左连接、右连接、内连接的区别

左连接

left join -- 返回左表中所有的数据以及右表中字段相同字段的值

两张表查询语句：

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;

多表查询语句：

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name
LEFT JOIN table2
ON table2.column_name = table3.column_name;

右连接

right join -- 返回右表中所有的数据以及左表中字段中相同字段的值

两张表查询语句：

SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name = table2.column_name;

多表查询语句：

SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name = table2.column_name
RIGHT JOIN table2
ON table2.column_name = table3.column_name;

内连接

inner join -- 返回左右两个表中字段相同的数据值

两张表查询语句：

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;

多表查询语句：

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name
INNER JOIN table2
ON table2.column_name = table3.column_name;