SQL Join two tables by unrelated date - sql

Question

I’m looking to join two tables that do not have a common data point, but common value (date). I want a table that lists the date and total number of hired/terminated employees on that day. Example is below:
Table 1
Hire Date Employee Number Employee Name
--------------------------------------------
5/5/2018 10078 Joe
5/5/2018 10077 Adam
5/5/2018 10078 Steve
5/8/2018 10079 Jane
5/8/2018 10080 Mary
Table 2
Termination Date Employee Number Employee Name
----------------------------------------------------
5/5/2018 10010 Tony
5/6/2018 10025 Jonathan
5/6/2018 10035 Mark
5/8/2018 10052 Chris
5/9/2018 10037 Sam
Desired result:
Date Total Hired Total Terminated
--------------------------------------
5/5/2018 3 1
5/6/2018 0 2
5/7/2018 0 0
5/8/2018 2 1
5/9/2018 0 1
Getting the total count is easy, just unsure as the best approach from the standpoint of "adding" a date column

Answer 1

If you need all dates within some window then you need to join the data to a calendar. You can then left join and sum flags for data points.
DECLARE #StartDate DATETIME = (SELECT MIN(ActionDate) FROM(SELECT ActionDate = MIN(HireDate) FROM Table1 UNION SELECT ActionDate = MIN(TerminationDate) FROM Table2)AS X)
DECLARE #EndDate DATETIME = (SELECT MAX(ActionDate) FROM(SELECT ActionDate = MAX(HireDate) FROM Table1 UNION SELECT ActionDate = MAX(TerminationDate) FROM Table2)AS X)
;WITH AllDates AS
(
SELECT CalendarDate=#StartDate
UNION ALL
SELECT DATEADD(DAY, 1, CalendarDate)
FROM AllDates
WHERE DATEADD(DAY, 1, CalendarDate) <= #EndDate
)
SELECT
CalendarDate,
TotalHired = SUM(CASE WHEN H.HireDate IS NULL THEN NULL ELSE 1 END),
TotalTerminated = SUM(CASE WHEN T.TerminationDate IS NULL THEN NULL ELSE 1 END)
FROM
AllDates D
LEFT OUTER JOIN Table1 H ON H.HireDate = D.CalendarDate
LEFT OUTER JOIN Table2 T ON T.TerminationDate = D.CalendarDate
/* If you only want dates with data points then uncomment out the where clause
WHERE
NOT (H.HireDate IS NULL AND T.TerminationDate IS NULL)
*/
GROUP BY
CalendarDate

Answer 2

I would do this with a union all and aggregations:
select dte, sum(is_hired) as num_hired, sum(is_termed) as num_termed
from (select hiredate as dte, 1 as is_hired, 0 as is_termed from table1
union all
select terminationdate, 0 as is_hired, 1 as is_termed from table2
) ht
group by dte
order by dte;
This does not include the "missing" dates. If you want those, a calendar or recursive CTE works. For instance:
with ht as (
select dte, sum(is_hired) as num_hired, sum(is_termed) as num_termed
from (select hiredate as dte, 1 as is_hired, 0 as is_termed from table1
union all
select terminationdate, 0 as is_hired, 1 as is_termed from table2
) ht
group by dte
),
d as (
select min(dte) as dte, max(dte) as max_dte)
from ht
union all
select dateadd(day, 1, dte), max_dte
from d
where dte < max_dte
)
select d.dte, coalesce(ht.num_hired, 0) as num_hired, coalesce(ht.num_termed) as num_termed
from d left join
ht
on d.dte = ht.dte
order by dte;

Answer 3

Try this one
SELECT ISNULL(a.THE_DATE, b.THE_DATE) as Date,
ISNULL(a.Total_Hire,0) as Total_Hire,
ISNULL (b.Total_Terminate,0) as Total_terminate
FROM (SELECT Hire_date as the_date, COUNT(1) as Total_Hire
FROM TABLE_HIRE GROUP BY HIRE_DATE) a
FULL OUTER JOIN (SELECT Termination_Date as the_date, COUNT(1) as Total_Terminate
FROM TABLE_TERMINATE GROUP BY HIRE_DATE) a
ON a.the_date = b.the_date