如何在SQL中对多个列求和

Question

我有数据库，有12列数字数据 - 每个月一列。

列名是

Jan, Feb, Mar. Apr, May, Jun, Jul, Aug, Sep, Oct, Nov and Dec.

我知道如何编写代码来汇总前3个月（1月，2月和3月）并返回Total总值。

SELECT Jan+Feb+Mar AS Total FROM MyData

问题：如何编写通用查询以给出前N个月的总和？

我是否需要编写代码来生成SQL语句，还是有其他方法？

Answer 1

我认为你需要动态SQL来自动生成前N个月的查询。但是你也可以定义一个包含所有总和的视图，例如：

CREATE VIEW sums AS (
    SELECT Jan AS total1,
    SELECT Jan + Feb AS total2,
    SELECT Jan + Feb + Mar AS total3,
    ...
    SELECT Jan + Feb + Mar + ... + Nov + Dec AS total12
    FROM yourTable
)

然后，您可以查询视图并访问所需的任何总和。

Answer 2

您可以生成一个可以使用sp_executesql command执行的字符串

DECLARE @query NVARCHAR(MAX);
SET @query = 'SELECT #selectColumns# FROM tableName'

--- write your logic to get how many columns you need based on value of `N`
--- make their summation  as stringToReplace (i.e. = 'Jan + Feb + Mar ...')
--- and replace them with #selectColumns# in @query

EXECUTE(@query)

PS：如果你使用的是MSSQL Server之外的数据库，你应该能够找到一些方法来执行字符串作为SQL命令。

例如，对于MySQL，有prepare

PREPARE stmt1 FROM @query; 
EXECUTE stmt1; 
DEALLOCATE PREPARE stmt1; 

对于Oracle，你将拥有EXEC SQL EXECUTE IMMEDIATE :query;

Answer 3

如果您正在使用SQL Server，那么您可以使用APPLY运算符生成月份序列。

SELECT
       SUM(a.Total) Totals 
FROM table t
CROSS APPLY (
        VALUES (1, 'Jan', Jan), (2, 'Feb', Feb), (3, 'Mar', Mar),
               ....
               (12, 'Dec', Dec)
)a(Seq, Months, Total)
WHERE a.Seq <= 3 -- pass Nos of Months to get the sum of Totals