Question

SQL。如下表所示，如何将表连接到自身以获得所需的结果。逻辑是我要为同一产品和上一年的相应月份设置单位。

在源表上对键a.[year]=b.[year]+1本身进行简单的左联接（当然还有逐月以及从产品到产品）会导致丢失我们上一年有值且没有值的数据现在。

Answer 1

完全加入就足够了

  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

尽管您的预期结果与2018年第2个月的描述略有不同，但产品2不存在，其先前值为2933。

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

如果您需要像这样过滤掉期货，则可以在谓词的地方添加一个附加内容，例如：

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

Answer 2

年份月份

使用cross join生成行，使用left join引入数据，然后使用lag()获得“上一个”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 3

我会选择LAG和一个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];

这对您的设计有一些猜测（假设您有一个产品表）。

Answer 4

您可以使用CROSS JOIN在数据中生成年份，月份和产品的所有可能组合。如果存在特定组合的数据，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       |

Answer 5

如果您希望2017年和2018年的行都没有卖出，以及2017年3月的预期结果，则需要生成月，年并加入产品以获取空值。

此查询适用于月份和年份，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);

SQL联接表本身以获取上一年的数据

5 个答案: