The join on the grammar

The various connections are shown below (red is the returned data) :

There are three main types of connections

  • INNER JOIN: Returns rows if there is at least one match in the table
  • LEFT JOIN: Returns all rows from the LEFT table even if there is no match in the right table
  • RIGHT JOIN: Returns all rows from the RIGHT table even if there is no match in the left table

Take INNER JOIN: as an example

SELECT s.studentno,studentname,subjectno,studentresult FROM 
student AS s INNER JOIN result ASR # alias preventselectStudentno is not determined from which tableON s.`studentno`=R. 'studentno' # intersection, both tables have studentno #onandwhereAlmost the same, are added conditions, the former called connection query, the latter called equivalent queryCopy the code

Select * from student; select * from student; select * from student;

If there are more than two lists of procedures (methodology)

  • Join a query with two tables
  • Two tables join to form one table (logically)
  • Join the table and then join the table.
  • And so on