如果我想从 table1 中带回所有“名称”并且仅在第 2 列中列出输出以包含值（如果它是“Apple”），我正在尝试计算 SQL 的方式。如果它不是“Apple”或者是“Null”，那么第 2 列也应该是“Null”
表 1
| ID(pk) | Name |
|---|---|
| 24 | Boris |
| 25 | Dominic |
| 26 | Rishi |
| 27 | Elizabeth |
| 28 | Ben |
表 2
| ID(fk) | Description |
|---|---|
| 27 | Apple |
| 27 | Orange |
| 27 | Pear |
| 26 | Apple |
| 26 | Pear |
| 25 | Pear |
| 24 | Orange |
需要的输出
| Name | Description |
|---|---|
| Boris | |
| Dominic | |
| Rishi | Apple |
| Elizabeth | Apple |
| Sajid |
【问题讨论】：
如果第二个表中没有重复项（在您确认是这种情况的问题下方的评论中），这是外连接的简单应用。
这里我将测试数据包含在with 子句中。你不需要它 - 你有实际的桌子。删除with 子句，检查主查询并根据需要更改表名和列名。
with
table_1 (id, name) as (
select 24, 'Boris' from dual union all
select 25, 'Dominic' from dual union all
select 26, 'Rishi' from dual union all
select 27, 'Elizabeth' from dual union all
select 28, 'Ben' from dual
)
, table_2 (id, description) as (
select 27, 'Apple' from dual union all
select 27, 'Orange' from dual union all
select 27, 'Pear' from dual union all
select 26, 'Apple' from dual union all
select 26, 'Pear' from dual union all
select 25, 'Pear' from dual union all
select 24, 'Orange' from dual
)
select t1.name, t2.description
from table_1 t1 left outer join table_2 t2
on t2.id = t1.id and t2.description = 'Apple'
order by t1.id
;
NAME DESCRIPTION
--------- -----------
Boris
Dominic
Rishi Apple
Elizabeth Apple
Ben
【讨论】：
你可以通过多种方式解决它，一个例子：
SELECT t1.name
, MAX(CASE t2.descripton WHEN 'Apple' THEN t2.descripton END)
FROM t1
JOIN t2
USING(id)
GROUP BY t1.name;
CASE 表达式会将除 Apple 之外的所有内容都映射为 null。然后，MAX 会将每个名称的集合减少为 Apple 或 null。
【讨论】：
我认为下面的查询对你有用：
Select Name, CASE WHEN ID IN (SELECT DISTINCT ID FROM table2 WHERE LOWER(description) = 'apple') THEN 'Apple' ELSE Null END as Description FROM table1;
【讨论】：