SQL 加入 table 自身以获取上一年的数据

Question

SQL。我怎样才能将 table 连接到自身以获得所需的结果，如下面的 table 所示。逻辑是我想要相同产品和上一年相应月份的单位。

源 table 上键 a.[year]=b.[year]+1 上的简单左连接（当然还有每个月和每个产品）会导致我们在前年现在没有了

Answer 1

完全连接就足够了

  select distinct
    coalesce(a.year, b.year+1) as year
    , coalesce(a.month, b.month) as month
    , coalesce(a.product, b.product) as product
    , a.units as units
    , b.units as units_prev
  from yourtable a
  full join yourtable b on a.[year] = b.[year]+1 and a.[month] = b.[month] and a.product = b.product

您的预期结果与 2018 年第 2 月的描述略有不同，产品 2 的先验值为 2933 不存在。

DB Fiddle : https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=d01dc5bd626854b083be0864f2d5b0e4

结果：

year    month   product units   units_prev
2017    1       1       1721    
2017    2       1       4915    
2017    4       2       2933    
2017    5       1       5230    
2018    1       1               1721
2018    1       2       7672    
2018    2       1       5216    4915
2018    3       1       8911    
2018    4       2               2933
2018    5       1               5230
2019    1       2               7672
2019    2       1               5216
2019    3       1               8911

如果您需要像那样过滤掉 futures，那么您可以添加一个额外的 where 谓词，例如：

where coalesce(a.year, b.year+1) <= year(getdate())

Answer 2

我会选择 LAG 和 calendar table。

SELECT C.[Year],
       C.[Month],
       YPT.product,
       YST.units,
       YST.LAG(YST.units) OVER (PARTITION BY YTP.[product],C.[month] ORDER BY C.[year]) AS UnitsPrev
FROM CalendarTable C
     CROSS JOIN YourProductTable YPT
     LEFT JOIN YourSourceTable YST ON C.[Year] YST.[Year]
                                  AND C.[Month] = YST.[Month]
                                  AND YPT.Product = YST.Product
WHERE C.[day] = 1
  AND C.[date] BETWEEN {SomeStartDate} AND {SomeEndDate];

这对您的设计进行了一些猜测（假设您有一个产品 table）。

Answer 3

如果您想要 2017 年和 2018 年都没有售出任何东西的行，就像您想要的 2017 年 3 月的结果一样，您还需要生成月份、年份并加入产品以获得空值。

此查询针对月份和年份进行查询，希望您也可以根据需要添加产品

DECLARE @startMonth INT=1
DECLARE @endMonth INT=12
DECLARE @startYear INT=2017
DECLARE @endYear INT=2018
;
WITH months AS (
    SELECT @startMonth AS m
    UNION ALL
    SELECT m+1 FROM months WHERE m+1<=@endMonth
),
years AS (
    SELECT @startYear AS y
    UNION ALL
    SELECT y+1 FROM years WHERE y+1<=@endYear
),
monthYears AS (
    SELECT m, y
    FROM months, years
)
SELECT  thisYear.[Year], thisYear.[Month], thisYear.[Product], thisYear.[Units], prevYear.[Units] as units_prev
FROM 
    (SELECT [Product], my.y as [Year], my.m as [Month], [Units]
    FROM monthYears my
    LEFT JOIN sales on my.m = [Month] and my.y = [Year]) as thisYear
LEFT OUTER JOIN     
    (SELECT [Product], my.y as [Year], my.m as [Month], my.y + 1 as NextYear, [Units]
    FROM monthYears my
    LEFT JOIN sales on my.m = [Month] and my.y = [Year])  as prevYear 
    on thisYear.Product = prevYear.Product
        and (thisYEAR.[Year]) = prevYear.[NextYear]
        and thisYEAR.[Month] = prevYear.[Month]
ORDER BY thisYear.[Year], thisYear.[Month], thisYear.[Product] 
option (maxrecursion 12);

Answer 4

年月

使用cross join生成行，left join引入数据，然后lag()获取"previous"值：

select y.year, m.month, p.product, t.units,
       lag(t.units) over (partition by p.product, m.month order by y.year) as prev_units
from (select distinct year from t) y cross join
     (select distinct month from t) m cross join
     (select distinct product from t) p left join
     t
     on t.year = y.year and t.month = m.month and t.product = p.producct;

Answer 5

您可以使用 CROSS JOIN 在您的数据中生成年、月和产品的所有可能组合。如果存在特定组合的数据，一个简单的 LEFT JOIN 将为您提供值或 NULL。

DECLARE @t TABLE (year int, month int, product int, unit int);
INSERT INTO @t VALUES
(2017, 1, 1, 1721),
(2017, 2, 1, 4915),
(2017, 5, 1, 5230),
(2018, 2, 1, 5216),
(2018, 3, 1, 8911),
(2017, 4, 2, 2933),
(2018, 1, 2, 7672);

SELECT ally.year, allm.month, allp.product, curr.units, prev.units AS units_prev
FROM (SELECT DISTINCT year FROM @t) AS ally
CROSS JOIN (SELECT DISTINCT product FROM @t) AS allp
CROSS JOIN (SELECT DISTINCT month FROM @t) AS allm
LEFT JOIN @t AS curr ON curr.year = ally.year AND curr.product = allp.product AND curr.month = allm.month
LEFT JOIN @t AS prev ON prev.year = ally.year - 1 AND prev.product = allp.product AND prev.month = allm.month

结果：

| year | month | product | units | units_prev |
|------|-------|---------|-------|------------|
| 2017 | 1     | 1       | 1721  | NULL       |
| 2017 | 2     | 1       | 4915  | NULL       |
| 2017 | 3     | 1       | NULL  | NULL       |
| 2017 | 4     | 1       | NULL  | NULL       |
| 2017 | 5     | 1       | 5230  | NULL       |
| 2017 | 1     | 2       | NULL  | NULL       |
| 2017 | 2     | 2       | NULL  | NULL       |
| 2017 | 3     | 2       | NULL  | NULL       |
| 2017 | 4     | 2       | 2933  | NULL       |
| 2017 | 5     | 2       | NULL  | NULL       |
| 2018 | 1     | 1       | NULL  | 1721       |
| 2018 | 2     | 1       | 5216  | 4915       |
| 2018 | 3     | 1       | 8911  | NULL       |
| 2018 | 4     | 1       | NULL  | NULL       |
| 2018 | 5     | 1       | NULL  | 5230       |
| 2018 | 1     | 2       | 7672  | NULL       |
| 2018 | 2     | 2       | NULL  | NULL       |
| 2018 | 3     | 2       | NULL  | NULL       |
| 2018 | 4     | 2       | NULL  | 2933       |
| 2018 | 5     | 2       | NULL  | NULL       |

SQL 加入 table 自身以获取上一年的数据

SQL join table to itself to get data for previous year

sql

tsql

sql-server

join

self-join