cno = C.Cno AND C.Cname = ‘Management’;