mysql加入日期，其中一个表的日期是一个月前

Question

I have 2 tables of which I want to join on the date. While the date are of the same format, I want to join t1.date_1 to t2.date_2 but where t2.date_2 is a month before. Please see my query below for context and the output I want:

t1
date_1      count_t1
2015-01-01   10
2015-02-01   20
2015-03-01   30



t2
date_2      count_t2
2014-12-01   40
2015-01-01   50
2015-02-01   60
2015-03-01   70

output i want:
date_1       percent_before
2015-01-01    0.25            <--10/40
2015-02-01    0.40            <--20/50
2015-03-01    0.5             <--30/60

my query:
select (t1.count_1/t2.count_2) as percent_before
from table1 t1 join table2 t2
on t1.date_1 = (t2.date_2 - 1 month);

Answer 1

You need the interval keyword:

select t1.date_1, (t1.count_1 / t2.count_2) as percent_before
from table1 t1 join
     table2 t2
     on t1.date_1 = (t2.date_2 - interval 1 month);

Answer 2

For people using teradata and cannot use interval:

select t1.date_1, (t1.count_1 / t2.count_2) as percent_before
from table1 t1 join
     table2 t2
     on t1.date_1 = add_months(t2.date_2 - 1 month);