Giving alias in JOIN command in sqldatasource - asp.net

Question

I am using JOIN command to connect two tables in SqlDataSource.
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:CS %>"
SelectCommand="SELECT UserName as UN, AboutMe, WebPage, Email FROM Users JOIN ProfileImages ON ProfileImages.UserName = Users.UN WHERE (UN = #UserName)" >
</asp:SqlDataSource>
In MS SQL Server Management Studio this command is ok. But in the SqlDataSource it return errors - Invalid column name 'UN', Ambiguous column name 'UserName'
How do I alias this command right in SqlDataSource to not return these errors?

Answer 1

Your query is a bit wrong
You used alias i a wrong way
First:
You can use alias in the following sections:
Fields. Example Username as UN
Tables names: UserTable as US
In the order by section example: Order BY UN Desc
This because the sql runs the query operations in the following sequence:
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
ORDER BY clause
So because the FROM clause runs first the sql doesn't know about the alias in the SELECT clause
Take a look on a correct query
SELECT
UserName as UN,
AboutMe,
WebPage,
Email
FROM Users
JOIN ProfileImages ON ProfileImages.UserName = Users.UserName
WHERE (Users.UserName = #UserName)

Answer 2

I'm not an expert at SQL, but if both ProfileImages and Users contain the column 'UserName', then in your select columns you need to specify the table from which the UserName is pulled. So something like:
SELECT Users.UserName as UN, Users.AboutMe, Users.WebPage, Users.Email
FROM Users
JOIN ProfileImages ON ProfileImages.UserName = Users.UserName
WHERE (Users.UserName = #UserName)

Answer 3

You cannot use the column alias in the where clause.
SELECT UserName as UN, AboutMe, WebPage, Email
FROM Users usersAlias JOIN ProfileImages piAlias ON piAlias.UserName = usersAlias.UserName
WHERE (usersAlias.UserName = #UserName)
It makes sense when you look at the order of execution:
http://blog.sqlauthority.com/2009/04/06/sql-server-logical-query-processing-phases-order-of-statement-execution/
Since the order of execution is:
These phases and their orders are given as follows:
FROM
ON
OUTER
WHERE
GROUP BY
CUBE | ROLLUP
HAVING
SELECT
DISTINCT
10 ORDER BY
TOP
When these are being processed (the "from" (#1) and "on" (#2) and "where" (#4))..they have no knowledge about the "select" (#8) (aka, the alias name for UN), the column alias in the select clause cannot be used in the WHERE and FROM (and ON) clauses.