MYSQL - 了解这个 SELECT 和 UNION ALL 查询

Question

我有这段代码，来自Tom Mac，但我想进行一些更改，但我不明白它是如何工作的。

我正在尝试通过添加UNION ALL SELECT date1, date2 FROM person 来同时添加两个日期列，并尝试将其插入现有的选择函数之间。

我将如何将同一张表中的这两列添加到我的结果中？

这是我正在尝试调整的提供的代码：

select person as Names,
sum(splitScore) as Split_Count,
sum(activeScore) as Active_Count
from
(
select Name1 as person, 
case when Name2 is null then 1 else 0.5 end as splitScore,
case when Active='True' and Name2 is null then 1 when Active='True' and Name2 is not null then 0.5 else 0 end as activeScore 
from person where Name1 is not null
union all
select Name2 as person, 
case when Name1 is null then 1 else 0.5 end as splitScore,
case when Active='True' and Name1 is null then 1 when Active='True' and Name1 is not null then 0.5 else 0 end as activeScore 
from person where Name2 is not null
) t inner join roster s on s.name = t.log
group by person
;

Answer 1

要连接的两个查询必须包含相同顺序的相同字段。因此，请参阅下面的示例以及其他字段：

select person as Names,
sum(splitScore) as Split_Count,
sum(activeScore) as Active_Count,
newfield1, newfield2
from
(
select Name1 as person, 
case when Name2 is null then 1 else 0.5 end as splitScore,
case when Active='True' and Name2 is null then 1 when Active='True' and Name2 is not  null then 0.5 else 0 end as activeScore, 
newfield1, newfield2 
from person where Name1 is not null
union all
select Name2 as person, 
case when Name1 is null then 1 else 0.5 end as splitScore,
case when Active='True' and Name1 is null then 1 when Active='True' and Name1 is not null then 0.5 else 0 end as activeScore, 
newfield1, newfield2 
from person where Name2 is not null
) t inner join roster s on s.name = t.log
group by person
;

标准的 UNION 可能如下所示：

SELECT field1, field2 FROM table1
UNION
SELECT field1, field2 from table2

您的查询显然具有额外的复杂性