从SQL查询中的一个表中选择两个日期范围会给出膨胀的数字

Question

我正在尝试输出两个日期范围作为在单个SQL查询中按第三个分组的列。查询有效，但输出的数字显着膨胀 - 查询有问题吗？

SELECT
  a.channel AS channel,
  SUM(b.sessions) AS 'thisYear',
 SUM(c.sessions) AS 'lastYear'
FROM overview a

LEFT JOIN overview b ON
 b.date= a.date
 AND b.date BETWEEN '2015-01-01' AND '2015-12-31'

JOIN overview c ON
 c.date= a.date
 AND c.date BETWEEN '2014-01-01' AND '2014-12-31'

WHERE a.channel!=""

group by a.channel;

当前输出给出：

channel thisYear    lastYear
Direct  36891   118335
Organic Search  40231   98531
Paid Search 19962   NULL
Referral    14782   17266
Social  4228    23399

但这些数字明显高于应有的数字。 有没有更好的方法呢？

由于

Answer 1

SELECT
  a.channel AS channel,
sum(case when date BETWEEN '2015-01-01' AND '2015-12-31' then sessions else 0 end) AS 'thisYear',
...
FROM overview a
...

更好的是，考虑使用year（datecolumn）= 2015，除非您遇到性能问题（尽管我无法想象您在包含这些内容的表中有数百万行）。