我将使用一个具体的但假设的示例。
每个订单通常只有一个订单项 :
命令:
OrderGUID OrderNumber
========= ============
{FFB2...} STL-7442-1
{3EC6...} MPT-9931-8A
LineItems:
LineItemGUID Order ID Quantity Description
============ ======== ======== =================================
{098FBE3...} 1 7 prefabulated amulite
{1609B09...} 2 32 spurving bearing
但偶尔会有一个包含两个订单项的订单:
LineItemID Order ID Quantity Description
========== ======== ======== =================================
{A58A1...} 6,784,329 5 pentametric fan
{0E9BC...} 6,784,329 5 differential girdlespring
通常在向用户显示订单时:
SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
INNER JOIN LineItems
ON Orders.OrderID = LineItems.OrderID
我想在订单上显示单个项目。 但是,如果这个偶然的订单包含两个(或多个)项目,这些订单将看起来是重复的 :
OrderNumber Quantity Description
=========== ======== ====================
STL-7442-1 7 prefabulated amulite
MPT-9931-8A 32 spurving bearing
KSG-0619-81 5 panametric fan
KSG-0619-81 5 differential girdlespring
我真正想要的是让SQL Server 只选择一个 ,因为这样就足够了 :
OrderNumber Quantity Description
=========== ======== ====================
STL-7442-1 7 prefabulated amulite
MPT-9931-8A 32 differential girdlespring
KSG-0619-81 5 panametric fan
如果我喜欢冒险,可以向用户显示一个省略号,以表明有多个:
OrderNumber Quantity Description
=========== ======== ====================
STL-7442-1 7 prefabulated amulite
MPT-9931-8A 32 differential girdlespring
KSG-0619-81 5 panametric fan, ...
所以问题是如何
- 消除“重复”行
- 仅连接到其中一行,以避免重复
第一次尝试
我的第一个天真的尝试是仅加入“ TOP 1 ”行项目:
SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
INNER JOIN (
SELECT TOP 1 LineItems.Quantity, LineItems.Description
FROM LineItems
WHERE LineItems.OrderID = Orders.OrderID) LineItems2
ON 1=1
但这给出了错误:
列或前缀“订单”不
与表名或别名匹配
在查询中使用。
大概是因为内部选择看不到外部表。
#1楼
我知道这个问题是在不久前回答的,但是当处理大型数据集时,嵌套查询的成本可能很高。 这是另一种解决方案,其中嵌套查询将只运行一次,而不是针对返回的每一行。
SELECT
Orders.OrderNumber,
LineItems.Quantity,
LineItems.Description
FROM
Orders
INNER JOIN (
SELECT
Orders.OrderNumber,
Max(LineItem.LineItemID) AS LineItemID
FROM
Orders INNER JOIN LineItems
ON Orders.OrderNumber = LineItems.OrderNumber
GROUP BY Orders.OrderNumber
) AS Items ON Orders.OrderNumber = Items.OrderNumber
INNER JOIN LineItems
ON Items.LineItemID = LineItems.LineItemID
#2楼
尝试了十字架,效果很好,但需要更长的时间。 调整行列以具有最大数量并添加组,以保持速度并删除多余的记录。
这是调整后的查询:
SELECT Orders.OrderNumber, max(LineItems.Quantity), max(LineItems.Description)
FROM Orders
INNER JOIN LineItems
ON Orders.OrderID = LineItems.OrderID
Group by Orders.OrderNumber
#3楼
SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
JOIN LineItems
ON LineItems.LineItemGUID =
(
SELECT TOP 1 LineItemGUID
FROM LineItems
WHERE OrderID = Orders.OrderID
)
在SQL Server 2005
及更高版本中,您可以将INNER JOIN
替换为CROSS APPLY
:
SELECT Orders.OrderNumber, LineItems2.Quantity, LineItems2.Description
FROM Orders
CROSS APPLY
(
SELECT TOP 1 LineItems.Quantity, LineItems.Description
FROM LineItems
WHERE LineItems.OrderID = Orders.OrderID
) LineItems2
请注意,没有ORDER BY
TOP 1
不是确定性的:此查询为每个订单提供一个订单项,但未定义为哪个订单项。
即使基础没有变化,多次调用查询也可以为同一订单提供不同的订单项。
如果要确定顺序,则应在最里面的查询中添加ORDER BY
子句。
#4楼
您可以这样做:
SELECT
Orders.OrderNumber,
LineItems.Quantity,
LineItems.Description
FROM
Orders INNER JOIN LineItems
ON Orders.OrderID = LineItems.OrderID
WHERE
LineItems.LineItemID = (
SELECT MIN(LineItemID)
