SQL 按 GROUP 查询当前行和上一行之间的差异

Question

我是 SQL 的新手。我有以下 table 结构。对于给定的 SITEID 和给定的 YEAR，我试图找到添加的 "new" CLASS，即前一年不存在的 CLASS。每个站点可以有多个类。

<table><tbody><tr><th>SITEID</th><th>YEAR</th><th>CLASS</th></tr><tr><td>1</td><td>2007</td><td>A</td></tr><tr><td>1</td><td>2007</td><td>B</td></tr><tr><td>1</td><td>2008</td><td>A</td></tr><tr><td>1</td><td>2008</td><td>B</td></tr><tr><td>1</td><td>2008</td><td>C</td></tr></tbody></table>

在上述情况下，我要查找的最终输出是：

<table><tbody><tr><th>SITEID</th><th>YEAR</th><th>CLASS</th></tr><tr><td>1</td><td>2007</td><td> </td></tr><tr><td>1</td><td>2008</td><td>C</td></tr></tbody></table>

非常感谢您的帮助。谢谢。

Answer 1

SELECT SITEID, YEAR, max(CLASS) AS CLASS FROM [dbo].[TestTable] GROUP BY YEAR,SITEID

Answer 2

您可以使用 window 函数：

select t.*
from (select t.*, lag(year) over (partition by siteid, class order by year) as prev_year
      from t
     ) t
where prev_year is null or prev_year <> year - 1

这将 return "A" 和 "B" 用于 2007 年。虽然这不是您提供的结果，但您的问题似乎是要问的。

Answer 3

请试试这个（请将 temp table 替换为您的 table）：

如果您使用的是 SQL 服务器，那么您可以使用通用 Table 表达式 (CTE)，如下所示：

 IF (OBJECT_ID('tempdb..#temp_table') IS NOT NULL)
    BEGIN
      DROP TABLE #temp_table
    END;

CREATE TABLE #temp_table (SiteId INT NOT NULL, [year] INT, [class] [Char] NOT NULL)

INSERT INTO #temp_table (SiteId, [year], [class])
       values 
(1, 2007,   'A'),
(1, 2007,   'B'),
(1, 2008,   'A'),
(1, 2008,   'B'),
(1, 2008,   'C')


;with temp_cte as
  (SELECT  siteid, [year], [year]-1 as [yearbefore]
            ,STUFF((SELECT '|' + CAST([class] AS VARCHAR(MAX)) [text()]
            FROM #temp_table 
            WHERE SiteId = t.SiteId and [year] = t.[year]
            FOR XML PATH(''), TYPE)
            .value('.','NVARCHAR(MAX)'),1,2,' ') [class_list]
            FROM #temp_table t
            GROUP BY SiteId, [year]
)
select c.SiteId
       , c.[year]
       , case when t.yearbefore = null then null else right(REPLACE(c.class_list, t.class_list, ''), LEN(REPLACE(c.class_list, t.class_list, ''))-1) end as [class_added]
       , case when t.yearbefore = null then null else stuff(REPLACE(c.class_list, t.class_list, ''), 1, charindex('|', REPLACE(c.class_list, t.class_list, '')), '') end as [class_added_using_stuff]
 from temp_cte c
left join temp_cte t on c.[yearbefore] = t.[year]

如果您没有使用 SQL 服务器（下面的也适用于 SQL 服务器），那么您可以使用连接（可能需要针对正在使用的 RDBMS 调整查询) 如下:

select distinct t1.SiteId, t1.[year]
    , case when t1.[year] = m.[year] then null else t1.[class] end as class_added
    from #temp_table t1
    left join #temp_table t2 on t1.SiteId = t2.SiteId and t1.class = t2.class and t1.[year] -1 = t2.[year]
    left join (select top 1 * from #temp_table order by [year] ) m on t1.[year] = m.[year]
where t2.SiteId is null

MYSQL更新：

CREATE TABLE test_table(
    SiteId INT NOT NULL,
    year INT,
    class CHAR(1)
);

INSERT INTO 
    test_table(SiteId, year, class)
VALUES
    (1, 2007,   'A'),
    (1, 2007,   'B'),
    (1, 2008,   'A'),
    (1, 2008,   'B'),
    (1, 2008,   'C');
select distinct t1.SiteId, t1.year
    , case when t1.year = m.year then null else t1.class end as class_added
    from test_table t1
    left join test_table t2 on t1.SiteId = t2.SiteId and t1.class = t2.class and t1.year -1 = t2.year
    left join (select * from test_table order by year limit 1) m on t1.year = m.year
where t2.SiteId is null;

我的SQLFiddlehere (http://sqlfiddle.com/#!9/c570d57/1/0)

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