c# - 返回 bool 值以在 C# 中处理的 sql 命令

Question

是否可以从 sql 命令返回一个 bool 值 - 然后在 C# 中使用它？

我目前有一个数据库，我在其中插入条目。我想在插入之前知道该条目是否存在，如果存在则做某事......

private void create_user_Click(object sender, EventArgs e)
{
    int exist;
    using (DbConnection connection = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\toilet\source\repos\WindowsFormsApp2\WindowsFormsApp1\Database1.mdf;Integrated Security=True"))
    {
        connection.Open();
        using (DbCommand command = new SqlCommand("WHERE EXIST (SELECT id FROM [information] WHERE id == @given_id)"))
        {
            command.Parameters.Add(new SqlParameter("@given_id", create_user_username_textbox.Text));
            command.Connection = connection;
            exist = command.ExecuteNonQuery();
        }
        MessageBox.Show(Convert.ToString(exist));
    }

    //login_screen login = new login_screen();
    //this.Hide();
    //login.ShowDialog();
    //this.Close();

}

这样做会在 SqlCommand

中的 WHERE 附近出现语法错误

Answer 1

尝试使用有效的 sql 语句并使用 ExecuteScalar 获取一些值以比较它是否存在。例如:

 using (DbCommand command = new SqlCommand("SELECT 1 FROM [information] WHERE id = @given_id"))
 {
     command.Parameters.Add(new SqlParameter("@given_id", create_user_username_textbox.Text));
     command.Connection = connection;

     // get the result from the sql statement 
     var result = command.ExecuteScalar();

     // if the result is not null, convert it to an integer and compare it. Otherwise it is false (no records).
     exists = result != null ? (int) result > 0 : false;

 }