SQL Server 2016 learning records - set query

Learning goals ：

One 、 Related concepts of set query
Two 、 Several common operations of set query

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Learning content ：

One 、 Classification of set queries

Two 、 Several common operations of set query

1、 And operate

• form

< Query block >
union
< Query block >

• To participate in union The number of columns of each result of the operation must be the same
• The data type of the corresponding item must be the same

-- Check the elective course 1 Course No. and 2 The student number of course No 
select sno from sc where  cno=1
union
select sno from sc where cno=2;

2、 Alternating operation and differential operation

standard sql Set intersection and difference operations are not provided in , But it can be achieved indirectly by other methods

-- Check the elective course 1 The students of course No. 1 have gathered and selected 2 The intersection of the student set of course No 
select sno from sc 
	where  cno=1  and sno In
	(select sno from sc 
		where cno=2);

-- Query computer science department students with age not older than 19 The difference set of a year old student 
select * from student 
	where sdept='CS' and sage>19;

4、 Sort the results of set operations

• order by Clause can only be used to sort the final query results , Intermediate results cannot be sorted
• In any case ,order by Clause can only appear at the end
• When sorting the results of set operations ,order by The sorting attribute can be specified numerically in the clause

-- Query computer science department students with age not older than 19 The union of year-old students , The results are arranged in ascending order of student number 
select * from student 
	where sdept='CS' 
union
select * from student 
	where sage<=19
	order by sno;