Question

SELECT ExchangeRatePrice = CASE pp.Price
     WHEN NULL THEN 0
       ELSE (CASE WHEN c.CurrencyId = 1 THEN pp.Price 
       ELSE CONVERT(DECIMAL(9, 2), (pp.Price * c.ExchangeRate)) END)
     END , 

     price as OriginalPriceInDB,
     10 * Price as CalculatedPrice,
    c.currencyid as Currency

FROM ProductPrice pp, currency c

alt text http://img682.imageshack.us/img682/3692/exchangerate.png

我想在CalculatedPrice中使用计算列（ExchangeRatePrice）。我可以直接使用它吗？我需要再次转换它吗？

我使用10 * Price只是为了向您展示示例，因为如果我在这里使用'ExchangeRatePrice'，则会出现错误'无效的列名'

Answer 1

引用预先计算SQL中的值通常意味着在内部查询中执行计算（AKA内联视图）：

SELECT x.exchangerateprice,
       x.OriginalPriceInDB,
       10 * x.exchangerateprice AS CalculatedPrice,
       x.currencyid
  FROM (SELECT CASE 
                 WHEN pp.price IS NULL THEN
                   0
                 WHEN c.CurrencyId = 1 THEN 
                   pp.Price
                 ELSE 
                   CONVERT(DECIMAL(9, 2), (pp.Price * c.ExchangeRate)) 
               END AS ExchangeRatePrice,
              price as OriginalPriceInDB,
              c.currencyid as Currency
         FROM PRODUCTPRICE pp, 
              CURRENCY c) x

这相当于使用WITH子句（SQL Server 2005+中提供） - 性能没有区别。

您可以复制逻辑：

SELECT CASE 
         WHEN pp.price IS NULL THEN
           0
         WHEN c.CurrencyId = 1 THEN 
           pp.Price
         ELSE 
           CONVERT(DECIMAL(9, 2), (pp.Price * c.ExchangeRate)) 
       END AS ExchangeRatePrice,
       price as OriginalPriceInDB,
       CASE 
          WHEN pp.price IS NULL THEN
            0
          WHEN c.CurrencyId = 1 THEN 
            pp.Price
          ELSE 
            CONVERT(DECIMAL(9, 2), (pp.Price * c.ExchangeRate)) 
        END * 10 AS CalculatedPrice,
        c.currencyid as Currency
   FROM PRODUCTPRICE pp, 
        CURRENCY c

..但这意味着重复＆amp;某人未更新两个实例以保持同步的固有风险。

Answer 2

是。使用“WITH”（公用表格表达式）

With MainT as (
    SELECT CASE pp.Price
        WHEN NULL 
        THEN 0
        ELSE (
            CASE WHEN c.CurrencyId = 1 THEN pp.Price
            ELSE CONVERT(DECIMAL(9, 2), (pp.Price * c.ExchangeRate)) END
            )
        END ExchangeRatePrice,
        price as OriginalPriceInDB,
        c.currencyid as Currency
    FROM ProductPrice pp, currency c
)
select *,  10*ExchangeRatePrice as CalculatedPrice
from MainT

Answer 3

在ORDER BY子句之前，不能使用列别名。

但您可以使用子查询中定义的列别名。

SELECT x * 10 AS y
FROM (SELECT 123 AS x) tbl

可以在另一个计算列中使用计算列吗？

3 个答案: