Question

我正在使用带有C＃的PostgreSQL数据库和Npgsql库。

现在我可以选择表格中的最后一行，但我无法弄清楚如何为它分配C＃变量。我知道我的选择有效，因为我之前已成功编辑了我的最后一个条目。

您可以在下面找到我的代码。请注意，我没有粘贴其他方法，因为我认为它们无关紧要。

public void myMethod()
{
    this.OpenConn(); //opens the connection

    string sql = "SELECT id FROM information_schema.tables WHERE table_schema = 'public' AND table_name = 'customers' ORDER BY id DESC, LIMIT 1";

    using (NpgsqlCommand command = new NpgsqlCommand(sql, conn))
    {
        int id = 0; //instead of '0' I want it to be equal to the ID value from the row
        //something like "int id = sqlSelection.id;" -- this obviously doesn't work

        this.CloseConn(); //close the current connection
    }
}

Answer 1

您可以使用特定的 DataReader ：

来实现此目标

public void myMethod()
{
    this.OpenConn(); //opens the connection

    string sql = "SELECT id FROM information_schema.tables WHERE table_schema = 'public' AND table_name = 'customers' ORDER BY id DESC, LIMIT 1";

    using (NpgsqlCommand command = new NpgsqlCommand(sql, conn))
    {
        int val;
        NpgsqlDataReader reader = command.ExecuteReader();
        while(reader.Read()){
           val = Int32.Parse(reader[0].ToString());
           //do whatever you like
        }

        this.CloseConn(); //close the current connection
    }
}

有用的笔记

在某些情况下， ExecuteScalar 是一个不错的选择
Npgsql documentation

Answer 2

使用也可以使用以下代码变体;

 using (var command = new NpgsqlCommand(sql, conn))
 {
        int id = 0; 
        var reader = command.ExecuteReader();
        while(reader.Read())
        { 
           var id = Int32.Parse(reader["id"].ToString());
        }
        this.CloseConn(); 
 }

Answer 3

您可以使用ExecuteScalarSync方法。

public void myMethod()
{
    this.OpenConn(); //opens the connection

    string sql = "SELECT id FROM information_schema.tables WHERE table_schema = 'public' AND table_name = 'customers' ORDER BY id DESC, LIMIT 1";

    using (NpgsqlCommand command = new NpgsqlCommand(sql, conn))
     {
       int id= (int)DBHelperRepository.ExecuteScalarSync(sqlString, CommandType.Text);

       this.CloseConn(); //close the current connection
     }
}

如何从C＃中的PostgreSQL中获取所选行的值？

3 个答案: