filling the data in gridview by using join query

That's because Default2 is null

Try this SQL

select users.username,groups.name,roles.userid
from users,groups,roles
inner join users on roles.userid=users.userid
inner join groups on roles.groupid=users.groupid

Hi,

Here is your answer.

SELECT U.Username,G.[Name] AS [GroupName]
FROM Users U
    INNER JOIN Roles R ON U.Userid = R.UserId
    INNER JOIN [Group] G ON R.GroupId = G.GroupId

for table3 i gave name Group which you didn't mention in your question.
And the reason for this error is your query will return an error and if you are not using using proper try catch block and assign data source to your server control Default2 which was bind to null object.

and also try your sql query to sql server's enterprise manager.

Is that query correctly framed

SqlDataAdapter da = new SqlDataAdapter("select users.username,groups.name from users,groups inner join users on roles.userid=users.userid inner join groups on roles.groupid=group.groupid", MAconn);

you have placed roles instead of group i guess