What does the @ do in a SQL statement?

Question

For example, when creating a command text for a SqlServer ce statement, the example has you use

command.CommandText = "INSERT INTO Region (RegionID, RegionDescription) VALUES (@id, @desc)";

I understand how to run sql queries, but have always just passed the database system a string with sql statements. This is new to me, and I would like to understand. Does the @ mean that it is a variable, so when it parses the string, it will insert the value of the variable id into the string where it says @id? I know in php, double quotes allow the parser to parse a variable value within a string, and am wondering if this is similar.
However, to test it, I made a simple program to test this theory.

string id = "FOO";
MessageBox.Show("@id"); 

It showed the literal @id, so this has left me confused. Thanks ahead for any help! I just feel the need to understand what I am typing, not just mindlessly following the examples.

Answer 1

In this case @ defines the variable that you want to pass into your SQL statement.

For instance:

SqlParameter param  = new SqlParameter();
param.ParameterName = "@id";
param.Value         = 666;

And then you add it to your command like that:

command.Parameters.Add(param);

Then, in your select @id and @desc will use values passed as a value of SqlParameter.

Therefore, if you just output "@id" it will be interpreted as a normal string. It only makes sense while used inside a SQL query.

Answer 2

The @ symbol, in this case, is used to parameterize a SQL command, not for general-purpose variable substitution within a string.

command.CommandText = "INSERT INTO Region (RegionID, RegionDescription) " +
                      "VALUES (@id, @desc)";
command.Parameters.AddWithValue("@id", 42);
command.Parameters.AddWithValue("@desc, "Description of region forty-two.");

Answer 3

@ has a special meaning in the SQL (T-SQL) query (ie: it is used to parametrize the query), but in a C# string you just get the literal @. Actually, command.CommandText is still a string with the @ characters; the substitution takes place only when needed (ie: query is executed).

---

If you want interpolation in C#, use String.Format(), eg:

string name = "John";
string greet = String.Format("Hello, {0}", name);

// this also works
Console.WriteLine("Hello, {0}", name);