Question

我有一个从我的数据库填充的数据表。我有一个添加和更新数据的功能，但现在我想检查数据是否存在。但我必须检查两件事

如果数据库中存在数据，请检查它是启用还是禁用，如果启用，则应返回message =“数据存在于表中”，否则“数据已禁用。”

    bool nameExists;

        connection statement
        {
            command.Connection = connection;
            command.CommandText =
                @"SELECT TOP 1 0 " +
                @"FROM [dbo].[products] p WITH (NOLOCK) " +
                @"WHERE [p].[name] = @name";
            command.Parameters.AddWithValue("name", name);
            command.CommandTimeout = 100;
            connection.Open();

            using (SqlDataReader reader = command.ExecuteReader())
            {
                nameExists = reader.HasRows;
            }
            connection.Close();
        }

如何增强查询以检查上述条件？

注意如果您认为这篇文章不是一篇好文章，请不要降级此帖子而是发表评论。如果我的帖子中存在我的语法错误，也请原谅我。

Answer 1

如果有一个名为Enabled的列是int数据类型，并且0表示已禁用且1表示已启用，那么这可能会执行您想要的操作：< / p>

declare @Enabled int;
set @Enabled = (
  select top 1
    p.Enabled 
  from dbo.products p with (nolock)
  where p.name = @name
  );

select 
    msg = convert(varchar(32), case 
      when @Enabled is null 
        then 'Data does not exist'
      when @Enabled = 0
        then 'Data is disabled'
      when @Enabled = 1
        then 'Data exist in the table'
    end);

Answer 2

您可以使用SqlCommand.ExecuteScalar Method ()。它总是返回第一行的第一列，如果没有返回行，则返回null。

using (var connection = new SqlConnection(connectionString))
{
    string sql = "SELECT enabled FROM dbo.products WITH(nolock) WHERE name = @name";
    using (var command = new SqlCommand(sql, connection)) {
        command.Parameters.AddWithValue("name", name);
        command.CommandTimeout = 100;
        connection.Open();
        object enabled = command.ExecuteScalar();
        if (enabled == null) {
            ... // Product does not exist.
        } else if ((int)enabled == 0) {
            ... // Product is disabled.
        } else {
            ... // Product is enabled.
        }
    }
}

此外，using语句会自动关闭并处理连接和命令。

Answer 3

您可以更改如下：

        bool nameExists;

        connection statement
        {
            SqlCommand command = new SqlCommand();
            command.Connection = connection;
            command.CommandText =
                @"SELECT TOP 1 Enabled " +
                @"FROM [dbo].[products] p WITH (NOLOCK) " +
                @"WHERE [p].[name] = @name";
            command.Parameters.AddWithValue("name", name);
            command.CommandTimeout = 100;
            connection.Open();

            using (SqlDataReader reader = command.ExecuteReader())
            {
                nameExists = reader.HasRows;

                if (nameExists)
                {
                    reader.Read();
                    //bool Enabled = (reader.GetInt32(0) == 1);
                    bool Enabled = reader.GetBooean(0);
                    if (Enabled)
                        return "Data exist in the table";
                    else
                        return "Data is disabled.";
                }
                else
                {
                    throw new Exception ("Data not exist in the table");
                }
            }
            connection.Close();
        }

检查条件是否存在重复

3 个答案: