I have fairly strong feelings when it comes to the stored procedures versus dynamic SQL argument, but one thing is clear: you should never, ever use concatenated SQL strings in your applications. Give me parameterized SQL, or give me death. There are two good reasons you should never do this.

First, consider this naive concatenated SQL:

SELECT email, passwd, login_id, full_name
FROM members
WHERE email = 'x';

Code like this opens your app to SQL injection attacks, and it's a huge, gaping vulnerability. Steve Friedl's SQL Injection Attacks by Example provides an excellent visual blow-by-blow of what can happen when you write code this naive. Here's the Reader's Digest version:

SELECT email, passwd, login_id, full_name
FROM members
WHERE email = 'x' OR full_name LIKE '%Bob%';

I know what you're thinking. No, escaping the strings doesn't protect you; see Steve's article.

Second, parameterized SQL performs better. A lot better. Consider the parameterized version of the above:

SqlConnection conn = new SqlConnection(_connectionString);
conn.Open();
string s = "SELECT email, passwd, login_id, full_name " +
"FROM members WHERE email = @email";
SqlCommand cmd = new SqlCommand(s);
cmd.Parameters.Add("@email", email);
SqlDataReader reader = cmd.ExecuteReader();

This code offers the following pure performance benefits:

• Fewer string concatenations
• No need to worry about any kind of manual string escaping
• A more generic query form is presented to db, so it's likely already hashed and stored as a pre-compiled execution plan
• Smaller strings are sent across the wire

Non-parameterized SQL is the GoTo statement of database programming. Don't do it, and make sure your coworkers don't either.