SQL Server 2016 learning record - nested query

Learning goals ：

One 、 The basic concept of nested query
Two 、 Classification of nested queries （ Predicate classification ）
1、 with IN Subquery of predicate
2、 Subqueries with comparison operators
3、 with ANY or ALL Subquery of predicate
4、 with EXISTS Subquery of predicate

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Learning content ：

One 、 The basic concept of nested query
Query block ： One select The statement becomes a query block
nested queries ： Nesting a query block in the... Of another query block where Clause or having Query in the condition of phrase

Restrictions on subqueries ：

Two 、 Classification of nested queries （ Predicate classification ）

1、 with IN Subquery of predicate
Between parent query and child query IN Connect , The subquery result is a collection

-- Query and “ Liu Chen ” Students in the same department 
--1. determine “ Liu Chen ” Family name 
--2. Find all in IS Students in the Department 
select sno,sname,sdept from student
where sdept In
			(select sdept from student 
			  where sname=' Liu Chen ');

Use self connection to realize the above question ：

select s1.sno,s1.sname,s1.sdept from student s1,student s2
where s1.sdept=s2.sdept and s2.sname=' Liu Chen ';

-- Query the elective course named “ The information system ” Student ID and name of 
select sno,sname from student
where sno In 
		(select sno from sc
			where cno In 
			 (select cno from course 
				where cname=' The information system ')
				);

2、 Subqueries with comparison operators
Scope of application ： When you know exactly that the inner query returns a single value , Available comparison operators and ANY or ALL Predicates are used in conjunction with

-- Find out the course number that each student has exceeded the average score of his elective course 
select sno,cno from sc x
where grade >
		(select avg(grade) from sc y
			where y.sno=x.sno);
-- perhaps   The inner layer can not be aliased 
select sno,cno from sc x
where grade >
		(select avg(grade) from sc
			where sno=x.sno);

The subquery must follow the comparison character

3、 with ANY or ALL Subquery of predicate
ANY Any value
ALL All worth

-- Query the ratio of other systems IS Student number and name of all students in the Department who are older 
select sname,sage from student
where sage > ALL 
			(select sage from student
				where sdept='IS');

4、 with EXISTS Subquery of predicate

with EXISTS The subquery of the predicate does not return any actual data , He only produces logical truth ‘true’, Or a logical false value ‘false’

-- Query all non elective courses 1 Student name of course No 
select * from student s;
select * from sc;
select sname from student s
	where not exists
			(select * from sc
				where s.sno=sno and cno=1);

-- Check the names of the students who have taken all the courses 

select sname from student s
	where not exists 
		(select * from course c
			where not exists 
				(select * from sc
					where s.sno=sno and c.cno=cno));

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