SQL Server - Cross join with where / on condition

Question

I have a table with monthly data for employees that I'd like to do cross joins but only after making sure they're in the same month. Table looks like this:

  Month      Emp_id
1/1/2017      123
1/1/2017      234
1/1/2017      345
1/1/2017      456
2/1/2017      123
2/1/2017      234
2/1/2017      345
  ...

I'd like to do something like this:

select *
from t1
cross join t1 as t2
   on t1.Month = t2.Month

Is there a way to do this?

EDIT:

To elaborate, if I have 100 employees in each month and 12 months I'm looking to get an output table with 120,000 rows (100 * 100 Cartesian for each month, times 12 (1 for each month) instead of doing a full Cartesian which would be 1,440,000.

Answer 1

Yes, it is called an inner join:

select *
from t1 inner join
     t1 t2
     on t1.Month = t2.Month

You could express the same thing using where and cross join, but I think an inner join is better than:

select *
from t1 cross join
     t1 t2
where t1.Month = t2.Month;

Note that you are using select *, which means that you will have duplicate column names and not know which t1 they are coming from. If that is an issue, ask another question.