where子句中的列别名给出了无效的列名错误

Question

This is a simple question that I'm hung up on. I want to know if I can create a column alias and then use it in my WHERE clause, ie:

SELECT TRACTOR, CONVERT(VARCHAR, ORDER) AS NUMBER
FROM TABLE
WHERE NUMBER = '4'

Keep in mind this is just an example of what I'm trying to do. The query I'm running is a bit more complex, but just the basic idea of how to create a variable and then use it in a clause.

My question is about the aliased columns in the where clause. I have a query that looks for an invoice number in one database and matches it the first 7 digits of a field in another database. The query worked fine when we only had 6 digits but now that we have 7, I'm getting an error and I'm trying to rewrite the query in a different manner.

Answer 1

Re-using aliased columns is a great use case for CROSS APPLY :

SELECT t.TRACTOR, CxA.Num
FROM TABLE t
CROSS APPLY
  (SELECT CONVERT(VARCHAR(10), ORDER)) CxA(Num)
WHERE CxA.Num = '4'

Anything in a CROSS APPLY can be referenced in the SELECT , WHERE , ORDER BY , etc with some limitations (normally if you have aggregation in the CROSS APPLY expression).

Answer 2

Take a look at: http://sqlmag.com/t-sql/working-variables-t-sql-part-1

The basic syntax of variables would be like so:

declare @id int
select @id = 1

select *
from myTable
where id = @id

EDIT: in case you're actually asking about column aliases in your where clause, and not sql variables, take a look at this SO question: Referring to a Column Alias in a WHERE Clause

EDITx2:

for your specific case (assuming asking about column aliases) you could do the following:

SELECT *
FROM (
    SELECT TRACTOR, CONVERT(VARCHAR(100), ORDER) AS NUMBER
    FROM TABLE
) someTableAlias
WHERE NUMBER = '4'

Though you may want to rethink your column names... i think number and order are both reserved words, and should be avoided where possible. you can get around these for the most part by using [order] and [number] I believe, but still best to avoid reserved words as columns/tables/whatevers.