查询语句在 SQL Server 中有效，但在 C# 中无效

Question

我有这个问题。

SELECT DISTINCT 
    nome as Nome, Cognome As Cognome, DataConsegna as Data, 
    'RichiesteAttrezzatura' [Tipo Richiesta] 
FROM 
    RichiesteAttrezzatura 

UNION 

SELECT DISTINCT 
    nome AS Nome, Cognome as Cognome, DataConsegna as Data, 
    'RichiesteMateriali' [Tipo Richiesta] 
FROM 
    RichiesteMateriali 

UNION 

SELECT DISTINCT 
    nome AS Nome, Cognome as Cognome, giorno AS Data, 
    'RichiesteVestiario' [Tipo Richiesta] 
FROM 
    RichiesteVestiario

如标题所示，问题是当我执行它时它在 SQL Server 中工作，但是当我在 C# 中尝试时它只返回一个空表。

根据我在网上和这个论坛上的理解，问题出在单引号上。

这是我的功能

adonet db;

DataProvider()
{ 
    db = new adonet();
    db.CreaConnessione();
}

public DataTable getRichiesteForDGV(int id)
{
    DataTable dt = new DataTable();

    string richiesteVestiario = "RichiesteVestiario";
    string richiesteMateriali = "RichiesteMateriali";
    string richiesteAttrezzatura = "RichiesteAttrezzatura";

    string sql = "SELECT DISTINCT nome as Nome, Cognome As Cognome, DataConsegna as Data, '" + richiesteAttrezzatura + "' [Tipo Richiesta] FROM RichiesteAttrezzatura " +
                "UNION SELECT DISTINCT nome AS Nome, Cognome as Cognome, DataConsegna as Data, '" + richiesteMateriali + "' [Tipo Richiesta] FROM RichiesteMateriali " +
                "UNION SELECT DISTINCT nome AS Nome, Cognome as Cognome, giorno AS Data, '" + richiesteVestiario + "' [Tipo Richiesta] FROM RichiesteVestiario";

    db.EseguiQueryWithParams(sql, new SqlParameter("@id", id));

    return dt;
}

public DataTable EseguiQueryWithParams(string sql, params SqlParameter[] parameters)
    {
        ApriConnessione();
        string nometab = "dump";
        cmd.CommandText = sql;

        cmd.CommandType = System.Data.CommandType.Text;
        cmd.Parameters.AddRange(parameters);
        try
        {
            adp = new SqlDataAdapter(cmd);
            if (dset.Tables[nometab] != null)
                dset.Tables[nometab].Clear();

            adp.Fill(dset, nometab);
            DataTable d = dset.Tables[nometab];
            //d.Columns;
            return d;
        }
        catch (Exception e)
        {
            throw e;
            //throw new System.Exception("Errore nella lettura della tabella");

        }
        finally
        {
            adp.Dispose();
            cmd.Dispose();
            ChiudiConnessione();
        }

    }

感谢任何帮助。我已经更新了帖子，因为我知道问题出在我的 C# 上

Answer 1

首先，我要花一点时间建议对 EseguiQueryWithParams() 进行一些更改，这些更改太大而无法作为评论。

对于初学者来说，它处理连接的方式is not good practice. 阅读链接，直到你理解为止。相反，您需要一个通用方法或属性来告诉您连接字符串，仅此而已。对方法的其他更改在代码中应该都可以理解：

private DataTable EseguiQuery(string sql, params SqlParameter[] parameters)
{
    // The method should only supply the string. 
    // DO NOT TRY TO REUSE THE SAME CONNECTION OBJECT!
    string connectionString = Connessione();
    var dset = new DataSet(); //creating a new dataset for this query also means there's no need to check and clear it further down

    //Using blocks guarantee the items are disposed, even if an exception is thrown
    using (var connection = new SqlConnection(connectionString))
    using (var cmd = new SqlCommand(sql, connection))
    using (var adp = new SqlDataAdapater(cmd))
    {
        cmd.CommandText = sql; //cmd.CommandType already has "Text" as the default value
        if (parameters is object && parameters.Length > 0)
        {   //conditional test lets us skip the parameters argument when calling the method
            cmd.Parameters.AddRange(parameters);
        }
        adp.Fill(dset); // Fill() opens and closes the connection automatically
        return dset.Tables[0]; 
    }
} // No try/catch anywhere, because the old try/catch didn't do anything

看看它的代码少了多少（17 行对 31 行，清理 cmets 之后），但它完成了所有相同的事情。甚至打开和关闭连接都被覆盖了，即使在哪里也不太明显。此外，它还允许您跳过传递参数，如下所示：

var result = db.EseguiQuery(sql)

另一个重要的变化是使这个方法私有。对其他 Execute/Esegui 方法执行相同操作。 getRichiesteForDGV() 以及与 DB 对话的其他方法也应该是由 db 变量表示的类型的（公共）成员。通过这种方式，您可以很好地分离数据访问和用户界面之间的关注点。如果不使用该类型的公共成员，您的应用程序就无法访问数据库。

最后，我删除了名称中的 WithParams 部分，因为我认为提供不期望它们的重载是不好的做法。参数非常重要，您希望向希望使用它们的开发人员清楚地传达信息。而且由于表名并不重要，您可以使用默认的Text CommandType 调用存储过程，方法是在它们前面加上exec，这可以完全取代旧的ExecuteQuery() 方法。

---

解决了所有这些问题，我们终于可以解决实际问题了。 EseguiQueryWithParams() 返回一个 DataTable，但这个返回值被忽略。它从未与在getRichiesteForDGV() 中声明和返回的dt 变量相关联。修复它，连同我建议的更改，如下所示：

public DataTable getRichiesteForDGV(int id)
{
    string richiesteVestiario = "RichiesteVestiario";
    string richiesteMateriali = "RichiesteMateriali";
    string richiesteAttrezzatura = "RichiesteAttrezzatura";

    string sql = $"SELECT DISTINCT nome as Nome, Cognome As Cognome, DataConsegna as Data, '{richiesteAttrezzatura}' [Tipo Richiesta] FROM RichiesteAttrezzatura " +
                "UNION SELECT DISTINCT nome AS Nome, Cognome as Cognome, DataConsegna as Data, '{richiesteMateriali}' [Tipo Richiesta] FROM RichiesteMateriali " +
                "UNION SELECT DISTINCT nome AS Nome, Cognome as Cognome, giorno AS Data, '{richiesteVestiario}' [Tipo Richiesta] FROM RichiesteVestiario";

    // this should be a member of the `db` class now, so "db." is implied
    return EseguiQuery(sql);
}