SQL 查询到 C# 中的标量结果

Question

In some programming contexts getting a scalar value from a sql query is easy:

RowCount = Connection.Execute("SELECT Count(*) FROM TableA").Fields(0).Value

In C#, given a SqlConnection variable conn that is already open, is there a simpler way to do this same thing without laboriously creating a SqlCommand, a DataReader, and all in all taking about 5 lines to do the job?

Answer 1

SqlCommand has an ExecuteScalar method that does what you want.

 cmd.CommandText = "SELECT COUNT(*) FROM dbo.region";
 Int32 count = (Int32) cmd.ExecuteScalar();

Answer 2

If you can use LINQ2SQL (or EntityFramework) you can simplify the actual query asking to

using (var context = new MyDbContext("connectionString"))
{
    var rowCount = context.TableAs.Count();
}

If LINQ2SQL is an option that has lots of other benefits too compared to manually creating all SqlCommands, etc.

Answer 3

There is ExecuteScalar which saves you at least from the DataReader :

static public int AddProductCategory(string newName, string connString)
{
    Int32 newProdID = 0;
    string sql =
        "INSERT INTO Production.ProductCategory (Name) VALUES (@Name); "
        + "SELECT CAST(scope_identity() AS int)";
    using (SqlConnection conn = new SqlConnection(connString))
    {
        SqlCommand cmd = new SqlCommand(sql, conn);
        cmd.Parameters.Add("@Name", SqlDbType.VarChar);
        cmd.Parameters["@name"].Value = newName;
        try
        {
            conn.Open();
            newProdID = (Int32)cmd.ExecuteScalar();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
    return (int)newProdID;
}

(Example taken from this MSDN documentation article )

Answer 4

Investigate Command.ExecuteScalar:

using(var connection = new SqlConnection(myConnectionString))
{
    connection.Open();
    using(var command = connection.CreateCommand())
    {
        command.CommandType = CommandType.Text;
        command.CommandText = mySql;
        var result = (int)command.ExecuteScalar();
    }
}

If you're feeling really lazy, encapsulate it all in an extension method, like we do.

EDIT: As requested, an extension method:

public static T ExecuteScalar<T> (this SqlConnection connection, string sql)
{
    if (connection == null)
    {
        throw new ArgumentNullException("connection");
    }

    if (string.IsNullOrEmpty(sql))
    {
        throw new ArgumentNullException("sql");
    }

    using(var command = connection.CreateCommand())
    {
        command.CommandText = sql;
        command.CommandType = CommandType.Text;
        return (T)command.ExecuteScalar();
    }
}

Note, this version assumes you've properly built the SQL beforehand. I'd probably create a separate overload of this extension method that took two parameters: the stored procedure name and a List. That way, you could protect yourself against unwanted SQL injection attacks.

Answer 5

You do not need a DataReader. This example pulls back the scalar value:

Object result;
using (SqlConnection con = new SqlConnection(ConnectionString)) {
       con.Open();
       using (SqlCommand cmd = new SqlCommand(SQLStoredProcName, con)) {
       result = cmd.ExecuteScalar();
      }
}