(D) under the Data Query Language (DQL) (nested subqueries)

(1) set membership

                       

Note denote the set of parentheses:

枚举集合
select * from s where sname in ('张军','王红')；
子查询的查询结果,注意(sno,cno)这种用法，子查询中不用这么写

查询哪些同学没有选择哪些课程
select sno,cno 
  from s,c
  where (sno,cno) not in (select sno,cno from sc);

 

Note: ① query result is a set of tuples when not a single finger, here in need rather than == comparison.

           ② enumerated set with (), such as (sno, cno), is a collection of sub-query results.

(2) sub-queries

① uncorrelated subquery

Subquery is not dependent on the parent query, execution starts sequentially from the innermost layer, from inside to outside.

--成绩在70分以上的学生信息
select sno, sname from studen
   where sno in 
       (select sno from garde  where score=70)

② correlated subquery

Subquery dependent parent query, from the outer first come first tuple (a tuple out) , and an inner layer based on its relevant inner query attribute value processing, it is true if the region where the tuple into the result set, remove a turn.

--查询选修了c1号课程的学生的学号和姓名。
select sno,sname from s
  where exists
    (select * from sc
       where cno = 'c1'
         and sno = s.sno)

The difference is that two different scopes tuples of variables.

(3) Comparative (symbol preceding clause, written) between the set

You know exactly when the subquery result is a single value, certain sub-query comparison operator followed.

SELECT sno，sname，dno FROM S
    WHERE dno  =
          (SELECT dno
           FROM S
           WHERE sname= ‘ 刘晨’)；

 

(4) some (subquery) and all (subquery)

查询比d1系某个学生年龄小的学生信息
SELECT sname，age FROM    S
       WHERE age < some (SELECT  age  FROM   S  WHERE dno= ' d1’)
            AND dno <> ' d1' ;
查询比d1系任意学生年龄小的学生信息
SELECT sname，age FROM    S
       WHERE age < all(SELECT  age  FROM   S  WHERE dno= ' d1’)
            AND dno <> ' d1' ;
或者
SELECT sname，age FROM    S
       WHERE age < (SELECT  max(age)  FROM   S  WHERE dno= ' d1’)
            AND dno <> ' d1' ;

some, all functions may be used instead of the aggregate (4) the presence of predicate

找出平均成绩最高的学生学号
select   sno  from     SC
    group by  sno  having    avg(score) >=  all（select    avg(score)
				                                    from     SC
     		                                          group  by  sno）

(5) exists (exist)

①exists basic usage

After the child is not relevant in the query, exist is correlated subquery .

                      

It can be seen from the result of the determination exists a sub-set of the query whether there tuple , and do not care what tuples exist. Traditionally considered a target column expression often used *, because the subquery with EXISTS only concerned about whether there is a tuple, column names are given no practical significance. But * will bring query performance issues, recommended constant (select 1 column is 1, how many rows there are that many 1) after 1 exists with no brain .

                                           

By this question look at the tuple variable scope, sub-query sno refers sc in sno.

② represents all with exists

                         

I chose to identify all classes of students, all of the students chose courses, equivalent to the absence of a course the student is not selected.

select sname from s
       where not exists
         (select cno from c where not exists
            (select 1 from sc where sc.sno = s.sno and sc.cno = c.cno))

从所有的课程表c中选出一门课，这门课没有出现在sname的选课表上，以上情况不会发生即sname把课都选了。

               

Query the name of the student election at least a full course of study the student chosen number is 002.

p represents the predicate: 002 students enrolled in the course y, q predicate X students enrolled in courses y.

After the above deduction, find all the elective courses students take in 002 students is equivalent to find a student is not present 002 enrolled in a course, but he had not even signed.

select scx.sno from sc scx where not exists(
选出一名学生X
       select 1 from sc scy where scy.sno = '002' and not exists(
       选出002选的课程scy.cno
            select 1 from sc scz where scz.sno = scx.sno and scz.cno = scy.cno))
               如果学生X没有选择课程K的话返回 true