SQL : 如何重用 count(*) 计算值？

Question

我正在使用 Postgres 9.4。

我的查询中有一个 select 语句，如下所示：

SELECT date_part('year'::text, c.date) AS yyyy,
    to_char(c.date, 'MM'::text) AS monthnumber,
    to_char(c.date, 'TMMonth'::text) AS monthname,
    l.id AS lineID,
    n.id AS networkID,
    l.name AS lineName,
    count(c.*) AS count,
    count(distinct(c.date)) AS number_of_journeys,

我想重复使用最后两列（计算的列）来计算另一列，如下所示：

SELECT date_part('year'::text, c.date) AS yyyy,
    to_char(c.date, 'MM'::text) AS monthnumber,
    to_char(c.date, 'TMMonth'::text) AS monthname,
    l.id AS lineID,
    n.id AS networkID,
    l.name AS lineName,
    count(c.*) AS count,
    count(distinct(c.date)) AS number_of_journeys,
    count / number_of_journeys AS frequentation_moyenne

可以这样做吗？

谢谢

Answer 1

不在 SQL 服务器中，您必须使用其中之一：

SELECT date_part('year'::text, c.date) AS yyyy,
to_char(c.date, 'MM'::text) AS monthnumber,
to_char(c.date, 'TMMonth'::text) AS monthname,
l.id AS lineID,
n.id AS networkID,
l.name AS lineName,
count(c.*) AS count,
count(distinct(c.date)) AS number_of_journeys,
count(c.*) / count(distinct(c.date)) AS frequentation_moyenne

或

Select yyyy, monthnumber, monthname, lineID, networkID, lineName, count, number_of_journery, count / number_of_journeys AS frequentation_moyenne
from
(SELECT date_part('year'::text, c.date) AS yyyy,
to_char(c.date, 'MM'::text) AS monthnumber,
to_char(c.date, 'TMMonth'::text) AS monthname,
l.id AS lineID,
n.id AS networkID,
l.name AS lineName,
count(c.*) AS count,
count(distinct(c.date)) AS number_of_journeys)

SQL : 如何重用 count(*) 计算值？

SQL : How to reuse count(*) computed value?

sql

postgresql