The meaning of JOIN is just like the English word "join". It connects two tables and can be roughly divided into inner join, outer join, right join, left join and natural join.
First create two tables, the following are used for examples
CREATE TABLE t_blog(
id INT PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(50),
typeId INT
);
SELECT * FROM t_blog;
+----+-------+--------+
| id | title | typeId |
+----+-------+--------+
| 1 | aaa | 1 |
| 2 | bbb | 2 |
| 3 | ccc | 3 |
| 4 | ddd | 4 |
| 5 | eee | 4 |
| 6 | fff | 3 |
| 7 | ggg | 2 |
| 8 | hhh | NULL |
| 9 | iii | NULL |
| 10 | jjj | NULL |
+----+-------+--------+
-- 博客的类别
CREATE TABLE t_type(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
SELECT * FROM t_type;
+----+------------+
| id | name |
+----+------------+
| 1 | C++ |
| 2 | C |
| 3 | Java |
| 4 | C# |
| 5 | Javascript |
+----+------------+To understand various JOINs first To understand the Cartesian product. The Cartesian product combines every record in table A with every record in table B. Therefore, when there are n records in table A and m records in table B, the result of the Cartesian product operation will be n*m records. In the following example, t_blog has 10 records, t_type has 5 records, and the Cartesian product of both of them has 50 records. There are five ways to produce a Cartesian product as follows.
SELECT * FROM t_blog CROSS JOIN t_type;
SELECT * FROM t_blog INNER JOIN t_type;
SELECT * FROM t_blog,t_type;
SELECT * FROM t_blog NATURE JOIN t_type;
select * from t_blog NATURA join t_type;
+----+-------+--------+----+------------+
| id | title | typeId | id | name |
+----+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | C++ |
| 1 | aaa | 1 | 2 | C |
| 1 | aaa | 1 | 3 | Java |
| 1 | aaa | 1 | 4 | C# |
| 1 | aaa | 1 | 5 | Javascript |
| 2 | bbb | 2 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 2 | bbb | 2 | 3 | Java |
| 2 | bbb | 2 | 4 | C# |
| 2 | bbb | 2 | 5 | Javascript |
| 3 | ccc | 3 | 1 | C++ |
| 3 | ccc | 3 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 3 | ccc | 3 | 4 | C# |
| 3 | ccc | 3 | 5 | Javascript |
| 4 | ddd | 4 | 1 | C++ |
| 4 | ddd | 4 | 2 | C |
| 4 | ddd | 4 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 4 | ddd | 4 | 5 | Javascript |
| 5 | eee | 4 | 1 | C++ |
| 5 | eee | 4 | 2 | C |
| 5 | eee | 4 | 3 | Java |
| 5 | eee | 4 | 4 | C# |
| 5 | eee | 4 | 5 | Javascript |
| 6 | fff | 3 | 1 | C++ |
| 6 | fff | 3 | 2 | C |
| 6 | fff | 3 | 3 | Java |
| 6 | fff | 3 | 4 | C# |
| 6 | fff | 3 | 5 | Javascript |
| 7 | ggg | 2 | 1 | C++ |
| 7 | ggg | 2 | 2 | C |
| 7 | ggg | 2 | 3 | Java |
| 7 | ggg | 2 | 4 | C# |
| 7 | ggg | 2 | 5 | Javascript |
| 8 | hhh | NULL | 1 | C++ |
| 8 | hhh | NULL | 2 | C |
| 8 | hhh | NULL | 3 | Java |
| 8 | hhh | NULL | 4 | C# |
| 8 | hhh | NULL | 5 | Javascript |
| 9 | iii | NULL | 1 | C++ |
| 9 | iii | NULL | 2 | C |
| 9 | iii | NULL | 3 | Java |
| 9 | iii | NULL | 4 | C# |
| 9 | iii | NULL | 5 | Javascript |
| 10 | jjj | NULL | 1 | C++ |
| 10 | jjj | NULL | 2 | C |
| 10 | jjj | NULL | 3 | Java |
| 10 | jjj | NULL | 4 | C# |
| 10 | jjj | NULL | 5 | Javascript |
+----+-------+--------+----+------------+Inner joinINNER JOIN is the most commonly used connection operation. From a mathematical point of view, this is to calculate the intersection of the two tables; from a Cartesian product point of view, it is to filter out the records that meet the conditions of the ON clause from the Cartesian product. There are four writing methods: INNER JOIN, WHERE (equivalent join), STRAIGHT_JOIN, and JOIN (INNER omitted).
SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId=t_type.id;
SELECT * FROM t_blog,t_type WHERE t_blog.typeId=t_type.id;
SELECT * FROM t_blog STRAIGHT_JOIN t_type ON t_blog.typeId=t_type.id; --注意STRIGHT_JOIN有个下划线
SELECT * FROM t_blog JOIN t_type ON t_blog.typeId=t_type.id;
+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
+----+-------+--------+----+------+The meaning of left join LEFT JOIN is to find the intersection of two tables plus the remaining data in the left table. Still speaking from the perspective of the Cartesian product, we first select the records for which the ON clause condition is true from the Cartesian product, and then add the remaining records in the left table (see the last three items).
SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id;
+----+-------+--------+------+------+
| id | title | typeId | id | name |
+----+-------+--------+------+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
+----+-------+--------+------+------+Similarly, RIGHT JOIN is to find the intersection of two tables plus the remaining data in the right table. Once again described from the perspective of Cartesian product, the right join is to select the records whose ON clause condition is true from the Cartesian product, and then add the remaining records in the right table (see the last item).
SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;
+------+-------+--------+----+------------+
| id | title | typeId | id | name |
+------+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
| 6 | fff | 3 | 3 | Java |
| 7 | ggg | 2 | 2 | C |
| NULL | NULL | NULL | 5 | Javascript |
+------+-------+--------+----+------------+Outer join is to find the union of two sets. From the perspective of the Cartesian product, it is to select the records whose ON clause condition is true from the Cartesian product, then add the remaining records in the left table, and finally add the remaining records in the right table. MySQL does not support OUTER JOIN, but we can achieve it by UNIONing the results of the left join and the right join.
SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
UNION
SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;
+------+-------+--------+------+------------+
| id | title | typeId | id | name |
+------+-------+--------+------+------------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
| NULL | NULL | NULL | 5 | Javascript |
+------+-------+--------+------+------------+In the connection SQL statement in MySQL, the syntax format of the ON clause is: table1.column_name = table2.column_name. When the schema design adopts the same naming style for the columns of the joined table, the USING syntax can be used to simplify the ON syntax, in the format: USING(column_name).
So, the function of USING is equivalent to ON. The difference is that USING specifies an attribute name to connect two tables, while ON specifies a condition. In addition, when SELECT *, USING will remove the columns specified by USING, but ON will not. Examples are as follows.
SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId =t_type.id;
+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
+----+-------+--------+----+------+
SELECT * FROM t_blog INNER JOIN t_type USING(typeId);
ERROR 1054 (42S22): Unknown column 'typeId' in 'from clause'
SELECT * FROM t_blog INNER JOIN t_type USING(id); -- 应为t_blog的typeId与t_type的id不同名，无法用Using，这里用id代替下。
+----+-------+--------+------------+
| id | title | typeId | name |
+----+-------+--------+------------+
| 1 | aaa | 1 | C++ |
| 2 | bbb | 2 | C |
| 3 | ccc | 3 | Java |
| 4 | ddd | 4 | C# |
| 5 | eee | 4 | Javascript |
+----+-------+--------+------------+Natural join is a simplified version of the USING clause. It finds the same columns in the two tables and uses them as join conditions to join. There are left natural joins, right natural joins and ordinary natural joins. In the t_blog and t_type examples, the same column in the two tables is id, so id will be used as the connection condition.
In addition, be sure to distinguish the differences between the following three statements.
NATURAL JOIN: SELECT * FROM t_blog NATURAL JOIN t_type;
Descartes product: SELECT * FROM t_blog NATURA JOIN t_type;
Descartes product: SELECT * FROM t_blog NATURE JOIN t_type;
SELECT * FROM t_blog NATURAL JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog,t_type WHERE t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type USING(id);
+----+-------+--------+------------+
| id | title | typeId | name |
| 1 | aaa | 1 | C++ |
| 2 | bbb | 2 | C |
| 3 | ccc | 3 | Java |
| 4 | ddd | 4 | C# |
| 5 | eee | 4 | Javascript |
SELECT * FROM t_blog NATURAL LEFT JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type USING(id);
| 6 | fff | 3 | NULL |
| 7 | ggg | 2 | NULL |
| 8 | hhh | NULL | NULL |
| 9 | iii | NULL | NULL |
| 10 | jjj | NULL | NULL |
SELECT * FROM t_blog NATURAL RIGHT JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type USING(id);
+----+------------+-------+--------+
| id | name | title | typeId |
| 1 | C++ | aaa | 1 |
| 2 | C | bbb | 2 |
| 3 | Java | ccc | 3 |
| 4 | C# | ddd | 4 |
| 5 | Javascript | eee | 4 |The above is the detailed content of How to use JOIN in MySql. For more information, please follow other related articles on the PHP Chinese website!
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