Get to know MySQL connection query for the first time

Classified by function :

Internal connection ： Equivalent connection   ：  The result is the intersection of multi table queries

                 Non equivalent connection

                 Self join

External connection ： The left outer join ： Left table is the main table

                 Right connection ： The right table is the main one

                 Full outer join

Cross connect

sql92 standard : Only internal connections are supported

grammar ：

SELECT  Query list  
FROM  surface 1, surface 2...
WHERE  Connection condition 
AND    Connection condition 
...
(AND  filter ) 
(GROUP BY  Clause 
 HAVING  filter 
 ORDER BY  Clause );

sql99 standard ：

grammar :

Internal connection ：inner（ It can be omitted ）

External connection ： Left lateral ：left （outer）

              Right outside ：right （outer）

              Total external ：full （outer）

cross ：cross

SELECT  Query list 
FROM  surface 1
 Connection type  join  surface 2
on  How to connect 
....
(WHERE  filter 
GROUP BY  Clause 
HAVING  filter 
ORDER BY  Clause );

Such as ：

SELECT COUNT(*)
FROM boy
INNER JOIN girl 
ON girl.boy_id = boy.id
WHERE boy.age > 25;

Be careful ：

1. The query result of the external connection is all the records in the main table , If you match it from the right of the table , The value of the match , If not, display null

2. The left outer join and the right outer join can achieve the same effect by exchanging the order of the two tables