SQL：按多列分组并为剩余的多列选择第一个非空值答案

【问题标题】：SQL: Group by multiple columns and select first non null values for remaining multiple columnsSQL：按多列分组并为剩余的多列选择第一个非空值
【发布时间】：2022-10-09 16:04:11
【问题描述】：

当左表作为输入时，如何获得右表作为输出。我想按 prod 和 geo 列分组，并为所有规则列获取第一个非空值。

我试过这种方法：

SELECT prod, geo,
(SELECT rule_1 FROM temp WHERE rule_1 IS NOT NULL LIMIT 1) as rule_1,
(SELECT rule_2 FROM temp WHERE rule_2 IS NOT NULL LIMIT 1) as rule_2,
(SELECT rule_3 FROM temp WHERE rule_3 IS NOT NULL LIMIT 1) as rule_3,
(SELECT rule_4 FROM temp WHERE rule_4 IS NOT NULL LIMIT 1) as rule_4
FROM temp
GROUP BY prod, geo

但不知何故，这仅适用于第一个产品地理组合，不适用于后续组合

【问题讨论】：

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我已经添加了我尝试过的代码。您能否为现有代码提出一些新方法以解决我面临的问题。

标签： sql

【解决方案1】：

Alias the table in your query and update the filter 在子查询中排除不属于当前组的记录。

SELECT prod,
       geo,

  (SELECT rule_1
   FROM test
   WHERE rule_1 IS NOT NULL
     -- exclude records not of the current grouping
     AND t.prod = test.prod
     AND t.geo = test.geo
   LIMIT 1) AS rule_1,

  (SELECT rule_2
   FROM test
   WHERE rule_2 IS NOT NULL
     AND t.prod = test.prod
     AND t.geo = test.geo
   LIMIT 1) AS rule_2,

  (SELECT rule_3
   FROM test
   WHERE rule_3 IS NOT NULL
     AND t.prod = test.prod
     AND t.geo = test.geo
   LIMIT 1) AS rule_3,

  (SELECT rule_4
   FROM test
   WHERE rule_4 IS NOT NULL
     AND t.prod = test.prod
     AND t.geo = test.geo
   LIMIT 1) AS rule_4
FROM test AS t -- alias the table.
GROUP BY prod,
         geo;

【讨论】：