如何防止仅针对列名过度使用 SELECT？

Question

如标题所示，有什么方法可以让我不必为列名调用那么多查询吗？

select val,  
(select val1 FROM data WHERE size='a' AND cat ='1' ) as cat1_a,  
(select val1 FROM data WHERE size='a' AND cat ='2' ) as cat2_a,  
(select val1 FROM data WHERE size='b' AND cat ='1' ) as cat1_b,  
(select val1 FROM data WHERE size='b' AND cat ='2' ) as cat2_b,  
(select val1 FROM data WHERE size='c' AND cat ='1' ) as cat1_c,  
(select val1 FROM data WHERE size='c' AND cat ='2' ) as cat2_c  
from data

Answer 1

它并没有真正节省字符/输入，但您可以使用连接。

select t1.val, a1.val1 as cat1_a, a2.val1 as cat2_a
from data t1
left join data a1
    on t1.val=a1.val
    and a1.size = 'a'
    and a1.cat = '1'
left join data a2
    on t1.val=a2.val
    and a2.size = 'a'
    and a2.cat = '2'

我不知道你的数据是什么样的，但你可以尝试一些不同的结果 https://dbfiddle.uk/?rdbms=sqlserver_2019&fiddle=a95ad5bd5c593c72354c24f9f867694a

如何防止仅针对列名过度使用 SELECT？

How to prevent over using SELECT just for column name?

sql

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