Question

I have a simple query in my code (shown below) written by my colleague. What does t mean here? Also what is the role of the ; inside the query? I am dead sure that t is not any table, nor any field anywhere in my database. And guess what this query works!!

string query = @"SELECT COUNT(*) FROM (SELECT AttemptNo FROM attempt_exercise 
                 WHERE FK_UId = @uId AND FK_EId = @eId AND Mode = @mode)t;
                ";

The code follows like this (for any other info if required):

MySqlCommand cmd = new MySqlCommand(query, _conn);
cmd.Parameters.AddWithValue("@uId", uId);
cmd.Parameters.AddWithValue("@eId", eId);
cmd.Parameters.AddWithValue("@mode", mode);
attemptCount = Convert.ToInt32(cmd.ExecuteScalar());
_conn.Close();
return attemptCount;

Answer 1

Your colleague created a query (SELECT COUNT(*)) with a subquery that he named t. This t is just a temporary table name which refers to

SELECT AttemptNo FROM attempt_exercise 
  WHERE FK_UId = @uId AND FK_EId = @eId AND Mode = @mode

He could have feasibly named it temp to be a bit more explicit. The reason that this becomes like a table is because, in MySQL, a SELECT query returns rows of data which act like a table. So, this inner query gets the AttemptNo, and creates a temporary table t. The outer query then counts this data.

The ; inside the query is to make it a full statement when the string query is called by the program. If this weren’t included, the String query wouldn’t contain a valid MySQL statement. The final ; is to complete the assignment for the variable.