SQL Server AS statement aliased column within WHERE statement - sql

Question

I want to execute a query in which I rename one of the columns using the 'AS' statement and reuse that aliased column name within the 'WHERE' statement. Below is an example:
SELECT lat AS latitude
FROM poi_table
WHERE latitude < 500
The problem here is that SQL Server does not like this query because of the WHERE clause and the AS statement name being referenced in the WHERE clause. Can anyone explain why this is happening and what I can do to remedy my situation?
Suppose I were to have a formula that I have aliased in the SELECT portion of the query, how do I tackle that?
SELECT *,
( 6371*1000 * acos( cos( radians(42.3936868308) ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(-72.5277256966) ) + sin( radians(42.3936868308) ) * sin( radians( lat ) ) ) )
AS distance
FROM poi_table
WHERE distance < 500;

Answer 1

SQL doesn't typically allow you to reference column aliases in WHERE, GROUP BY or HAVING clauses. MySQL does support referencing column aliases in the GROUP BY and HAVING, but I stress that it will cause problems when porting such queries to other databases.
When in doubt, use the actual column name:
SELECT t.lat AS latitude
FROM poi_table t
WHERE t.lat < 500
I added a table alias to make it easier to see what is an actual column vs alias.
Update
A computed column, like the one you see here:
SELECT *,
( 6371*1000 * acos( cos( radians(42.3936868308) ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(-72.5277256966) ) + sin( radians(42.3936868308) ) * sin( radians( lat ) ) ) ) AS distance
FROM poi_table
WHERE distance < 500;
...doesn't change that you can not reference a column alias in the WHERE clause. For that query to work, you'd have to use:
SELECT *,
( 6371*1000 * acos( cos( radians(42.3936868308) ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(-72.5277256966) ) + sin( radians(42.3936868308) ) * sin( radians( lat ) ) ) ) AS distance
FROM poi_table
WHERE ( 6371*1000 * acos( cos( radians(42.3936868308) ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(-72.5277256966) ) + sin( radians(42.3936868308) ) * sin( radians( lat ) ) ) ) < 500;
Be aware that using a function on a column (IE: RADIANS(lat)) will render an index useless, if one exists on the column.

Answer 2

SQL Server is tuned to apply the filters before it applies aliases (because that usually produces faster results).
You could do a nested select statement. Example:
SELECT Latitude FROM
(
SELECT Lat AS Latitude FROM poi_table
) A
WHERE Latitude < 500
I realize this may not be what you are looking for, because it makes your queries much more wordy.
A more succinct approach would be to make a view that wraps your underlying table:
CREATE VIEW vPoi_Table AS
SELECT Lat AS Latitude FROM poi_table
Then you could say:
SELECT Latitude FROM vPoi_Table WHERE Latitude < 500

Answer 3

I am not sure why you cannot use "lat" but, if you must you can rename the columns in a derived table.
select latitude from (SELECT lat AS latitude FROM poi_table) p where latitude < 500

Answer 4

This would work on your edited question !
SELECT * FROM (SELECT <Column_List>,
( 6371*1000 * acos( cos( radians(42.3936868308) ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(-72.5277256966) ) + sin( radians(42.3936868308) ) * sin( radians( lat ) ) ) )
AS distance
FROM poi_table) TMP
WHERE distance < 500;

Answer 5

Logical Processing Order of the SELECT statement
The following steps show the logical processing order, or binding
order, for a SELECT statement. This order determines when the objects
defined in one step are made available to the clauses in subsequent
steps. For example, if the query processor can bind to (access) the
tables or views defined in the FROM clause, these objects and their
columns are made available to all subsequent steps. Conversely,
because the SELECT clause is step 8, any column aliases or derived
columns defined in that clause cannot be referenced by preceding
clauses. However, they can be referenced by subsequent clauses such as
the ORDER BY clause. Note that the actual physical execution of the
statement is determined by the query processor and the order may vary
from this list.
FROM
ON
JOIN
WHERE
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
Source: http://msdn.microsoft.com/en-us/library/ms189499%28v=sql.110%29.aspx

Answer 6

Both accepted answer and Logical Processing Order explain why you could not do what you proposed.
Possible solution:
use derived table (cte/subquery)
use expression in WHERE
create view/computed column
From SQL Server 2008 you could use APPLY operator combined with Table valued Constructor:
SELECT *, s.distance
FROM poi_table
CROSS APPLY (VALUES(6371*1000*acos(cos(radians(42.3936868308))*cos(radians(lat))*cos(radians(lon)-radians(-72.5277256966))+sin(radians(42.3936868308))*sin(radians(lat))))) AS s(distance)
WHERE distance < 500;
LiveDemo

Answer 7

I am not sure why you cannot use "lat" but, if you must you can rename the columns in a derived table.
select a.latitude from (SELECT lat AS latitude FROM poi_table) a where latitude < 500