asp.net check is primary key exists before inserting into table

Question

looking for a simple way to check if a primary key already exists in the table before inserting into it. where Fname is the primary key in the table.

SqlConnection conn = new SqlConnection(connStr);
conn.Open();
SqlCommand cmd = new SqlCommand();
cmd.Connection = conn;
cmd.CommandText = "INSERT INTO [table] ([firstName], [lastName]) VALUES ('" + txtFName.Text + "','" + txtLName.Text + "')";        

cmd.ExecuteNonQuery();
conn.Close();

Answer 1

As a general rule, you should just try to insert. The dbms will return an error if they key exists. Trap that error.

You have to trap errors anyway, because there are a lot of things besides primary key constraints that can prevent a row from being inserted. Among them

• Any other kind of constraint, like a foreign key constraint, a check constraint, etc.
• Disk error.
• Network error.
• Hurricane.

You don't lose any efficiency by trapping the error.

• Inserting (no error) requires one round-trip to the database.
• Inserting (with error, which you trap) requires one round-trip to the database, not counting whatever you might have to do to correct that error. You'd have to do all the correcting if you checked first anyway.
• Check-then-insert requires two round-trips to the database.

Some platforms can merge data if you try to insert a row for which a primary key already exists. The most famous (or most notorious, depending on your point of view) is MySQL's ON DUPLICATE KEY UPDATE.