SQL Server 2016 learning records - connection query

Learning goals ：

SELECT — Link query
Link query ： Queries involving multiple tables at the same time
The condition used to join two tables becomes a join condition or join predicate
sql The main types of join queries in ：

• （1） Cross connect （ The generalized Cartesian product ）
• （2） Equivalent connection
• （3） Self connect
• （4） Composite conditional connection
• （5） Internal connection
• （6） External connection

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Learning content ：

sql The main types of join queries in ：

（1） Cross connect （ The generalized Cartesian product ）
Connection without predicate , Rarely used

SELECT student.*,sc.* FROM student,sc
SELECT student.*,sc.* FROM student cross join sc

Many are meaningless , So I seldom use

（2） Equivalent connection
The connection operator is = The connection of

-- Inquire about each student and their elective courses 
select student.*,sc.* from student,sc where student.sno=sc.sno;

-- Check the student number of computer major students , full name , major , Elective course number , Course name , And achievements 
select sc.sno,sname,sdept,sc.cno,cname,grade
      from student,sc,course
	  where student.sno=sc.sno and course.cno=sc.cno
	        and sdept='CS';

（3） Self connect
Representation ：

• You need to alias the table to show the difference
• Because all attribute names have the same name , So you have to use the alias prefix

-- Query the course name of the direct prerequisite course of each course 
select a1.cname,a2.cname from course a1,course a2 where a1.cpno=a2.cno;

-- Query the course number of the indirect prerequisite course of each course （ That is, the prerequisite course of the pre course ）
select a1.cno,a2.cno from course a1,course a2 where a1.cpno=a2.cpno;

-- Query at the same time 2 Course No. and 3 The student number of course No 
select a1.sno from sc a1,sc a2 where a1.sno=a2.sno and a1.cno=2 and a2.cno=3;

• （4） Composite conditional connection

where Clause contains multiple join conditions , It is called qualified connection
classification ：
The two tables are connected by multiple attributes
Connect itself by multiple attributes
Multiple table joins

-- Check options 1 No. and the result is 90 Names of students with scores above , Gender and department 
select sname,ssex,sdept from student,sc
where student.Sno=sc.Sno /* Join predicate */
	and cno=1
	and grade>90;

-- Check the student number of each student 、 full name 、 The name of the elective course is the grade 
select student.sno,sname,cname,grade from student,sc,course
where student.sno=sc.sno
	and sc.cno=course.cno;

• （5） Internal connection

inner join perhaps join
Inner connection is actually equivalent connection , He USES “=、>、<、<>” The equal operator matches the row query statements in the two tables according to the values of the columns common to each table

-- Query all student information and elective courses 
select student.*,sc.* from student join sc
on student.sno=sc.sno;

• （6） External connection

Left connection （left join） Or left outer connection （left outer join）
The right connection （right join） Or right outer connection （right outer join）
Full connection （full join） Or all external connections （full outer join）

-- All information of all students and their course selection records 
select student.*,sc.* from student left join sc
on student.sno=sc.sno;

-- Course selection records and corresponding course information of all students 
select sc.*,course.* from sc right join course
on sc.cno=course.cno;

select sc.*,course.* from sc full join course
on sc.cno=course.cno;
// Returns all rows in the left and right tables , When a row has no matching row in another table , Return null value