c# - Access 类型不匹配

Question

有两个表:

MyTable1:
  id       AutoNumber
  typecode Text

MyTable2
  id      AutoNumber
  pid     Number
  freq    Number

using System.Data.OleDb;
namespace AccessSelect
{
    class Program
    {
        static void Main(string[] args)
        {
            var sql = @"select 'x' from mytable1 where typecode=@typeCode and EXISTS (
                        select 'x' from mytable2 where (freq=0 OR freq=@freq) and mytable1.id=mytable2.pid)";

            using (OleDbConnection conn = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=gpi.mdb;Jet OLEDB:Database Password=???"))
            {
                conn.Open();
                OleDbCommand cmd = new OleDbCommand(sql, conn);

                cmd.Parameters.Add(new OleDbParameter("@typeCode", "KK3000"));
                cmd.Parameters.Add(new OleDbParameter("@freq", 50));

                var o = cmd.ExecuteScalar();
            }
        }
    }
}

我不断收到“标准表达式中的数据类型不匹配”异常。

如果我更改 SQL 以包含值:

select 'x' from mytable1 where typecode='KK3000' and EXISTS (
 select 'x' from mytable2 where (freq=0 OR freq=50) and  mytable1.id=mytable2.pid)

我没有得到错误......

知道哪里出了问题吗？

Answer 1

来自 MSDN :

The OLE DB .NET Provider does not support named parameters for passing parameters to an SQL statement or a stored procedure called by an OleDbCommand when CommandType is set to Text. In this case, the question mark (?) placeholder must be used. For example:

因此将您的查询更改为:

select 'x' from mytable1 where typecode = ? 
     and EXISTS (select 'x' from mytable2 where (freq=0 OR freq = ?) and mytable1.id=mytable2.pid)

并且您必须按照它们在查询中出现的相同顺序添加参数。