目录

INNER JOIN（内连接）

LEFT JOIN（左连接）

RIGHT JOIN（右连接）

FULL JOIN（全连接）

示例图形化解释JOIN的不同类型

INNER JOIN：

LEFT JOIN：

RIGHT JOIN：

FULL JOIN：

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INNER JOIN（内连接）

返回两个表中匹配的行。

表达方式1：

SELECT * 
FROM table1 
INNER JOIN table2 
ON table1.column = table2.column;

表达方式2：

SELECT * 
FROM table1, table2 
WHERE table1.column = table2.column;

LEFT JOIN（左连接）

返回左表中的所有行以及与右表匹配的行。

表达方式1：

SELECT * 
FROM table1 
LEFT JOIN table2 
ON table1.column = table2.column;

表达方式2：

SELECT * 
FROM table1 
LEFT OUTER JOIN table2 
ON table1.column = table2.column;

RIGHT JOIN（右连接）

返回右表中的所有行以及与左表匹配的行。

表达方式1：

SELECT * 
FROM table1 
RIGHT JOIN table2 
ON table1.column = table2.column;

表达方式2：

SELECT * 
FROM table1 
RIGHT OUTER JOIN table2 
ON table1.column = table2.column;

FULL JOIN（全连接）

返回左右表中的所有行。

SELECT * 
FROM table1 
FULL JOIN table2 
ON table1.column = table2.column;

示例图形化解释JOIN的不同类型

INNER JOIN

+----+-------+-----+-------+
| ID | Name  | Age | Grade |
+----+-------+-----+-------+
| 1  | John  | 24  | A     |
| 2  | Alice | 22  | B     |
| 3  | Bob   | 25  | A     |
+----+-------+-----+-------++--------+--------+
| ID     | City   |
+--------+--------+
| 1      | London |
| 2      | Paris  |
| 4      | Berlin |
+--------+--------+After INNER JOIN:+----+-------+-----+-------+--------+--------+
| ID | Name  | Age | Grade | ID     | City   |
+----+-------+-----+-------+--------+--------+
| 1  | John  | 24  | A     | 1      | London |
| 2  | Alice | 22  | B     | 2      | Paris  |
+----+-------+-----+-------+--------+--------+

LEFT JOIN

+----+-------+-----+-------+
| ID | Name  | Age | Grade |
+----+-------+-----+-------+
| 1  | John  | 24  | A     |
| 2  | Alice | 22  | B     |
| 3  | Bob   | 25  | A     |
+----+-------+-----+-------++--------+--------+
| ID     | City   |
+--------+--------+
| 1      | London |
| 2      | Paris  |
| 4      | Berlin |
+--------+--------+After LEFT JOIN:+----+-------+-----+-------+--------+--------+
| ID | Name  | Age | Grade | ID     | City   |
+----+-------+-----+-------+--------+--------+
| 1  | John  | 24  | A     | 1      | London |
| 2  | Alice | 22  | B     | 2      | Paris  |
| 3  | Bob   | 25  | A     | NULL   | NULL   |
+----+-------+-----+-------+--------+--------+

RIGHT JOIN

+----+-------+-----+-------+
| ID | Name  | Age | Grade |
+----+-------+-----+-------+
| 1  | John  | 24  | A     |
| 2  | Alice | 22  | B     |
| 3  | Bob   | 25  | A     |
+----+-------+-----+-------++--------+--------+
| ID     | City   |
+--------+--------+
| 1      | London |
| 2      | Paris  |
| 4      | Berlin |
+--------+--------+After RIGHT JOIN:+----+-------+-----+-------+--------+--------+
| ID | Name  | Age | Grade | ID     | City   |
+----+-------+-----+-------+--------+--------+
| 1  | John  | 24  | A     | 1      | London |
| 2  | Alice | 22  | B     | 2      | Paris  |
| NULL | NULL  | NULL   | NULL    | 4      | Berlin |
+----+-------+-----+-------+--------+--------+

FULL JOIN

+----+-------+-----+-------+
| ID | Name  | Age | Grade |
+----+-------+-----+-------+
| 1  | John  | 24  | A     |
| 2  | Alice | 22  | B     |
| 3  | Bob   | 25  | A     |
+----+-------+-----+-------++--------+--------+
| ID     | City   |
+--------+--------+
| 1      | London |
| 2      | Paris  |
| 4      | Berlin |
+--------+--------+After FULL JOIN:+----+-------+-----+-------+--------+--------+
| ID | Name  | Age | Grade | ID     | City   |
+----+-------+-----+-------+--------+--------+
| 1  | John  | 24  | A     | 1      | London |
| 2  | Alice | 22  | B     | 2      | Paris  |
| 3  | Bob   | 25  | A     | NULL   | NULL   |
| NULL | NULL  | NULL   | NULL    | 4      | Berlin |
+----+-------+-----+-------+--------+--------+