Sql server MS SQL Server-使用SELECT语句的计算字段

我想问您是否有一条语句可以使用同一SELECT语句的计算字段：

例如：

表测试：

机器数量值 500 20

SELECT Machine, 
    Amount*Value AS TestFormula
    TestFormula*12 AS TestFormulaYear
FROM Test

重用此计算字段的正确语句是什么

提前感谢,

---

Kevin

至少在sql server中，您可以通过子查询来完成：

SELECT  Machine 
,       TestFormula
,       TestFormula*12 AS TestFormulaYear
FROM    (
           SELECT Machine 
           ,      Amount*Value AS TestFormula
           FROM   Test
        ) T

---

至少在sql server中，您可以通过子查询执行此操作：

SELECT  Machine 
,       TestFormula
,       TestFormula*12 AS TestFormulaYear
FROM    (
           SELECT Machine 
           ,      Amount*Value AS TestFormula
           FROM   Test
        ) T

---

对于您向我们展示的简单示例，我建议您重复该表达式

SELECT
    Machine, 
    Amount*Value AS TestFormula,
    Amount*Value*12 AS TestFormulaYear
FROM Test;

---

其他答案已经说明了如何使用子查询来真正重用列，但与我上面所写的相比，这并不是很好。

对于您向我们展示的简单示例，我建议重复该表达式

SELECT
    Machine, 
    Amount*Value AS TestFormula,
    Amount*Value*12 AS TestFormulaYear
FROM Test;

其他答案已经说明了如何使用子查询来真正重用列，但与我上面所写的相比，这并不是很有效。

您可以使用子查询来重用值：

WITH formula AS (
    SELECT Machine, 
        Amount*Value AS TestFormula
    FROM Test
    )
SELECT Machine, 
    TestFormula
    TestFormula*12 AS TestFormulaYear
FROM formula;

如果带有CTE的批处理包含多个语句，则前面的语句必须以分号终止。

您可以使用a来重用该值：

WITH formula AS (
    SELECT Machine, 
        Amount*Value AS TestFormula
    FROM Test
    )
SELECT Machine, 
    TestFormula
    TestFormula*12 AS TestFormulaYear
FROM formula;

---

如果带有CTE的批处理包含多条语句，则前面的语句必须以分号终止。

假设这是T-SQL：

您不能在

SELECT

语句中引用列的别名，否。如果查看，您会注意到SELECT是要处理的查询的第8部分。这意味着只有

orderby

才能引用列的别名

如果需要对计算值进行进一步计算，则需要使用CTE、子查询或重新声明计算。例如：

重复计算：

SELECT [Column] * 10 As Expression,
       [Column] * 10 * 5 AS Expression2
FROM [Table];

WITH Formula AS(
    SELECT [Column] * 10 As Expression
    FROM [Table])
SELECT Expression,
       Expression * 5 AS Expression2
FROM Formula;

CTE:

SELECT [Column] * 10 As Expression,
       [Column] * 10 * 5 AS Expression2
FROM [Table];

WITH Formula AS(
    SELECT [Column] * 10 As Expression
    FROM [Table])
SELECT Expression,
       Expression * 5 AS Expression2
FROM Formula;

子查询：

SELECT Expression,
       Expression * 5 AS Expression2
FROM (SELECT [Column] * 10 As Expression
      FROM [Table]) Formula;

---

假设这是T-SQL：

您不能在

SELECT

语句中引用列的别名，否。如果查看，您会注意到SELECT是要处理的查询的第8部分。这意味着只有

orderby

才能引用列的别名

如果需要对计算值进行进一步计算，则需要使用CTE、子查询或重新声明计算。例如：

重复计算：

SELECT [Column] * 10 As Expression,
       [Column] * 10 * 5 AS Expression2
FROM [Table];

WITH Formula AS(
    SELECT [Column] * 10 As Expression
    FROM [Table])
SELECT Expression,
       Expression * 5 AS Expression2
FROM Formula;

CTE:

SELECT [Column] * 10 As Expression,
       [Column] * 10 * 5 AS Expression2
FROM [Table];

WITH Formula AS(
    SELECT [Column] * 10 As Expression
    FROM [Table])
SELECT Expression,
       Expression * 5 AS Expression2
FROM Formula;

子查询：

SELECT Expression,
       Expression * 5 AS Expression2
FROM (SELECT [Column] * 10 As Expression
      FROM [Table]) Formula;

---

如果您希望设置一个语句，以便在更改公式时更新许多列，我想您可以声明公式并使用动态SQL。如果您想确保许多列都得到了正确更新，那么这样做有一个好处：

Declare @TestFormula as nvarchar(100) = '([Amount]*[Value])'
Declare @TestFormulaYear as nvarchar(100) = '(12*' + @TestFormula + ')'
declare @sql as nvarchar(max)
set @sql = 'SELECT [Machine], ' + @TestFormula + ' AS TestFormula, ' + @TestFormulaYear + ' AS TestFormulaYear
FROM (values(500, 20, 20)) a([Machine], [Amount], [Value])'
exec(@sql)

---

如果您希望设置一个语句，以便在更改公式时更新许多列，我想您可以声明公式并使用动态SQL。如果您想确保许多列都得到了正确更新，那么这样做有一个好处：

Declare @TestFormula as nvarchar(100) = '([Amount]*[Value])'
Declare @TestFormulaYear as nvarchar(100) = '(12*' + @TestFormula + ')'
declare @sql as nvarchar(max)
set @sql = 'SELECT [Machine], ' + @TestFormula + ' AS TestFormula, ' + @TestFormulaYear + ' AS TestFormulaYear
FROM (values(500, 20, 20)) a([Machine], [Amount], [Value])'
exec(@sql)