Question

I am doing a password authentication query like this.

if exists(select UserID from users where UserName=@UserName and Password=@Password) 
    select UserID from users where UserName=@UserName and Password=@Password
else 
    select 0

but it seems to me that I am querying twice for a single result
Can I just do this

Select UserID Where Username = @UserName and Password = @Password

and when I get my results just check the reader for number of results?

if (!myReader.HasRows)
    MessageBox.Show("UserName not found or Password invalid");
else
    //do login stuff

Guess what I am asking is, which of these is better, or are they the same?

Answer 1

Use this:

SELECT  ISNULL
        (
        (
        SELECT  userId
        FROM    users
        WHERE   userName = @userName
                AND password = @password
        ),
        0
        )

Your solution with checking number of records would work too of course.

Note that it will always return exactly one record and will fail if userName is not unique and there are duplicates of @userName.

As noted by @Andriy M, COALESCE will not help here since it’s being internally rewritten into a CASE prone to the same problem.

Answering your question: yes, the original batch would access users twice.