SQL join, left join, right join usage

SQL Join Used to combine rows from two or more tables , Based on the common fields between these tables .
INNER JOIN： If there is at least one match in the table , Then go back to the line
LEFT JOIN： Even if there is no match in the right table , Also returns all rows from the left table
RIGHT JOIN： Even if there is no match in the left table , Also returns all rows from the right table
FULL JOIN： As long as there is a match in one of the tables , Then go back to the line

grammar
SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;
perhaps ：
SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;
INNER JOIN And JOIN It's the same .

Refer to the figure below ：

LEFT JOIN Keywords from the left table （table1） Go back to all the lines , Even if the right watch （table2） There is no match in . If there is no match in the right table , The result is NULL.
grammar
SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;
or ：
SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;
In some databases ,LEFT JOIN be called LEFT OUTER JOIN.

RIGHT JOIN Keywords from the right table （table2） Go back to all the lines , Even if the left watch （table1） There is no match in . If there is no match in the left table , The result is NULL.
grammar
SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name=table2.column_name;
or ：
SELECT column_name(s)
FROM table1
RIGHT OUTER JOIN table2
ON table1.column_name=table2.column_name;
In some databases ,RIGHT JOIN be called RIGHT OUTER JOIN.

FULL OUTER JOIN Key words only need left table （table1） And the right watch （table2） There is a match... In one of the tables , Then go back to the line .
FULL OUTER JOIN Keywords combine LEFT JOIN and RIGHT JOIN Result .
grammar
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;
FULL OUTER JOIN Keyword returns all rows in the left and right tables . If table1 The rows in the table are in table2 There is no match or table2 The rows in the table are in table1 There is no match in the table , These lines will also be listed .
author ：Ritchie_Li

this paper [ SQL Join,Left Join,Right Join usage ] Included in Game programming ️ - database , A game development favorite ~

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