Question

I have a Logins table which records when user is login, logout or loginFailed and its timestamp. Now I want to get the list of loginFailed after last login and the loginFailed happened within 24 hrs.

What I am doing now is get the last login timestamp first. then use second query to get the final list. do you think I should join those two queries together? Why not? Why yes? And how if yes?

var lastLoginTime = (from inRecord in db.Logins
                     where inRecord.Users.UserId == userId
                           && inRecord.Action == "I"
                     orderby inRecord.Timestamp descending
                     select inRecord.Timestamp).Take(1);

if (lastLoginTime.Count() == 1)
{
    DateTime lastInTime = (DateTime)lastLoginTime.First();

    DateTime since = DateTime.Now.AddHours(-24);
    String actionStr = "F";    

    var records = from record in db.Logins
                  where record.Users.UserId == userId
                        && record.Timestamp >= since
                        && record.Action == actionStr
                        && record.Timestamp > lastInTime
                  orderby record.Timestamp 
                  select record;
}

Answer 1

In the long run, I don’t think it’d matter. No matter how you actually build the query in LINQ to SQL, the ultimate sequence of events on the DB server will be

1. get lastInTime
2. use lastInTime as part of records filter

Now… doing it as part of a single query will save on roundtrips of the actual date-time, so you can get some performance that way. But I would suggest that you only try to merge them if you absolutely need to because your performance profiling suggested that query was a bottleneck.