Comparing Datetime / Timestamp Values From Two Databases for Employee Scheduling (MySQL) - mysql

Question

My goal is to compare a table of employee schedules from one database, to their actual clock in times in another.
Here is the query I'm starting with to simply find a singular clock-in time when searching the clock-in database:
SELECT *
FROM `users_log`
WHERE `user` = 'Employee'
AND `type` = 'Login'
ORDER BY ABS(`logintime` - UNIX_TIMESTAMP(`scheduled_logintime_from_2nd_database`)))
LIMIT 1
Not sure on the best way to do this (specifically because using the ORDER BY in a subquery is iffy). But ultimately I'd want some sort of parent query / join that would join the two databases on the Employee Name (user), and show both the scheduled logintime, and actual logintime from the two databases.
In the above example, for scheduled_logintime_from_2nd_database I'm just using a text string for testing, eventually that'd be the actual column from the other database. (note one is a timestamp, the other a datetime, hence the UNIX_TIMESTAMP function).

Answer 1

You're very close.
I suggest you proceed as follows:
First, do an appropriate JOIN operation to gather your two sources of data together. Something like this:
SELECT DATE(ul.logintime) day,
ul.name, ul.logintime,
UNIX_TIMESTAMP(sch.scheduled_logintime) sched_time
FROM users_log ul
JOIN schedule sch ON ul.name = sch.name
AND DATE(ul.logintime) = DATE(sch.scheduled_logintime)
This should get you a bunch of rows, one or more for each day and user, showing scheduled and actual times.
Then you can use that as a subquery, perhaps doing something like this:
SELECT name, day,
MAX(ABS(logintime - sched_time)),
MIN(ABS(logintime - sched_time))
FROM ( SELECT DATE(ul.logintime) day,
ul.name, ul.logintime,
UNIX_TIMESTAMP(sch.scheduled_logintime) sched_time
FROM users_log ul
JOIN schedule sch ON ul.name = sch.name
AND DATE(ul.logintime) = DATE(sch.scheduled_logintime)
)
GROUP BY name, day
That should give you each name's best and worst adherence to a schedule in each day.