Preventing duplicate database entries

Hi all.. I've been scouring the forums for about 6 hours to noavail. This is a really simple question. I'm trying to havea registration page that lets the user input name, email, desiredusername, and password. I want to check the username and emailfields to make sure ppl cannot sign up twice. So from what I'vegathered I have a couple of options:

1) i can set up a unique constraint on the database columns,
2) i can run a select statement before inserting,
3) i can store the whole database column in a variable then search through it.

My question is how to do option 2? All of my transactions are through a sqldatasource object in c#.

since you mentioned sqldatasource I am assuming you are working with SQL Server. You could write a stored proc and do a conditional INSERT.

CREATE PROC..

...

BEGIN

IF NOT EXISTS ( SELECT * FROM table WHERE username = @.username ANDemail=@.email...)

BEGIN

-- do the INSERT

END

ELSE

BEGIN

--the user already exists. return an appropriate message

END

|||Do #1, then catch the constraint error in sqldatasource_inserted event.|||Stored procedures are pretty exciting and cool.. I'm about to spendsome time reading up on them because there's a lot for me to learn (howto execute them, syntax, capabilities, etc.). Appreciate thereply ndinakar. I'll figure this out soon I'm sure.|||I've spent a considerable amount of time trying to set up a constrainton the email and usernames fields to no avail. This is how I'vebeen going about it:

1)in server explorer, double click my table
2)right click the email column, select Check Constraints
3) Add new

Now it asks for an Expression. No idea what to enter here I triedthe single word "UNIQUE" but that is wrong. And, all my web/forumsearches regarding the subject were unsuccessful- lots of regularexpression results returned.

Similarly, I tried playing around with the Indexes/Keys option insteadof check constraints option. I thought this would give me moreluck since it looks like you can select from a drop-down which columnsyou want to apply the effect to. Problem here is the drop-down isonly populated with one column, the PK.

I'm thinking I need to head over to the bookstore and park myself thereuntil I figure some things out- sick of staring at the monitor. Thanks for the reply.|||

You can also do it via script from Query Analyzer:

ALTERTABLE <tableName> ADD COSNTRAINT <constraint_name> UNIQUE(<column1>,<column2>,<column3>)

|||It's not a check constraint, it's really a unique index that you build.