Dapper官方教程翻译3：Dapper方法之Query

Query方法描述

Query方法也是一个扩展方法，可以被IDbConnection对象调用，可以执行查询语句，并且映射到结果。

可以映射的类型：

• Anonymous（匿名类型 var）
• Strongly Typed（强类型）
• Multi-Mapping (One to One)（一对一映射）
• Multi-Mapping (One to Many)（一对多映射）
• Multi-Type（多类型）

Query方法可使用的参数

示例：Query执行匿名类型

string sql = \"SELECT TOP 10 * FROM OrderDetails\";

using (var connection = new SqlCeConnection(\"Data Source=SqlCe_W3Schools.sdf\"))
{	
        //使用var进行类型推断
	var orderDetail = connection.Query(sql).FirstOrDefault();
        
	FiddleHelper.WriteTable(orderDetail);
}

示例：Query执行强类型查询

string sql = \"SELECT TOP 10 * FROM OrderDetails\";

using (var connection = new SqlCeConnection(\"Data Source=SqlCe_W3Schools.sdf\"))
{			
	var orderDetails = connection.Query<OrderDetail>(sql).ToList();

	Console.WriteLine(orderDetails.Count);

	FiddleHelper.WriteTable(orderDetails);
}

示例：执行一对一关系查询

string sql = \"SELECT * FROM Invoice AS A INNER JOIN InvoiceDetail AS B ON A.InvoiceID = B.InvoiceID;\";

using (var connection = My.ConnectionFactory())
{
    connection.Open();

    var invoices = connection.Query<Invoice, InvoiceDetail, Invoice>(
            sql,
            (invoice, invoiceDetail) =>
            {
                invoice.InvoiceDetail = invoiceDetail;
                return invoice;
            },
            splitOn: \"InvoiceID\")
        .Distinct()
        .ToList();
}

示例：执行一对多关系查询

string sql = \"SELECT TOP 10 * FROM Orders AS A INNER JOIN OrderDetails AS B ON A.OrderID = B.OrderID;\";

using (var connection = new SqlCeConnection(\"Data Source=SqlCe_W3Schools.sdf\"))
{			
	var orderDictionary = new Dictionary<int, Order>();


	var list = connection.Query<Order, OrderDetail, Order>(
	sql,
	(order, orderDetail) =>
	{
		Order orderEntry;

		if (!orderDictionary.TryGetValue(order.OrderID, out orderEntry))
		{
		orderEntry = order;
		orderEntry.OrderDetails = new List<OrderDetail>();
		orderDictionary.Add(orderEntry.OrderID, orderEntry);
		}

		orderEntry.OrderDetails.Add(orderDetail);
		return orderEntry;
	},
	splitOn: \"OrderID\")
	.Distinct()
	.ToList();

	Console.WriteLine(list.Count);

	FiddleHelper.WriteTable(list);
	FiddleHelper.WriteTable(list.First().OrderDetails);
}

示例：Query执行多类型

string sql = \"SELECT * FROM Invoice;\";

using (var connection = My.ConnectionFactory())
{
    connection.Open();

    var invoices = new List<Invoice>();

    using (var reader = connection.ExecuteReader(sql))
    {
        var storeInvoiceParser = reader.GetRowParser<StoreInvoice>();
        var webInvoiceParser = reader.GetRowParser<WebInvoice>();

        while (reader.Read())
        {
            Invoice invoice;

            switch ((InvoiceKind) reader.GetInt32(reader.GetOrdinal(\"Kind\")))
            {
                case InvoiceKind.StoreInvoice:
                    invoice = storeInvoiceParser(reader);
                    break;
                case InvoiceKind.WebInvoice:
                    invoice = webInvoiceParser(reader);
                    break;
                default:
                    throw new Exception(ExceptionMessage.GeneralException);
            }

            invoices.Add(invoice);
        }
    }
    
    My.Result.Show(invoices);
}