SQL 根据条件查询到 select 一列的多个实例

Question

我要加入 3 tables:

SELECT TABLE1.FIELD1
FROM TABLE1
JOIN TABLE2 ON TABLE1.ID = TABLE2.FK
JOIN TABLE3 ON TABLE2.ID = TABLE3.FK

我想根据某些条件 select TABLE2.FIELD1 的几列，例如 TABLE2.FIELD1 WHERE TABLE2.CONDITIONI、TABLE2.FIELD1 WHERE TABLE3.ANOTHERCONDITION.

所以最终结果是这样的：

TABLE1.FIELD1 | TABLE2.FIELD1 BASE ON CON1 | TABLE2.FIELD1 BASED ON CON2

我不确定我是否足够清楚，如果需要，请进一步说明。

示例报告：

Id |   Product | Income Quantity | Outcome Quantity

在此示例中，数量仅在第二个 table 中的一个字段中与产品 table 联合。它根据不同的条件在输出中渲染了两次，例如，收入是 header.flow == 1 的地方，而输出是 header.flow == -1.

的地方

Answer 1

不确定，但您似乎应该使用 CASE 语句

select 
case when <<cod1>> then
table1.field
when <<cod2>> then 
table2.field
end 
from your_table1
inner join your_table2 on <<condition>>

Answer 2

像这样

SELECT ID,
       CASE WHEN flow = 1 THEN Quantity ELSE NULL END as 'Income',
       CASE WHEN flow = -1 THEN Quantity ELSE NULL END as 'Outcome'
FROM Sample

只需在最后根据需要添加您的加入，并在 CASE 语句中更改您的条件

根据您的评论

I have to use group by for flow and SUM(quantity). and I'll have two columns for each product.

您可以试试这个：

SELECT s1.ID,
    (SELECT SUM(Quantity) FROM SAMPLE s2 WHERE s2.ID = S1.ID AND S2.FLOW = 1) as 'Income',
    (SELECT SUM(Quantity) FROM SAMPLE s2 WHERE s2.ID = S1.ID AND S2.FLOW = -1) as 'Outgoing'
FROM Sample S1
GROUP BY ID

也可以通过

实现

SELECT DISTINCT s1.ID,
    (SELECT SUM(Quantity) FROM SAMPLE s2 WHERE s2.ID = S1.ID AND S2.FLOW = 1) as 'Income',
    (SELECT SUM(Quantity) FROM SAMPLE s2 WHERE s2.ID = S1.ID AND S2.FLOW = -1) as 'Outgoing'
FROM Sample S1

SQL 根据条件查询到 select 一列的多个实例

SQL Query to select several instances of one column based on conditions

sql

sql-server

select

join

inner-join