Question

我想编写一个SQL查询来划分两个表的计数，然后将它们联接起来以显示相对于月份和年份的月份。

我编写了一个查询来对两个表的计数进行除法，但是我不确定如何组合两个表的日期字段，从而出现“月数与除数”。

 select 
    (a.count_one / b.count_two) as final_count, 
    a.Months, 
    b.Months 
from 
    (
        select count(*) as count_one, DATE_FORMAT(`date`, "%M %Y") AS `Months` 
        from first_table
        GROUP BY str_to_date(concat(date_format(`first_table `.`date`, '%Y-%m'), '-01'), '%Y-%m-%d')
    ) a,
    (
        select count(*) as count_two, DATE_FORMAT(`date`, "%M %Y") AS `Months` 
        from second_table
        GROUP BY str_to_date(concat(date_format(`second_table`.`date`, '%Y-%m'), '-01'), '%Y-%m-%d')
    ) b

期望的输出：

**Month and Year**    **Final_Count**
January 2016              126
February 2016             123
March 2016                 45
....                      ....
...                       ....
...                       ....

Answer 1

您需要加入条件：

select 
    (a.count_one / b.count_two) as final_count, 
    a.Months
from 
    (
        select count(*) as count_one, DATE_FORMAT(`date`, "%M %Y") AS `Months` 
        from first_table
        GROUP BY Months)
    ) a,
    (
        select count(*) as count_two, DATE_FORMAT(`date`, "%M %Y") AS `Months` 
        from second_table
        GROUP BY Montns)
    ) b
WHERE a.Months = b.Months

学习编写ANSI JOIN会更好：

select 
    (a.count_one / b.count_two) as final_count, 
    a.Months
from 
    (
        select count(*) as count_one, DATE_FORMAT(`date`, "%M %Y") AS `Months` 
        from first_table
        GROUP BY Months)
    ) a
INNER JOIN
    (
        select count(*) as count_two, DATE_FORMAT(`date`, "%M %Y") AS `Months` 
        from second_table
        GROUP BY Months)
    ) b
ON a.Months = b.Months

MYSQL查询可将两个表的count（*）相除，并按月连接数年

1 个答案: