T-SQL 查询以根据某一特定列删除输出中的重复记录

Question

我是运行 SQL Server 2014，我有以下 T-SQL 查询：

 USE MYDATABASE

 SELECT * 
 FROM RESERVATIONLIST
 WHERE [MTH] IN ('JANUARY 2015','FEBRUARY 2015')

上面代码中提到的

RESERVATIONLIST是一个视图。查询给了我以下输出（摘录）：

ID     NAME       DOA         DOD       Nights Spent       MTH
--------------------------------------------------------------------
251    AH     2015-01-12   2015-01-15      3           JANUARY 2015
258    JV     2015-01-28   2015-02-03      4           JANUARY 2015
258    JV     2015-01-28   2015-02-03      2           FEBRUARY 2015

以上输出包含大约 12,000 条记录。

我需要修改我的查询以消除所有重复的 ID 并提供以下结果：

 ID    NAME       DOA          DOD       Nights Spent       MTH
 --------------------------------------------------------------------
 251     AH     2015-01-12   2015-01-15      3           JANUARY 2015
 258     JV     2015-01-28   2015-02-03      4           JANUARY 2015

我试过类似的方法，但没有用：

USE MYDATABASE

SELECT * 
FROM RESERVATIONLIST
WHERE [MTH] IN ('JANUARY 2015', 'FEBRUARY 2015')    
GROUP BY [ID] 
HAVING COUNT ([MTH]) > 1

Answer 1

以下查询将为每个 ID return 一行：

SELECT * FROM 
(
    SELECT *,ROW_NUMBER() OVER (PARTITION BY ID ORDER BY (SELECT NULL)) rn FROM RESERVATIONLIST
    WHERE [MTH] IN ('JANUARY 2015','FEBRUARY 2015')
) T
WHERE rn = 1

注意：这将 return 具有相同 ID 的多行中的随机行。如果你想 select 一些特定的行，那么你必须在 order by 中定义它。例如:

SELECT * FROM 
(
    SELECT *,ROW_NUMBER() OVER (PARTITION BY ID ORDER BY DOA DESC) rn FROM RESERVATIONLIST
    WHERE [MTH] IN ('JANUARY 2015','FEBRUARY 2015')
) T
WHERE rn = 1

肯定会 return 具有 max(DOA) 的行。

Answer 2

您正在尝试执行 GROUP BY 语句，恕我直言，这是正确的方法。您应该制定所有常量列，然后汇总其他列。根据 DOD 和 DOA 的值，我可以看到两种解决方案：

SELECT ID,NAME,DOA,DOD,SUM([Nights Spent]) as Nights,
   min(MTH) as firstRes, max(MTH) as lastRes
FROM RESERVATIONLIST
GROUP BY ID,NAME,DOA,DOD

或

SELECT ID,NAME,min(DOA) as firstDOA,max(DOD) as lastDOD,SUM([Nights Spent]) as Nights,
   min(MTH) as firstRes, max(MTH) as lastRes
FROM RESERVATIONLIST
GROUP BY ID,NAME

T-SQL 查询以根据某一特定列删除输出中的重复记录

T-SQL Query to remove duplicate records in the output based on one particular column

sql-server

group-by

duplicates

having