容易遗忘的一些小代码之 ROW_NUMBER 和去重

ROW_NUMBER 的使用基本上倒不会遗忘了, 这是很久以前的代码库了, 整理出来对初学者有帮助.

如果去继续研究研究 SQL Server 2012 中的Window Function 新特性, 就会发现里面还有更多很强大的东东, 很多内容和这里的 ROW_NUMBER 的概念或者使用方式有些相似.

理解了ROW_NUMBER 以及 PARTITION BY 的使用再去看 2012的 Windows Function 新特性的话会比较容易些.

-- ROW_NUMBER function
 
DECLARE @DEMO TABLE
(
   Name  VARCHAR(15),
   ADDR1 VARCHAR(50),
   ADDR2 VARCHAR(50),
   CITY  VARCHAR(50),
   ZIP   CHAR(6)
)
-- Insert testing records
INSERT INTO @DEMO VALUES
('Zhang San','Leshan Road 33','','Shanghai','200081'),
('Li Si','Zhaojiabang Road 23','Nan song Street 9002','Shanghai','200083'),
('Wang Wu','Huashan Road 27','Weihai zhong Road 1039','Shanghai','200081'),
('Sun liu','Jiaoda dong Round 12','Gao liangqiao xiejie 19','Beijing','100023'),
('Yang Qi','Zhong guan cun nan A39','','Beijing','100009'),
('Zhu Ba','Cuihua nan 30','haiding road 32','Beijing','100103'),
('Huang Jiu','Lv you Road 33','Huan dong jie 39','Wuhan','420021')
 
-- Original query
SELECT *
FROM @DEMO

-- Only to set row number for each record.
SELECT ROW_NUMBER() OVER(ORDER BY addr.city) AS 'ID',
       addr.Name,
       addr.ADDR1,
       addr.CITY,
       addr.ZIP
FROM @DEMO AS addr

-- Set group first, then set row number for each record under the group - city
SELECT ROW_NUMBER() OVER (PARTITION BY addr.CITY
                          ORDER BY addr.CITY) AS 'ID',
       addr.Name,
       addr.ADDR1,
       addr.CITY,
       addr.ZIP
FROM @DEMO AS addr

-- Set group first, then set row number for each record under the group - city and zip
SELECT ROW_NUMBER() OVER (PARTITION BY addr.CITY,
                                       addr.ZIP
                          ORDER BY addr.CITY) AS 'ID',
       addr.Name,
       addr.ADDR1,
       addr.CITY,
       addr.ZIP
FROM @DEMO AS addr

---

通过了上面的这个小例子,再看看去重就很简单了.

-- Remove duplicated records
 
DECLARE @DupDemo TABLE
(
   A VARCHAR(15),
   B VARCHAR(15),
   C VARCHAR(15)
)
 
-- Insert testing records
INSERT INTO @DupDemo VALUES
('1','a','2'),
('1','a','1'),
('2','c','6'),
('2','c','3'),
('1','a','1'),
('3','b','3')
 
-- Original Records
SELECT *
FROM @DupDemo

-- Show the duplicated records, only to check column A and B.
SELECT ROW_NUMBER() OVER(PARTITION BY A,
                                      B
                    ORDER BY C) AS ID,
             A,
             B,
             C
      FROM @DupDemo

-- Delete the duplicated records
DELETE D
FROM(
      SELECT ROW_NUMBER() OVER(PARTITION BY A,
                                            B
                               ORDER BY C) AS ID,
             A,
             B,
             C
      FROM @DupDemo
)D
WHERE D.ID > 1
 
-- After delete operation
SELECT *
FROM @DupDemo

-- Show DEMO 2
DELETE FROM
@DupDemo
 
-- Insert test records
INSERT INTO @DupDemo VALUES
('1','a','2'),
('1','a','1'),
('2','c','6'),
('2','c','3'),
('1','a','1'),
('3','b','3');
 
-- With TEMP table
WITH Dup AS
(
   SELECT ROW_NUMBER() OVER(PARTITION BY A,
                                            B
                               ORDER BY C) AS ID,
             A,
             B,
             C
      FROM @DupDemo
)
 
DELETE FROM
Dup
WHERE ID > 1;
 
-- Show records after delete operation.
SELECT *
FROM @DupDemo