Can calculated column be used in another calculated column?

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

I want calculated column (ExchangeRatePrice) to use in CalculatedPrice. Can I use it straight of I need to convert it again?

I have used 10 * Price just to show you the example because if I use 'ExchangeRatePrice' here it will give error 'Invalid Column name'

Answer 1

Yes. Use "WITH" (Common Table Expressions)

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 2

You can't use column aliases until the ORDER BY clause.

But you can use column aliases defined in a subquery.

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

Answer 3

To reference pre-calculate a value in SQL usually means performing the calculation in an inner query (AKA inline view):

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

This is the equivalent to using the WITH clause (available in SQL Server 2005+) - there's no difference in performance.

You could duplicate the logic:

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

..but that means duplication & inherent risk of someone not updating both instances to keep in sync.