可以更改为 Case 语句的 Union 语句答案

【问题标题】：Union Statement that can be changed to Case statement可以更改为 Case 语句的 Union 语句
【发布时间】：2013-09-23 00:46:56
【问题描述】：

大家好！

我有这种代码，但它有点难看，我的一个朋友告诉我，我可以在这里实现案例语句，但我不知道我将如何或如何实现，代码很长，所以如果你能帮助我优化我的代码，我将不胜感激！

PS。请对我温柔，我是 T-sql 的新手 :) 谢谢你！

SELECT 
         SUM(CYJEWELRY) 'CY_Jewelry'
        ,SUM(CYAPPLICANCE) 'CY_Appliance'
        ,SUM(CYCELLPHONE) 'CY_Cellphone'
        ,SUM(PYJEWELRY) 'PY_Jewelry'
        ,SUM(PYAPPLIANCE) 'PY_Appliance'
        ,SUM(PYCELLPHONE) 'PY_Cellphone'
    FROM

        (   

            ---TOTAL NUNG A FORMAT 0,0,0,0,0,0
            --------------CURRENT YEAR JEWELRY
            SELECT COUNT (*) AS CYJEWELRY,0 AS CYAPPLICANCE,0 AS CYCELLPHONE,0 AS PYJEWELRY,0 AS PYAPPLIANCE,0 AS PYCELLPHONE
            FROM #TEMPTABLE1
            WHERE (fld_StorageGroupID  >= 3 and fld_StorageGroupID <= 14)

        UNION
            -----------CURRENT YEAR APPLIANCE

            SELECT 0,COUNT(*),0,0,0,0
            FROM #TEMPTABLE1
            WHERE fld_StorageGroupID = 1

        UNION
            ------------CURRENT YEAR CELLPHONE

            SELECT 0,0,COUNT(*),0,0,0
            FROM #TEMPTABLE1
            WHERE fld_StorageGroupID = 2

        UNION
            ---------------LAST YEAR JEWELRY
            SELECT  0,0,0,COUNT(*),0,0
            FROM #TEMPTABLE2
            WHERE (fld_StorageGroupID  >= 3 and fld_StorageGroupID <= 14)

        UNION
            -----------------------LAST YEAR APPLIANCE
            SELECT 0,0,0,0,COUNT (*),0
            FROM #TEMPTABLE2
            WHERE fld_StorageGroupID = 1

        UNION
            -------------------------LAST YEAR CELLPHONE
            SELECT 0,0,0,0,0,COUNT(*)
            FROM #TEMPTABLE2
            WHERE fld_StorageGroupID = 2
        )A

【问题讨论】：

COUNT(*) 是一种从表中获取行数的低效方法。指定一列（理想情况下是索引列或主键列），例如：COUNT(PK_Column_Name)

标签： tsql count sql-server-2008-r2 case

【解决方案1】：

假设您的数据有点像这样Sql Fiddle Example，请尝试使用SUM() 和CASE 对子查询进行此操作。

SELECT SUM(CASE WHEN fld_StorageGroupID  >= 3 and fld_StorageGroupID <= 14 ELSE 0 END) Col1And4,
       SUM(CASE WHEN fld_StorageGroupID = 1 THEN 1 ELSE 0 END)  Col2And5,
       SUM(CASE WHEN fld_StorageGroupID = 2 THEN 1 ELSE 0 END)  Col3And6
FROM #TEMPTABLE1
GROUP BY fld_StorageGroupID

由于您对子查询中的最后 3 列应用相同的过滤器，因此我在这里只完成了前 3 列。

编辑：

我认为这比上面的要好（注意： no need to use SUM() in the main query）。

Fiddle Example with data

select col1_4 CY_Jewelry,
       col2_5 CY_Appliance,
       col3_6 CY_Cellphone,
       col1_4 PY_Jewelry,
       col2_5 PY_Appliance,
       col3_6 PY_Cellphone
from (
select sum(case when id>= 3 and id <= 14 then 1 else 0 end) col1_4,
       sum(case when id = 2 then 1 else 0 end) col2_5,
       sum(case when id = 3 then 1 else 0 end) col3_6
from t
--group by id
) X

【讨论】：