通过他的名字更改列引用 ID 保持 NULL 值

Question

我有两个 table。 Table1:

+-------+----------+---------+---------+
| ID    | name     |   refT1 |  refT2  |
+-------+----------+---------+---------+
| 1     |  Name1   |   1     |    2    |
| 2     |  Name2   |   2     |    4    |
| 3     |  Name3   |  NULL   |   NULL  |
| 4     |  Name4   |   3     |   NULL  |
+-------+----------+---------+---------+

和Table 2:

+-------+--------+
| ID    |  name  |
+-------+--------+
| 1     |   A1   |
| 2     |   A2   |
| 3     |   A3   |
| 4     |   A4   |
+-------+--------+

我需要这个table

+-------+----------+---------+---------+
| ID    | name     |   A1    |    A1   |
+-------+----------+---------+---------+
| 1     |  Name1   |   A1    |    A2   |
| 2     |  Name2   |   A2    |    A4   |
| 3     |  Name3   |  NULL   |   NULL  |
| 4     |  Name4   |   A3    |   NULL  |
+-------+----------+---------+---------+

我写了一个sql命令，但是空值的行消失了，我想保留它们。

Answer 1

希望此查询对您有所帮助：

SELECT ID, name,
Case when Table_1.refT1 is null then NULL else Table_2.name end as A1,
Case when Table_1.refT2 is null then NULL else Table_2.name end as A2
 from 
Table_1
inner join
Table_2
on Table_1.ID = Table_2.ID

SQL Fiddle

Answer 2

做LEFT JOIN两次：

select t1.id, t1.name, t2.name as a1, t3.name as a2
from table1 t1
  left join table2 as t2 on t1.refT1 = t2.id
  left join table2 as t3 on t1.refT2 = t3.id

(Left join是把消失的都包括进来"null rows")

通过他的名字更改列引用 ID 保持 NULL 值

Change column refence id by his name keeping NULL values

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