Oracle subquery

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One 、 Subquery

exists()： Judge , If the subquery returns data , Then execute the external query

1、 Query the information of the youngest student

select * from student where age = (select min(age) from student);

2、 Query age and “ Zhang San ” Student information of the same size

select * from student where age = (select age from student where name = ' Zhang San ')

3、 Check age , Gender and “ Zhang San ” The same student information , barring “ Zhang San ”

select * from student where (age,sex) = (select age,sex from student where name = ' Zhang San ')
and name <> ' Zhang San '

4、 Query age and “ Zhang San ” perhaps “ Li Si ” Same student information

select * from student where age in (select age from student where name =' Zhang San ' or ' Li Si ')

5、 If there is a name “ Zhang San ” Of the students , Then find out all the student information , If no name is “ Zhang San ” Of the students , Just don't query anything

select * from student where exists(select * from student where name = ' Zhang San ')

Two 、 stay having Use subqueries in

1、 Find out the classes whose class average score is higher than the grade average score 、 average 、 Class size

select class,count(*),avg(grade) from student group by class having avg(grade) > (select avg(grade) from student)

3、 ... and 、 stay from Clause using subqueries

1、 Ask the students id After class grouping with the teacher sid Equal student id

select s.id from student s (select t.id,t.class from teacher t group by t.class) where s.id = t.sid

Four 、 summary

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The commands involved in the article must be typed several times each like me , Only in the process of knocking can you find out whether you really master the command .

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