left outer join

left outer join 会从左边的集合名（collection1_name）中返回所有的记录，即使在右边的集合名（collection2_name）中没有匹配的记录。

语法

as left outer join as [on condition]

参数

参数名参数类型描述是否必填
collection1_name/collection2_namestring集合名。是
select_set1/select_set2set结果集。是
alias1_name/alias2_namestring别名。是
conditionexpression集合之间关联条件。否

返回值

left outer join 会从左边的集合(collection1_name)那里返回所有的记录，即使在右边的集合(collection2_name)中没有匹配的记录。

示例

  • 集合 foo.persons 中记录如下。

    1. { "Id_P": 1, "LastName": "Adams", "FirstName": "John", "Address": "Oxford Street", "City": "London" }
    2. { "Id_P": 2, "LastName": "Bush", "FirstName": "George", "Address": "Fifth Avenue", "City": "New York" }
    3. { "Id_P": 3, "LastName": "Carter", "FirstName": "Thomas", "Address": "Changan Street", "City": "Beijing" }

  • 集合 foo.orders 中记录如下。

    1. { "Id_O": 1, "OrderNo": 77895, "Id_P": 3 }
    2. { "Id_O": 2, "OrderNo": 44678, "Id_P": 3 }
    3. { "Id_O": 3, "OrderNo": 22456, "Id_P": 1 }
    4. { "Id_O": 4, "OrderNo": 24562, "Id_P": 1 }
    5. { "Id_O": 5, "OrderNo": 34764, "Id_P": 65 }

  • 列出所有客户的订单, 如果该用户没有订单信息，则用null替代其订单信息。

    1. > db.exec("select t1.LastName, t1.FirstName, t2.OrderNo from foo.persons as t1 left outer  join foo.orders as t2 on t1.Id_P=t2.Id_P")
    2. { "LastName": "Adams", "FirstName": "John", "OrderNo": 22456 }
    3. { "LastName": "Adams", "FirstName": "John", "OrderNo": 24562 }
    4. { "LastName": "Bush", "FirstName": "George", "OrderNo": null }
    5. { "LastName": "Carter", "FirstName": "Thomas", "OrderNo": 77895 }
    6. { "LastName": "Carter", "FirstName": "Thomas", "OrderNo": 44678 }
    7. Return 5 row(s).
    8. Takes 0.11811s.