Question

假设您有一系列人们对他们喜欢的颜色的反应。此信息存储在SQL表中：

| id | favorite_color | friend_recommendation_id |
|----|----------------|--------------------------|
| 1  | green          |                          |
| 2  | blue           |                          |
| 3  | yellow         |                          |
| 4  | green          |                          |
| 5  | yellow         |                          |
| 6  | green          |                          |

我的目标是编写一个Postgres SQL查询，该查询会在friend_recommendation列中填充最新人员的ID，以使用与提供的个人相同的颜色进行回复。这将产生下表：

| id | favorite_color | friend_recommendation_id |
|----|----------------|--------------------------|
| 1  | green          |                          |
| 2  | blue           |                          |
| 3  | yellow         |                          |
| 4  | green          | 1                        |
| 5  | yellow         | 3                        |
| 6  | green          | 4                        |

请注意，身份6已填充4而不是1

我尝试过使用变量和子选择，但我们正在努力解决如何从父查询中应用每个结果的选择。

Answer 1

使用子查询来计算字段

<强> SQL Fiddle Demo

SELECT "id", "favorite_color", (SELECT MAX("id")
                                FROM colors c2
                                WHERE c2."favorite_color" = c1."favorite_color"
                                  AND c2."id" < c1."id"
                                ) as friend_recommendation_id 
FROM colors c1

<强>输出

| id | favorite_color | friend_recommendation_id |
|----|----------------|--------------------------|
|  1 |          green |                   (null) |
|  2 |           blue |                   (null) |
|  3 |         yellow |                   (null) |
|  4 |          green |                        1 |
|  5 |         yellow |                        3 |
|  6 |          green |                        4 |

也可以这样写：

SELECT c1."id", c1."favorite_color", MAX(c2."id") as friend_recommendation_id 
FROM colors c1
LEFT JOIN colors c2
       ON c2."favorite_color" = c1."favorite_color"
      AND c2."id" < c1."id"
GROUP BY c1."id", c1."favorite_color"
ORDER BY c1."id";

<强>更新

UPDATE colors target
SET "friend_recomendation_id"  = ( SELECT MAX("id")
                                   FROM colors c2
                                   WHERE c2."favorite_color" = target."favorite_color"
                                     AND c2."id" < target."id")

使用Postgres SQL从上一行设置多个列值

1 个答案: