Question

我有两张表格如下

TableA
A_id    name
----    ----
123     Test1   
124     Test2
125     Test3
126     Test4

TableB
B_id    fk_A_id    value     type_id
----    -------    -----     -------
1       123        op1       1
2       123        hello     2
3       123        abc       3
4       126        op2       1
5       126        hello     2
6       126        def       3

我使用查询加入两个表

select TableA.A_id as id, TableA.name as name, TBL2.type_name as type_name
from TableA
full join (
select fk_A_id, value as type_name
from TableB
where type_id = 2
)TBL2 on TableA.A_id = TBL2.fk_A_id

返回结果将是

id            name            type_name
--            ----            ---------
123           Test1           hello
124           Test2             
125           Test3
126           Test4           hello

我的问题是如何用“你好”填充空列。同样是TableB的返回结果，type_id =＆＃39; 2＆＃39;将在TableB上有类似的字符串

Answer 1

我想你想要left join，就像这样：

select a.A_id as id, a.name as name, b.type_name as type_name
from TableA a left join
     TableB b
     on a.A_id = b.fk_A_id and b.type_id = 2;

不需要子查询，full join过度杀伤。

如果您希望NULL值也为hello，请使用coalesce()：

select a.A_id as id, a.name as name,
       coalesce(b.type_name, 'hello') as type_name
from TableA a left join
     TableB b
     on a.A_id = b.fk_A_id and b.type_id = 2;

连接表并使用返回表值填充空列

1 个答案: