SQL, join workdate to pay rate by effective date - sql

Question

I have 2 tables, one with employee work dates and one with pay rates for each employee and when those rates took effect.
They look like this:
CREATE TABLE pay_rates
(
[EMP ID] [int] NOT NULL,
[RATE] [decimal](11, 6) NOT NULL,
[EFFECTIVE DATE] [datetime] NOT NULL
)
CREATE TABLE work_dates
(
[EMP ID] [int] NOT NULL,
[WORK DATE] [datetime] NOT NULL,
)
The pay_rates table will have many entries for each employee as their pay was adjusted over the years.
The work_dates table contains dates each employee worked.
Both of these tables contain other information, but I've simplified down to the relevant columns.
I would like to join the two such that I can see the what the pay rate would have been on each work date. More technically I want to join each record in work_dates to a record in pay_rates where with the largest EFFECTIVE DATE that is less than the WORK DATE and where the EMP ID matches.

Answer 1

Perhaps when you join pay_rates to work_dates, you could use not exists to require that no rate exists whose effective date is on or before the work date and is strictly after the effective date of the rate to which you're joining. Something like this:
declare #pay_rates table (
[EMP ID] [int] NOT NULL,
[RATE] [decimal](11, 6) NOT NULL,
[EFFECTIVE DATE] [datetime] NOT NULL
);
declare #work_dates table (
[EMP ID] [int] NOT NULL,
[WORK DATE] [datetime] NOT NULL
);
insert #pay_rates values
(1, 10.25, '20160615'),
(1, 10.65, '20170101'),
(1, 11.85, '20180101');
insert #work_dates values
(1, '20160701'),
(1, '20170101'),
(1, '20170701'),
(1, '20171231'),
(1, '20180102');
select
WorkDate.[EMP ID],
WorkDate.[WORK DATE],
Rate.RATE,
Rate.[EFFECTIVE DATE]
from
#work_dates WorkDate
inner join #pay_rates Rate on
WorkDate.[EMP ID] = Rate.[EMP ID] and
WorkDate.[WORK DATE] >= Rate.[EFFECTIVE DATE] and
not exists
(
select 1
from #pay_rates LaterRate
where
LaterRate.[EMP ID] = WorkDate.[EMP ID] and
LaterRate.[EFFECTIVE DATE] <= WorkDate.[WORK DATE] and
LaterRate.[EFFECTIVE DATE] > Rate.[EFFECTIVE DATE]
);
I have assumed here that you do not allow two pay rates to have taken effect on the same date for the same employee. In that case, the above query would give you two results for each work date that uses those rates, since it has no way to prioritize one over the other.

Answer 2

I think you need:
SELECT pr2.[emp id], pr2.rate
FROM pay_rates pr2
INNER JOIN
(SELECT wd.[emp id], MAX(pr.[effective date]) AS mx_date
FROM work_dates wd
LEFT JOIN pay_rates pr
ON (wd.[emp id] = pr.[emp id] AND wd.[work date] > pr.[effective date])
GROUP BY wd.[emp id]) sub
ON pr2.[emp id] = sub.[emp id] AND pr2.[effective date] = sub.mx_date
In the sub-query, select the largest effective day per emp id that is less than the employee's work date. Then join the result back to the pay rates table to pull in the rate.