在sql中组合多个计数查询答案

【问题标题】：combining multiple count queries in sql在sql中组合多个计数查询
【发布时间】：2017-02-22 19:38:03
【问题描述】：

我有一个用户表，其中包含 3 列，我试图对其进行计数，然后按 created_at 日期分组。

列：“created_at”、“answers_count”、“questions_count”

如何将这 3 个查询合并为一个，并将所有计数按相同的 created_at 日期分组？

这里有 3 个单独的查询：

-- daily new signups
SELECT date_trunc('day', users.created_at) AS signup_date, count(*) AS new_users
FROM users
GROUP BY signup_date
ORDER BY signup_date DESC;

-- new user answers_count by signup date
SELECT date_trunc('day', users.created_at) AS signup_date, count(*) AS answers_count
FROM users
WHERE users.answers_count > 0
GROUP BY signup_date
ORDER BY signup_date DESC;

-- new user questions_count by signup date 
SELECT date_trunc('day', users.created_at) AS signup_date, count(*) AS qs_received
FROM users
WHERE users.questions_count > 0
GROUP BY signup_date
ORDER BY signup_date DESC;

【问题讨论】：

您正在使用哪个数据库..？ mysql还是postgresql？

标签： mysql sql postgresql

【解决方案1】：

您应该尝试使用SUM() 和CASE 来完成此操作。

试试这样的：

SELECT date_trunc('day', users.created_at) AS signup_date, 
  count(*) AS new_users,
  sum(case when answers_count > 0 then 1 else 0 end) as answers_count,
  sum(case when questions_count > 0 then 1 else 0 end) as qs_received,
FROM users
GROUP BY signup_date
ORDER BY signup_date DESC;

【讨论】：

【解决方案2】：

您可以尝试使用count 和FILTER 来执行此操作。

SELECT date_trunc('day', users.created_at) AS signup_date, 
  count(*) AS new_users,
  count(*) FILTER (WHERE answers_count > 0) as answers_count,
  count(*) FILTER (WHERE questions_count > 0) as qs_received,
FROM users
GROUP BY signup_date
ORDER BY signup_date DESC;

【讨论】：