SQL：查明一个事件是否发生在另一个事件之前或之后

Question

我有两张这样的桌子：

CREATE TABLE XX 
(
    name VARCHAR(50),
    date DATE,
    a INT,
    b INT,
    c INT
);

INSERT INTO XX (name, date, a, b, c) 
VALUES 
('john', '2010-11-01', 1, 0, 0),
('john', '2010-10-01', 0, 1, 0),
('sara', '1999-02-01', 1, 0, 0),
('julie', '2015-09-01', 1, 0, 0),
('julie', '2015-09-01', 0, 1, 0);


  name       date a b c
  -----------------------
  john 2010-11-01 1 0 0
  john 2010-10-01 0 1 0
  sara 1999-02-01 1 0 0
 julie 2015-09-01 1 0 0
 julie 2015-09-01 0 1 0

还有另一张桌子

CREATE TABLE YY 
(
    name VARCHAR(50),
    yy_date DATE,
    yy CHAR(1)
);

INSERT INTO YY (name, yy_date, yy) 
VALUES 
('john', '2015-01-01', 'A'),
('john', '2016-01-01', 'A'),
('john', '2000-02-01', 'B'),
('john', '2010-03-01', 'C'),
('julie', '2017-09-01', 'A'),
('julie', '2010-09-01', 'B');
('tom', '2010-09-01', 'B');


  name  yy_date     yy
  --------------------
  john  2015-01-01  A
  john  2016-01-01  A
  john  2000-02-01  B
  john  2010-03-01  C
  julie 2017-09-01  A
  julie 2010-09-01  B
  tom   2010-09-01  B

我想编写一个 SQL 查询来回答以下问题：

在特定日期收到 type_yy 的客户，即使他们在该日期之前已经拥有相同类型的 xx

例如

• John，A：XX 发生在 YY 之前
• John, B：YY 发生在 XX 之前
• Sara：不适用（仅出现在一张表中）
• 朱莉，A：XX 发生在 YY 之前
• 朱莉，B：XX 发生在 YY 之后
• Tom：不适用（仅出现在一张表中）

最终结果应该是表YY，其中有一个额外的列，指示对于每个（名称，类型），XX是否发生在YY之前或XX是否发生在YY之后。

 name   yy_date yy  xx_vs_yy
 --------------------------------------------
 john   2015-01-01  A XX happened before YY   
john   2016-01-01  A XX happened before YY
 john   2000-02-01  B YY happened before XX
 john   2010-03-01  C        Not applicable
 julie  2017-09-01  A XX happened before YY
 julie  2010-09-01  B YY happened before XX
 tom    2010-09-01  B        Not applicable

是否可以使用多个 CTE、CASE WHEN、UNION ALL 和 MAX 语句来执行类似的操作？

WITH earliest_xx AS 
(
    SELECT name, MIN(date) as earliest_xx_date
    FROM XX
    GROUP BY name
),
yy_with_xx AS 
(
    SELECT 
        y.name, 
        y.yy_date, 
        y.yy, 
        e.earliest_xx_date
    FROM YY y
    LEFT JOIN earliest_xx e ON y.name = e.name
),
comparison_result AS 
(
    SELECT 
        name, 
        yy_date, 
        yy,
        CASE
            WHEN earliest_xx_date IS NULL THEN 'Not applicable'
            WHEN earliest_xx_date < yy_date THEN 'XX happened before YY'
            ELSE 'YY happened before XX'
        END as xx_vs_yy
    FROM yy_with_xx
)
SELECT *
FROM comparison_result
ORDER BY name, yy_date;

我对这个问题感到有点迷失......

---

我认为我在这方面取得了一些进展 - 需要添加一些内容以确保 XX 日期对应于 XX 和 YY 之间的相同类型（例如，a 到 a、b 到 b、c 到 c）：

WITH xx_mapped AS (
    SELECT name, date, 
           CASE 
               WHEN a = 1 THEN 'A'
               WHEN b = 1 THEN 'B'
               WHEN c = 1 THEN 'C'
           END AS xx_type
    FROM XX
),
earliest_xx AS (
    SELECT name, xx_type, MIN(date) as earliest_xx_date
    FROM xx_mapped
    GROUP BY name, xx_type
),
yy_with_xx AS (
    SELECT 
        y.name, 
        y.yy_date, 
        y.yy, 
        e.earliest_xx_date
    FROM YY y
    LEFT JOIN earliest_xx e ON y.name = e.name AND y.yy = e.xx_type
),
comparison_result AS (
    SELECT 
        name, 
        yy_date, 
        yy,
        CASE
            WHEN earliest_xx_date IS NULL THEN 'Not applicable'
            WHEN earliest_xx_date < yy_date THEN 'XX happened before YY'
            WHEN earliest_xx_date > yy_date THEN 'YY happened before XX'
            ELSE 'XX and YY happened on the same date'
        END as xx_vs_yy
    FROM yy_with_xx
)
SELECT *
FROM comparison_result
ORDER BY name, yy_date;

Answer 1

左连接应该得到你想要的结果：

select y.name, y.yy_date, y.yy,
  case when x.date is null then 'Not applicable'
       when x.date > y.yy_date then 'YY happened before XX'
       else 'XX happened before YY'
  end as xx_vs_yy
from yy y
left join xx x on x.name = y.name and (
  x.a = 1 and y.yy = 'A' or
  x.b = 1 and y.yy = 'B' or
  x.c = 1 and y.yy = 'C');

结果：

NAME    YY_DATE    YY  XX_VS_YY             
-----  ----------  --  ---------------------
john   2015-01-01  A   XX happened before YY
john   2016-01-01  A   XX happened before YY
john   2000-02-01  B   YY happened before XX
john   2010-03-01  C   Not applicable       
julie  2017-09-01  A   XX happened before YY
julie  2010-09-01  B   YY happened before XX
tom    2010-09-01  B   Not applicable