如何加入最新的行从一个表?

Question

我经常遇到问题的这种形式，并没有发现了一个很好的解决方案尚未：

假设我们有两个数据库表代表一个电子商务系统。

userData (userId, name, ...)
orderData (orderId, userId, orderType, createDate, ...)

为所有用户的系统，选择他们的用户信息，他们最近的以信息与type='1'，而他们最近的以信息与type='2'.我想这样做在一个查询。这里是一个例子的结果:

(userId, name, ..., orderId1, orderType1, createDate1, ..., orderId2, orderType2, createDate2, ...)
(101, 'Bob', ..., 472, '1', '4/25/2008', ..., 382, '2', '3/2/2008', ...)

Answer 1

这应该有效，你必须调整表/列名称：

select ud.name,
       order1.order_id,
       order1.order_type,
       order1.create_date,
       order2.order_id,
       order2.order_type,
       order2.create_date
  from user_data ud,
       order_data order1,
       order_data order2
 where ud.user_id = order1.user_id
   and ud.user_id = order2.user_id
   and order1.order_id = (select max(order_id)
                            from order_data od1
                           where od1.user_id = ud.user_id
                             and od1.order_type = 'Type1')
   and order2.order_id = (select max(order_id)
                             from order_data od2
                            where od2.user_id = ud.user_id
                              and od2.order_type = 'Type2')

对数据进行非规范化也可能是一个好主意。这类事情的成本相当高。因此，您可以将 last_order_date 添加到userData。

Answer 2

我已经提供了三种不同的方法解决此问题：

1. 采用枢轴
2. 使用情况的陈述
3. 使用内联的查询其条款

所有的解决方案的假设，我们确定的"最近"为基础 orderId 列。使用 createDate 列会增加复杂性由于时间戳冲突和严重阻碍性能因为 createDate 可能是不属于索引的关键。我只是测试这些查询使用MS SQL Server2005年，所以我不知道如果他们会的工作在你的服务器。

解决方案(1)和(2)执行几乎完全相同。事实上，他们都致相同数量的读数据库。

解决方案(3) 不 优选方法，当工作与大数据集。它一贯使得数以百计的逻辑读取多(1)和(2)。当筛选的一个具体的用户，办法，(3)可比的其他方法。在单个用户的情况下，下降cpu时有助于对更多的读取；然而，如磁盘驱动器变得更加繁忙和缓存未发生，这种微弱优势将消失。

结论

对于所呈现的情况下，使用枢轴的方法，如果它是支持你的DBMS。它需要较少的代码比的情况下，声明和简化，添加了种类型的未来。

请注意，在某些情况下，枢轴是没有足够的灵活性和特性的价值功能的使用情况的陈述是的路要走。

代码

方法(1)使用枢轴:

select 
    ud.userId, ud.fullname, 
    od1.orderId as orderId1, od1.createDate as createDate1, od1.orderType as orderType1,
    od2.orderId as orderId2, od2.createDate as createDate2, od2.orderType as orderType2

from userData ud
    inner join (
            select userId, [1] as typeOne, [2] as typeTwo
            from (select
                userId, orderType, orderId
            from orderData) as orders
            PIVOT
            (
                max(orderId)
                FOR orderType in ([1], [2])
            ) as LatestOrders) as LatestOrders on
        LatestOrders.userId = ud.userId 
    inner join orderData od1 on
        od1.orderId = LatestOrders.typeOne
    inner join orderData od2 on
        od2.orderId = LatestOrders.typeTwo

方法(2)使用的情况下声明：

select 
    ud.userId, ud.fullname, 
    od1.orderId as orderId1, od1.createDate as createDate1, od1.orderType as orderType1,
    od2.orderId as orderId2, od2.createDate as createDate2, od2.orderType as orderType2

from userData ud 
    -- assuming not all users will have orders use outer join
    inner join (
            select 
                od.userId,
                -- can be null if no orders for type
                max (case when orderType = 1 
                        then ORDERID
                        else null
                        end) as maxTypeOneOrderId,

                -- can be null if no orders for type
                max (case when orderType = 2
                        then ORDERID 
                        else null
                        end) as maxTypeTwoOrderId
            from orderData od
            group by userId) as maxOrderKeys on
        maxOrderKeys.userId = ud.userId
    inner join orderData od1 on
        od1.ORDERID = maxTypeTwoOrderId
    inner join orderData od2 on
        OD2.ORDERID = maxTypeTwoOrderId

