sql left join error using Month(date) in select statment

Question

This code is working:

select Date1,  Date2
from 
    (SELECT Date1 FROM Table1 GROUP BY Date1) t1
left join
    (SELECT Date2 FROM Table2 GROUP BY Date2) t2
on
    t1.Date1 = t2.Date2

But I need to use it extracting Month(...) so when I change code like this I've getting an error:

Unknown column 'Date1' in 'field list'

select Date1,  Date2
from 
    (SELECT Month(Date1) FROM Table1 GROUP BY Date1) t1
left join
    (SELECT Month(Date2) FROM Table2 GROUP BY Date2) t2
on
    t1.Date1 = t2.Date2

Actually the code is bigger and only this part of code does not let me to proceed..

Answer 1

Alias name for month is missing in the inner select query

select Date1,  Date2
from 
    (SELECT Month(Date1) as date1 --Alias missing Here
     FROM Table1 
     GROUP BY Date1) t1
left join
    (SELECT Month(Date2) as date2 --Alias missing here
     FROM Table2 
     GROUP BY Date2) t2
on
    t1.Date1 = t2.Date2

Answer 2

SQL Engine throws an error because it couldn't find this name entity in your subquery. As mentioned by this user you need to provide aliases in subquery.

Answer 3

The alias name is missing while using the Month():

select Date1,  Date2
from 
    (SELECT Month(Date1) month1 FROM Table1 GROUP BY Date1) t1
left join
    (SELECT Month(Date2) month2 FROM Table2 GROUP BY Date2) t2
on
    t1.Date1 = t2.Date2