如何将多个 SQL 数据列存储到不同的变量 C#

Question

我正在尝试将用于凭证 ID 和凭证金额的 sql 数据存储到一个变量中，并通过单击按钮将其显示到标签中。

protected void Button1_Click(object sender, EventArgs e)
{
    string voucherId = String.Empty;
    string voucherAmount = String.Empty;

    string queryVoucherId = "select voucherid from ReturnForm where email = '" + Session["username"] + "';";
    string queryVoucherAmount = "select voucheramount from ReturnForm where email = '" + Session["username"] + "';";

    int index = 0;

    using (SqlConnection con = new SqlConnection(str))
    {
        SqlCommand cmd = new SqlCommand(queryVoucherId, con);

        con.Open();

        SqlDataReader reader = cmd.ExecuteReader();

        while (reader.Read())
        {
            voucherId = reader[index].ToString();
            index++;
        }
    }

    using (SqlConnection con = new SqlConnection(str))
    {
        SqlCommand cmd = new SqlCommand(queryVoucherAmount, con);
        con.Open();
        SqlDataReader reader = cmd.ExecuteReader();

        while (reader.Read())
        {
            voucherAmount = reader[index].ToString();
            index++;
        }
    }

    if (txtVoucher.Text == voucherId)
    {
        Label3.Visible = true;
        Label3.Text = voucherAmount;
    }
}

当我点击按钮时，它给我一个错误，提示索引超出范围。

Answer 1

以@JSGarcia 的回答为基础——但使用参数作为一个总是应该——你会得到这个代码：

string email = Session['username'];
string query = $"SELECT voucherid, voucheramount FROM ReturnFrom WHERE Email = @email";

DataTable dt = new DataTable();

using (SqlConnection conn = new SqlConnection(connectionString))
using (SqlCommand cmd = new SqlCommand(query, conn))
using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
{
    // set the parameter before opening connection
    // this also defines the type and length of parameter - just a guess here, might need to change this
    cmd.Parameters.Add("@email", SqlDbType.VarChar, 100).Value = email;

    conn.Open();
    sda.Fill(dt);
    conn.Close();
}

就个人而言，我宁愿使用类似的数据类

public class VoucherData
{
    public int Id { get; set; }
    public Decimal Amount { get; set; }
}

然后从您的 SQL 查询中取回 List<VoucherData>（使用例如 Dapper）：

string query = $"SELECT Id, Amount FROM ReturnFrom WHERE Email = @email";

List<VoucherData> vouchers = conn.Query<VoucherData>(query).ToList();

我会尽量避免使用相当笨重且不太容易使用的DataTable 构造...

Answer 2

我强烈建议将您的 sql 查询合并为一个，将其写入数据表并从那里继续您的逻辑。恕我直言，代码更简洁：

string email = Session['username'];
string query = $"SELECT voucherid, voucheramount FROM ReturnFrom where Email = '{email}'";

DataTable dt = new DataTable();

using (SqlConnection conn = new SqlConnection(connectionString))
using (SqlCommand cmd = conn.CreateCommand())
using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
{
    cmd.CommandText = query;
    cmd.CommandType = CommandType.Text;
    conn.Open();
    sda.Fill(dt);
    conn.Close();
}

// Work with DataTable dt from here on
...

Answer 3

好吧，还有一个大提示？ 如果您要更新数据表，您通常只需要一个数据适配器。

如果你说不使用 sql 命令对象，你只需要一个新的连接对象。

sqlcommand 对象有：

a connection object - no need to create a separate one
a reader - no need to create a separate one.

请注意，我没有创建单独的连接对象，而是使用了命令对象中内置的连接对象。

既然参数在两种情况下都是相同的？那为什么不重用它呢！！

所以，我们得到了这个：

    void TestFun2()
    {
        String str = "some conneciton???";

        DataTable rstVouch = new DataTable();
        using (SqlCommand cmdSQL =
              new SqlCommand("select voucherid from ReturnForm where email = @email",
              new SqlConnection(str)))
        {
            cmdSQL.Parameters.Add("@email", SqlDbType.NVarChar).Value = Session["username"];
            cmdSQL.Connection.Open();

            rstVouch.Load(cmdSQL.ExecuteReader());

            // now get vouch amount
            cmdSQL.CommandText = "select voucheramount from ReturnForm where email = @email";

            DataTable rstVouchAmount = new DataTable();
            rstVouchAmount.Load(cmdSQL.ExecuteReader());

            if (rstVouch.Rows[0]["vourcherid"].ToString() == txtVoucher.Text)
            {
                Label3.Visible = true;
                Label3.Text = rstVouchAmount.Rows[0]["voucheramount"].ToString();
            }
        }
    }