众所周知,TOP子句可以通过控制返回行的数量来影响查询。
我们知道TOP子句能很容易的满足返回指定行数的子集,接下来有一些例子来展示什么情况下使用TOP子句来返回一个结果集;
为了去解释TOP子句的如何工作,我将列举几个实例,使你能够更容易理解并观察使用TOP子句的影响返回值得细微差别。
TOP 的语法
语法很简单,可以将TOP加在任何如 SELECT、DELETE, INSERT, or UPDATE 的语句中:
TOP (expression) [PERCENT]
[WITH TIES]
“expression”的值是一个数字,,如果PERCENT的可选项被启用则数字将被转换成一个float 类型,否则姜维BIGINT类型。可以指定数字也可以使用局部变量。
可选项WITH TIES ,用来包含具有系统值得数据,需要注意的是该选项支队带有Order by的子句有效。
现有数据:
-- 找到两个SalesAgent 根据SalesAmount倒序。先排序在选出前两个,如果没有Order by 则此数据会随即返回(没有主键)
SELECT TOP(2) SalesAgent, Region, SalesAmount
FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
运行结果:
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
Nick Potts East 9834212.87
Mary Johnson West 8723412.61
If you want to return a percentage of the top records in a set then you need to use the TOP clause with the PERCENT option. To demonstrate using the PERCENT option look at Listing 3.
-- 查询前百分之50的数据,按照SalesAmount
SELECT TOP(50) PERCENT SalesAgent, Region, SalesAmount
FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
查询结果:
SalesAgent Region SalesAmount
----------------------------- ---------- ---------------------------
Nick Potts East 9834212.87
Mary Johnson West 8723412.61
Sam Holder East 8723412.61
Stan Morris East 4562341.67
Lori Morin East 2000111.67
--不带百分比的查询
DECLARE @Number INT = 2;
SELECT TOP(@Number) SalesAgent, Region, SalesAmount
FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
-- 带百分比的
SET @Number = 50;
SELECT TOP(@Number) PERCENT SalesAgent, Region, SalesAmount
FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
按照值进行排序,如果有相同的则一并显示出来
-- 找出SalesAmount 最大的前两个。实际是3个
SELECT TOP(2) WITH TIES SalesAgent, Region, SalesAmount
FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
查询结果:
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
Nick Potts East 9834212.87
Mary Johnson West 8723412.61
Sam Holder East 8723412.61
正如我们看到的,返回了3行而不是2行,因为第三行和第二行的值是相同的。
如何使用TOP子句限制更新的行数,如下:
UPDATE TOP (2) dbo.HectorSales
SET SalesAmount = 100000.00
FROM dbo.HectorSales
WHERE Region = 'West'
更新后查询Region = 'West'的结果
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
John Smith West 100000.00
Mary Johnson West 100000.00
Doris Bean West 2000111.67
Martin Derrick West 120834.81
Don Olson West 508921.48
当然也可以使用其他不同的方式来更新,先查询出前2的数据,然后将符合前2的数据进行更新,如下:
UPDATE dbo.HectorSales
SET SalesAmount = 6666666.00
FROM
(SELECT TOP(2) SalesAgent FROM dbo.HectorSales
WHERE Region = 'West'
ORDER BY SalesAmount DESC) TS
WHERE dbo.HectorSales.SalesAgent = TS.SalesAgent
更新后的结果:
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
John Smith West 100000.00
Mary Johnson West 100000.00
Doris Bean West 6666666.00
Martin Derrick West 120834.81
Don Olson West 6666666.00
例如我打算将SalesAmount 最多的两个SalesAgent 插入到表dbo.TopTwoSales 的agent 里面。
INSERT TOP(2) INTO dbo.TopTwoHectorSales
SELECT * FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
SELECT * FROM dbo.TopTwoHectorSales;
结果集如下:
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
John Smith West 100000.00
Mary Johnson West 100000.00
通过查询结果我们发现插入的两行,并不是SalesAmount 最大的两行,因为我将TOP放在了Insert 后面,SQLServer 认为从子结果集中的前两行,这样的话实际上子结果集是随即的。为了纠正之前的问题,我这样写:
INSERT INTO dbo.TopTwoHectorSales
SELECT TOP(2) * FROM dbo.HectorSales
ORDER BY SalesAmount DESC;
SELECT * FROM dbo.TopTwoHectorSales;
改正后的结果:
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
Nick Potts East 9834212.87
Sam Holder East 8723412.61
BEGIN TRANSACTION;
DELETE FROM dbo.HectorSales
WHERE SalesAgent in (SELECT TOP(2) SalesAgent FROM dbo.HectorSales
ORDER by SalesAmount ASC);
SELECT * FROM dbo.HectorSales;
ROLLBACK;
结果如下:
SalesAgent Region SalesAmount
------------------------------ ---------- --------------------------
John Smith West 100000.00
Doris Bean West 6666666.00
Martin Derrick West 120834.81
Don Olson West 6666666.00
Sam Holder East 8723412.61
Nick Potts East 9834212.87
Lori Morin East 2000111.67
Stan Morris East 4562341.67
Report 12: Rows inserted into dbo.TopTwoHectorSales table using ORDER BY
当然如果想包含相同的值,使用WITH TIES
DELETE FROM dbo.HectorSales
WHERE SalesAgent in (SELECT TOP(2) WITH TIES SalesAgent FROM dbo.HectorSales
ORDER by SalesAmount ASC);
SELECT * FROM dbo.HectorSales;
与上一个相比,这个代码将会多删除掉一个SalesAmount 1000000.00数据
创建一个表,插入初始数据。
CREATE TABLE dbo.Sales (
AgentName varchar(30),
Region varchar(10),
SalesAmount decimal(10,2));
INSERT INTO dbo.Sales VALUES
('John Smith', 'West',1012302.01),
('Mary Johnson', 'West',2453202.89),
('Doris Bean', 'West',99001.43),
('Sam Holder', 'East',8723412.61),
('Nick Potts', 'East',9834212.87),
('Jason Thomas', 'East',13424.51);
将东西部最高的SalesAmount 筛选出来并联
SELECT AgentName, Region, SalesAmount FROM
(
SELECT TOP(1) AgentName, Region, SalesAmount
FROM dbo.Sales
WHERE Region = 'East'
ORDER BY SalesAmount DESC ) East
UNION
SELECT AgentName, Region, SalesAmount FROM
(
SELECT TOP(1) AgentName, Region, SalesAmount
FROM dbo.Sales
WHERE Region = 'West'
ORDER BY SalesAmount DESC ) West
ORDER BY SalesAmount DESC;
TOP关键字,让我们可以返回指定行数的数据,也能指定行数或者百分比的数据。为了确保结果集的一致性,一定要保证使用ORDER BY,假如你没有使用则将返回随机的指定行数数据。