方法(3)使用内联的查询其条款(基于史蒂夫*K.的答复):

select  ud.userId,ud.fullname, 
        order1.orderId, order1.orderType, order1.createDate, 
        order2.orderId, order2.orderType, order2.createDate
  from userData ud,
       orderData order1,
       orderData order2
 where ud.userId = order1.userId
   and ud.userId = order2.userId
   and order1.orderId = (select max(orderId)
                            from orderData od1
                           where od1.userId = ud.userId
                             and od1.orderType = 1)
   and order2.orderId = (select max(orderId)
                             from orderData od2
                            where od2.userId = ud.userId
                              and od2.orderType = 2)

脚本生成的表格和1000个用户拥有100订单中的每个：

CREATE TABLE [dbo].[orderData](
    [orderId] [int] IDENTITY(1,1) NOT NULL,
    [createDate] [datetime] NOT NULL,
    [orderType] [tinyint] NOT NULL, 
    [userId] [int] NOT NULL
) 

CREATE TABLE [dbo].[userData](
    [userId] [int] IDENTITY(1,1) NOT NULL,
    [fullname] [nvarchar](50) NOT NULL
) 

-- Create 1000 users with 100 order each
declare @userId int
declare @usersAdded int
set @usersAdded = 0

while @usersAdded < 1000
begin
    insert into userData (fullname) values ('Mario' + ltrim(str(@usersAdded)))
    set @userId = @@identity

    declare @orderSetsAdded int
    set @orderSetsAdded = 0
    while @orderSetsAdded < 10
    begin
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-06-08', 1)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-02-08', 1)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-08-08', 1)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-09-08', 1)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-01-08', 1)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-06-06', 2)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-02-02', 2)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-08-09', 2)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-09-01', 2)
        insert into orderData (userId, createDate, orderType) 
            values ( @userId, '01-01-04', 2)

        set @orderSetsAdded = @orderSetsAdded + 1
    end
    set @usersAdded = @usersAdded + 1
end

小段进行测试的查询性能MS SQL服务器除了SQL探查:

-- Uncomment these to clear some caches
--DBCC DROPCLEANBUFFERS
--DBCC FREEPROCCACHE

set statistics io on
set statistics time on

-- INSERT TEST QUERY HERE

set statistics time off
set statistics io off

Answer 3

抱歉，我面前没有oracle，但这是我在oracle中所做的基本结构：

SELECT b.user_id, b.orderid, b.orderType, b.createDate, <etc>,
       a.name
FROM orderData b, userData a
WHERE a.userid = b.userid
AND (b.userid, b.orderType, b.createDate) IN (
  SELECT userid, orderType, max(createDate) 
  FROM orderData 
  WHERE orderType IN (1,2)
  GROUP BY userid, orderType) 

Answer 4

T-SQL示例解决方案（MS SQL）：

SELECT
    u.*
    , o1.*
    , o2.* 
FROM
(
    SELECT
        , userData.*
        , (SELECT TOP 1 orderId.url FROM orderData WHERE orderData.userId=userData.userId AND orderType=1 ORDER BY createDate DESC)
            AS order1Id
        , (SELECT TOP 1 orderId.url FROM orderData WHERE orderData.userId=userData.userId AND orderType=2 ORDER BY createDate DESC)
            AS order2Id
    FROM userData
) AS u
LEFT JOIN orderData o1 ON (u.order1Id=o1.orderId)
LEFT JOIN orderData o2 ON (u.order2Id=o2.orderId)

在SQL 2005中，您还可以使用RANK（）OVER函数。 （但AFAIK完全是MSSQL特有的功能）

Answer 5

您可以为此进行联合查询。确切的语法需要一些工作，特别是逐个部分，但联盟应该能够做到。

例如：

SELECT orderId, orderType, createDate
FROM orderData
WHERE type=1 AND MAX(createDate)
GROUP BY orderId, orderType, createDate

UNION

SELECT orderId, orderType, createDate
FROM orderData
WHERE type=2 AND MAX(createDate)
GROUP BY orderId, orderType, createDate

Answer 6

他们最新的你是指当天的全新内容吗？如果createDate＆gt; =当前日期，您可以随时查看您的createDate并获取所有用户和订单数据。

SELECT * FROM
"orderData", "userData"
WHERE
"userData"."userId"  ="orderData"."userId"
AND "orderData".createDate >= current_date;

<强>已更新

以下是您在评论后想要的内容：

SELECT * FROM
"orderData", "userData"
WHERE
"userData"."userId"  ="orderData"."userId"
AND "orderData".type = '1'
AND "orderData"."orderId" = (
SELECT "orderId" FROM "orderData"
WHERE 
"orderType" = '1'
ORDER "orderId" DESC
LIMIT 1

