Database SQL language 02 connection query

JOIN

Example :

/*  Link query   If you need data from multiple data tables to query , Multiple queries can be implemented through join operators   Internal connection  inner join  Query the intersection of the result sets in two tables   External connection  outer join  The left outer join  left join ( Take the left table as a benchmark , The table on the right matches one by one , It doesn't match , Return the record in the left table , The table on the right shows NULL fill )  Right connection  right join ( Take the right table as a benchmark , The table on the left matches one by one , It doesn't match , Return the record in the right table , On the left side of the table NULL fill )  Equivalent connection and non equivalent connection   Self join  */

--  Query the information of the students who took the exam ( Student number , The student's name , Account No , fraction )
SELECT * FROM student;
SELECT * FROM result;

/* Ideas : (1): Analyze requirements , Make sure that the columns of the query come from two classes ,student result, Link query  (2): Determine which connection query to use ?( Internal connection ) */
SELECT s.studentno,studentname,subjectno,StudentResult
FROM student s
INNER JOIN result r
ON r.studentno = s.studentno
--  The right connection ( Can also be realized )
SELECT s.studentno,studentname,subjectno,StudentResult
FROM student s
RIGHT JOIN result r
ON r.studentno = s.studentno

--  Equivalent connection 
SELECT s.studentno,studentname,subjectno,StudentResult
FROM student s , result r
WHERE r.studentno = s.studentno

--  Left connection  ( I checked all the students , Those who don't take exams will find out )
SELECT s.studentno,studentname,subjectno,StudentResult
FROM student s
LEFT JOIN result r
ON r.studentno = s.studentno

--  Check out the absent students ( Left link application scenario )
SELECT s.studentno,studentname,subjectno,StudentResult
FROM student s
LEFT JOIN result r
ON r.studentno = s.studentno
WHERE StudentResult IS NULL

--  Thinking questions : Query the information of the students who took the exam ( Student number , The student's name , Subject name , fraction )
SELECT s.studentno,studentname,subjectname,StudentResult
FROM student s
INNER JOIN result r
ON r.studentno = s.studentno
INNER JOIN `subject` sub
ON sub.subjectno = r.subjectno