合并来自同一来源的两个计数查询

Question

我有两个查询，返回带有计数的服务项目列表。 一个用于整个数据库，另一个用于指定时间段。 它们各自可以很好地工作，但是我想将它们优化为单个查询。

这两个查询是：

SELECT service_type, count(service_type) from qba_customers group by service_type order by count(service_type) desc

SELECT service_type, count(service_type) from qba_customers WHERE created_on BETWEEN '2013-01-01' AND '2013-06-30' group by service_type order by count(service_type) desc

我尝试了一些失败的尝试，以下是我最初认为可以进行的工作：

SELECT service_type, COUNT(service_type) AS full_count, (count(service_type) WHERE created_on BETWEEN '2013-01-01' AND '2013-06-30') AS period_count FROM qba_customers GROUP BY service_type ORDER BY service_type DESC

提前致谢！

Answer 1

如果不是NULL，则所有聚合函数（包括COUNT ）将仅包含一个值。 要仅对日期范围内的行进行计数，请使用CASE构造作为count参数。 如果CASE返回一个值，则将其计数。 如果不返回值，则不计算：

SELECT
  service_type,
  COUNT(service_type) AS GrandTotal,
  COUNT(CASE WHEN created_on BETWEEN '2013-01-01' AND '2013-06-30' THEN 1 END) AS FirstHalf2013
FROM qba_customers
GROUP BY service_type
ORDER BY GrandTotal

对于FirstHalf2013列，如果created_on在目标范围内，则CASE返回一个非空值（我在这里使用1 ，但是它可以是任何非空值）。 如果created_on不在目标范围内，则CASE返回NULL ，这表示该行未被计数。

Answer 2

使用case语句尝试：

SELECT service_type, 
       count(service_type) as full_count,
       sum(case when created_on between '2013-01-01' and '2013-06-30' then 1 else 0 end) as period_count 
  FROM qba_customers 
 GROUP by service_type
 ORDER by service_type desc