）

Answer 7

我在MySQL中使用这样的东西：

SELECT
   u.*,
   SUBSTRING_INDEX( MAX( CONCAT( o1.createDate, '##', o1.otherfield)), '##', -1) as o2_orderfield,
   SUBSTRING_INDEX( MAX( CONCAT( o2.createDate, '##', o2.otherfield)), '##', -1) as o2_orderfield
FROM
   userData as u
   LEFT JOIN orderData AS o1 ON (o1.userId=u.userId AND o1.orderType=1)
   LEFT JOIN orderData AS o2 ON (o1.userId=u.userId AND o2.orderType=2)
GROUP BY u.userId

简而言之，使用MAX（）来获得最新的，通过将条件字段（createDate）添加到感兴趣的字段（otherfield）。 SUBSTRING_INDEX（）然后删除日期。

OTOH，如果您需要任意数量的订单（如果userType可以是任何数字，而不是有限的ENUM）;处理单独的查询会更好，如下所示：

select * from orderData where userId=XXX order by orderType, date desc group by orderType

为每个用户。

Answer 8

假设orderId随时间单调增加：

SELECT *
FROM userData u
INNER JOIN orderData o
  ON o.userId = u.userId
INNER JOIN ( -- This subquery gives the last order of each type for each customer
  SELECT MAX(o2.orderId)
    --, o2.userId -- optional - include if joining for a particular customer
    --, o2.orderType -- optional - include if joining for a particular type
  FROM orderData o2
  GROUP BY o2.userId
    ,o2.orderType
) AS LastOrders
  ON LastOrders.orderId = o.orderId -- expand join to include customer or type if desired

然后在客户端转动，或者如果使用SQL Server，则有一个PIVOT功能

Answer 9

以下是将类型1和2数据移动到同一行的一种方法：
（通过将类型1和类型2信息放入它们自己的选择中，然后在from子句中使用。）

SELECT
  a.name, ud1.*, ud2.*
FROM
    userData a,
    (SELECT user_id, orderid, orderType, reateDate, <etc>,
    FROM orderData b
    WHERE (userid, orderType, createDate) IN (
      SELECT userid, orderType, max(createDate) 
      FROM orderData 
      WHERE orderType = 1
      GROUP BY userid, orderType) ud1,
    (SELECT user_id, orderid, orderType, createDate, <etc>,
    FROM orderData 
    WHERE (userid, orderType, createDate) IN (
      SELECT userid, orderType, max(createDate) 
      FROM orderData 
      WHERE orderType = 2
      GROUP BY userid, orderType) ud2

Answer 10

我是这样做的。这是标准SQL，适用于任何品牌的数据库。

SELECT u.userId, u.name, o1.orderId, o1.orderType, o1.createDate,
  o2.orderId, o2.orderType, o2.createDate
FROM userData AS u
  LEFT OUTER JOIN (
    SELECT o1a.orderId, o1a.userId, o1a.orderType, o1a.createDate
    FROM orderData AS o1a 
      LEFT OUTER JOIN orderData AS o1b ON (o1a.userId = o1b.userId 
        AND o1a.orderType = o1b.orderType AND o1a.createDate < o1b.createDate)
    WHERE o1a.orderType = 1 AND o1b.orderId IS NULL) AS o1 ON (u.userId = o1.userId)
  LEFT OUTER JOIN (
    SELECT o2a.orderId, o2a.userId, o2a.orderType, o2a.createDate
    FROM orderData AS o2a 
      LEFT OUTER JOIN orderData AS o2b ON (o2a.userId = o2b.userId 
        AND o2a.orderType = o2b.orderType AND o2a.createDate < o2b.createDate)
    WHERE o2a.orderType = 2 AND o2b.orderId IS NULL) o2 ON (u.userId = o2.userId);

请注意，如果您有多个日期等于最新日期的类型的订单，您将在结果集中获得多行。如果您有两种类型的多个订单，您将在结果集中获得N x M行。所以我建议你在不同的查询中获取每种类型的行。

Answer 11

史蒂夫K绝对是对的，谢谢！我确实重写了他的答案，以解释某个特定类型可能没有订单的事实（我没有提及，所以我不能错过Steve K。）

以下是我最终使用的内容：

select ud.name,
       order1.orderId,
       order1.orderType,
       order1.createDate,
       order2.orderId,
       order2.orderType,
       order2.createDate
  from userData ud
  left join orderData order1
   on order1.orderId = (select max(orderId)
                            from orderData od1
                           where od1.userId = ud.userId
                             and od1.orderType = '1')
  left join orderData order2
   on order2.orderId = (select max(orderId)
                            from orderData od2
                           where od2.userId = ud.userId
                             and od2.orderType = '2')
 where ...[some limiting factors on the selection of users]...;