Argthtjtr

I am trying to get a count of IDS on a orders table for yesterday (not hardcode) where the date in the table corresponds to when the order was placed.

My table looks like this

orders (

order_id INT PRIMARY KEY

, user_id INT

, date_created DATE

, order_value FLOAT

, city_id INT

)

I have used this code to get todays date in the table -

Select *,DATEADD(dd, DATEDIFF(dd, 0, getdate()), 0) as Today

From orders

This works fine but when I try attempt a where clause below this

Select *,DATEADD(dd, DATEDIFF(dd, 0, getdate()), 0) as Today

From orders

where Today -1 = date_created;

I receive this error "Invalid column name 'Today'"
So my question is simply how can I get this table to recognise the the date I added as a new column and allow me to perform work on it?

Thanks in advance

Your query doesn't work because of the way queries are logically processed.

You need to either use a common table expression:

WITH c

    AS

     (

         SELECT *, DATEADD(dd, DATEDIFF(dd, 0, GETDATE()), 0) AS Today

         FROM   orders

     )

SELECT *

FROM   c

WHERE  c.Today - 1 = date_created;

Or you could use a derived table.

SELECT *

FROM   (   SELECT *, DATEADD(dd, DATEDIFF(dd, 0, GETDATE()), 0) AS Today

           FROM   orders ) AS c

WHERE  c.Today - 1 = date_created;

Neither one is "better".

I receive this error "Invalid column name 'Today'"

You can't reference the Today expression in the WHERE clause because the WHERE clause is evaluated before the SELECT clause according to the logical order of query processing.

Since your desired type is date, it would be better to use CAST or CONVERT rather than the DATEADD...DATEDIFF ugliness (common before SQL Server 2008 introduced the date datatype). Also, you need to use DATEADD to calculate yesterday's date instead of a subtraction operator.

The example below uses these techniques to get the orders from yesterday:

SELECT

      order_id

    , user_id

    , date_created

    , order_value

    , city_id

    , CAST(GETDATE() AS date) as Today

FROM dbo.orders

WHERE DATEADD(day, -1, CAST(GETDATE() AS date)) = date_created;

As others have indicated you're not able to use a calculation done in the SELECT clause in the WHERE clause due to the order of operations.

However, a nifty workaround I use from time to time is to do some calculations within a CROSS APPLY, and then those can be referenced in SELECT and WHERE. It is a helpful technique to avoid repeating code.

SELECT 

    A.*,

    S1.Today

FROM orders A

CROSS APPLY 

    (

        SELECT

            CAST(GETDATE() AS DATE) Today

    ) S1

WHERE   DATEADD(DAY, -1, S1.Today) = A.date_created;