是否可以根据 PostgreSQL 中的输入列生成列

Question

我在 PostgreSQL 中有一个这样的表，其中包含 3 个初始列，分别称为 Project、Size 和 StartDate（见下文）：

Project	Size	StartDate
Project1	88	2020-06-15
Project2	105	2020-03-01

我需要在 StartDate 列旁边添加 12 列，并将初始表格扩展到 15 列。所有新列都代表给定年份（2020 年）的月份，它们将包含 0 和基于从 Size 和 StartDate 列派生的条件逻辑计算的值。条件如下：如果 StartDate 属于特定月份，则该月获得 value=Size，之后每个月的 Value=Value-50 直到值 >0。请检查以下预期结果：

Project	Size	StartDate	Jan	Feb	Mar	Apr	Mai	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Project1	88	2020-06-15	0	0	0	0	88	38	0	0	0	0	0	0
Project2	105	2020-03-01	0	0	105	55	5	0	0	0	0	0	0	0

Answer 1

这可以通过很多CASE 表达式来完成：

SELECT project,
       size,
       startdate,
       CASE WHEN mon < 2
            THEN greatest(size - 50 * (1 - mon), 0)
            ELSE 0
       END AS jan,
       CASE WHEN mon < 3
            THEN greatest(size - 50 * (2 - mon), 0)
            ELSE 0
       END AS feb,
       CASE WHEN mon < 4
            THEN greatest(size - 50 * (3 - mon), 0)
            ELSE 0
       END AS mar,
       CASE WHEN mon < 5
            THEN greatest(size - 50 * (4 - mon), 0)
            ELSE 0
       END AS apr,
       CASE WHEN mon < 6
            THEN greatest(size - 50 * (5 - mon), 0)
            ELSE 0
       END AS may,
       CASE WHEN mon < 7
            THEN greatest(size - 50 * (6 - mon), 0)
            ELSE 0
       END AS jun,
       CASE WHEN mon < 8
            THEN greatest(size - 50 * (7 - mon), 0)
            ELSE 0
       END AS jul,
       CASE WHEN mon < 9
            THEN greatest(size - 50 * (8 - mon), 0)
            ELSE 0
       END AS aug,
       CASE WHEN mon < 10
            THEN greatest(size - 50 * (9 - mon), 0)
            ELSE 0
       END AS sep,
       CASE WHEN mon < 11
            THEN greatest(size - 50 * (10 - mon), 0)
            ELSE 0
       END AS oct,
       CASE WHEN mon < 12
            THEN greatest(size - 50 * (11 - mon), 0)
            ELSE 0
       END AS nov,
       CASE WHEN mon < 13
            THEN greatest(size - 50 * (12 - mon), 0)
            ELSE 0
       END AS dec
FROM (SELECT project,
             size,
             startdate,
             extract(month FROM startdate) AS mon
      FROM mytable) AS q;