Dynamic sql join in sql server

Question

I have a relationship between two tables. The two tables PKs are int types.

In one table (UserS), I need to supply the Username and get the corresponding ID (which is the PK). This is the standard ASP.NET user table when using forms authentication.

However, in a related table, I want to supply the ID I find from the Users table to get a value out.

Something like:

Run query to get ID for a username (simple select/where query) Return the result Run a subquery where I can pass in the above result - Get value where ID = result

This sounds a lot like dynamic sql. However, there might be a better suited and appropriate way of writing this query (on Sql Server 2k5).

How can I go about doing this and what gotchas lurk?

EDIT: I will try something along the lines of http://www.sqlteam.com/article/introduction-to-dynamic-sql-part-1

EDIT: Thanks for the tips everyone, I wrote this:

SELECT Discount.DiscountAmount FROM Discount INNER JOIN aspnet_Users ON Discount.DiscountCode = aspnet_Users.UserId And aspnet_Users.Username = 's'

Where 's' is to be replaced by a parameter.

Thanks

Answer 1

You don't have to use dynamic SQL for that.

You can use a lookup instead:

DECLARE @ID bigint
SELECT @ID = ID FROM Users WHERE Username = @Username

SELECT
  *
FROM
  TableB
WHERE
  ID = @ID

Then, you could add the PARAMETER @Username to your SqlCommand object, preventing the risks of SQL Injection.

Also, doing the lookup is preferable to a join, since the index is scanned a single time, for TableB.

Answer 2

This is just a simple join isn't it?

SELECT  x.*
FROM    user_table u
        INNER JOIN
                other_table x
                ON u.id = x.user_id
WHERE   u.name = @user_name

Answer 3

SELECT  values.value
FROM    form_users, values
WHERE   form_users.username = 'John Doe'
        AND values.user = form_users.userid

Answer 4

Right, i just would do this:

SELECT *
FROM TableB
JOIN Users ON Users.Id = TableB.ID 
WHERE Users.Username = @Username

Answer 5

SELECT
   *
FROM
   table2 t2
   JOIN
   UserS t1 ON t2.IDKey = t1.IDKey
WHERE
   UserS.Username = @Input

Answer 6

Regarding lookup vs joins - while it may seem more intuitive for the novice to use the lookup, you should go with a join. A lookup needs to be evaluated for every row in your primary result set, while a join is evaluated once. Joins are much more efficient, which means that they are faster.