按项目批次对 SQL 结果进行分组的最佳方法是什么？答案

【问题标题】：What's the best way to group SQL results by items batch?按项目批次对 SQL 结果进行分组的最佳方法是什么？
【发布时间】：2023-02-07 08:01:29
【问题描述】：

比如我有一个简单的表books：

author	book
Author-A	Book-A1
Author-A	Book-A2
Author-B	Book-B1
Author-C	Book-C1
Author-C	Book-C2

我需要计算每个作者的书，所以我会写：

select author, count(*) from books
group by author

# Author-A = 2
# Author-B = 1
# Author-C = 2

但是现在我需要按作者组来计算书籍：

groupA = ['Author-A', 'Author-C'],
groupB = ['Author-B']

select authorGroup, count(*) from books
group by {
  case author in groupA -> 'groupA'
  case author in groupB -> 'groupB'
} as authorGroup

# ['Author-A', 'Author-C'] = 4
# ['Author-B'] = 1

这些组可以不同并且来自另一个模块。编写此请求的最佳方式是什么？也许没有联合，例如：

select author as 'groupA', count(*) from books
where author in { groupA }
  union
select author as 'groupB', count(*) from books
where author in { groupB }

因为可能会有很多组请求 (~20-30)

【问题讨论】：

标签： sql postgresql

【解决方案1】：

通常的方法是JOIN 到一个映射表，如果需要，它可以是一个内联视图。

SELECT
  author_group.group_label,
  COUNT(*)
FROM
  books
INNER JOIN
(
  SELECT 'Author-A' AS author, 'Group-A' AS group_label
  UNION ALL
  SELECT 'Author-B' AS author, 'Group-B' AS group_label
  UNION ALL
  SELECT 'Author-C' AS author, 'Group-A' AS group_label
)
  AS author_group
    ON author_group.author = book.author
GROUP BY
  author_group.group_label

【讨论】：