我要评论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 | +----+-------+-----+-------+--------+--------+
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