SqlServer技巧：使用APPLY替代游标解决逐行运算问题

--数据准备
--表值函数 dbo.SplitString：按照分隔符将文本转换为表
declare @str1 varchar(max)
set @str1 = ‘7,8,9‘
SELECT * FROM dbo.SplitString(@str1,‘,‘,1)

--示例：某表有Col1,Col2两列，想将Col2中的文本拆分为多行，并在拆分后保留Col1和Col2的对应关系
CREATE TABLE #TMP(
NAME VARCHAR(100) NULL,
STRING VARCHAR(MAX) NULL
)
TRUNCATE TABLE #TMP
INSERT INTO #TMP(NAME, STRING)
VALUES(‘TEST111‘,‘2,3,4‘),
(‘TEST222‘,‘6,7‘),
(‘TEST222‘,null)

--根据某列一行变多行，并保留其他列的对应关系
SELECT C.NAME, A.Value
FROM #TMP C
CROSS APPLY dbo.SplitString(C.STRING,‘,‘,1) A --去除NULL列

SELECT C.NAME, A.Value
FROM #TMP C
OUTER APPLY dbo.SplitString(C.STRING,‘,‘,1) A --保留NULL列

--每个客户的前三笔交易
--方法1：用ROW_NUMBER()
SELECT C.custid, C.companyname, A.orderid, A.orderdate
FROM Sales.Customers C
LEFT JOIN(
SELECT ROW_NUMBER()OVER(PARTITION BY custid ORDER BY orderdate DESC, orderid DESC) AS RN
, custid, orderid, empid, orderdate, requireddate
FROM Sales.Orders
) A ON C.custid=A.custid
WHERE A.RN <=3
ORDER BY custid, RN

--方法2：用APPLY + TOP
SELECT C.custid, C.companyname, A.orderid, A.orderdate
FROM Sales.Customers C
CROSS APPLY(
SELECT TOP 3 orderid, empid, orderdate, requireddate
FROM Sales.Orders O
WHERE O.custid=C.custid
ORDER BY orderdate DESC, orderid DESC) A
ORDER BY C.custid