FROM LineItems
WHERE OrderID = Orders.OrderID
)
这需要对一个索引(或主键) LineItems.LineItemID
和索引LineItems.OrderID
或这将是缓慢的。
#5楼
编辑:没关系,夸斯诺伊有一个更好的答案。
对于SQL2K,如下所示:
SELECT
Orders.OrderNumber
, LineItems.Quantity
, LineItems.Description
FROM (
SELECT
Orders.OrderID
, Orders.OrderNumber
, FirstLineItemID = (
SELECT TOP 1 LineItemID
FROM LineItems
WHERE LineItems.OrderID = Orders.OrderID
ORDER BY LineItemID -- or whatever else
)
FROM Orders
) Orders
JOIN LineItems
ON LineItems.OrderID = Orders.OrderID
AND LineItems.LineItemID = Orders.FirstLineItemID
#6楼
@Quassnoi的答案很好,在某些情况下(特别是如果外部表很大),使用窗口函数可能是更有效的查询,如下所示:
SELECT Orders.OrderNumber, LineItems2.Quantity, LineItems2.Description
FROM Orders
LEFT JOIN
(
SELECT LineItems.Quantity, LineItems.Description, OrderId, ROW_NUMBER()
OVER (PARTITION BY OrderId ORDER BY (SELECT NULL)) AS RowNum
FROM LineItems
) LineItems2 ON LineItems2.OrderId = Orders.OrderID And RowNum = 1
有时您只需要测试哪个查询可以提供更好的性能。
#7楼
我通过使用LEFT JOIN和GROUP BY Orders.OrderNumber解决了类似的问题。 有没有理由不这样做呢?
SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
LEFT JOIN LineItems
ON Orders.OrderID = LineItems.OrderID
GROUP BY Orders.OrderNumber
我将用您自己的问题的答案回答您的回答问题:
Orders LineItems
+-------------+ +---------+----------+---------------+
| OrderNumber | | OrderID | Quantity | Description |
+-------------+ +---------+----------+---------------+
| 22586 | | 22586 | 17 | Trunion |
+-------------+ | 22586 | 3 | Girdle Spring |
+---------+----------+---------------+
将两者在OrderNumber上结合在一起可以得到:
OrderNumber Quantity Description
----------- -------- -------------
22586 17 Trunion
22586 3 Girdle Spring
2 row(s) affected
我们希望它只返回一行的地方:
OrderNumber Quantity Description
----------- -------- -------------
22586 17 Trunion
1 row(s) affected
这就是为什么我使用GROUP BY Orders.OrderNumber的原因,每个OrderNumber只返回一行。
#8楼
关联子查询是依赖于外部查询的子查询。 就像SQL中的for循环一样。 子查询将为外部查询中的每一行运行一次:
select * from users join widgets on widgets.id = (
select id from widgets
where widgets.user_id = users.id
order by created_at desc
limit 1
)
#9楼
我最喜欢的运行此查询的方式是使用不存在子句。 我相信这是运行这种查询的最有效方法:
select o.OrderNumber,
li.Quantity,
li.Description
from Orders as o
inner join LineItems as li
on li.OrderID = o.OrderID
where not exists (
select 1
from LineItems as li_later
where li_later.OrderID = o.OrderID
and li_later.LineItemGUID > li.LineItemGUID
)
但是我没有针对这里建议的其他方法测试过这种方法。
#10楼
,使用通用表表达式的另一个方法:
with firstOnly as (
select Orders.OrderNumber, LineItems.Quantity, LineItems.Description, ROW_NUMBER() over (partiton by Orders.OrderID order by Orders.OrderID) lp
FROM Orders
join LineItems on Orders.OrderID = LineItems.OrderID
) select *
from firstOnly
where lp = 1
或者,最后您是否想显示所有连接的行?
逗号分隔版本:
select *
from Orders o
cross apply (
select CAST((select l.Description + ','
from LineItems l
where l.OrderID = s.OrderID
for xml path('')) as nvarchar(max)) l
) lines
#11楼
从SQL Server 2012起,我认为这可以解决问题:
SELECT DISTINCT
o.OrderNumber ,
FIRST_VALUE(li.Quantity) OVER ( PARTITION BY o.OrderNumber ORDER BY li.Description ) AS Quantity ,
FIRST_VALUE(li.Description) OVER ( PARTITION BY o.OrderNumber ORDER BY li.Description ) AS Description
FROM Orders AS o
INNER JOIN LineItems AS li ON o.OrderID = li.OrderID