使用if语句从多个表中进行MySQL SELECT

Question

I have got two tables in a database to track who is currently onsite. There is a list of all staff members with their staff number, and a clock in/out log.

What I am trying to do is create a daily report of who is on site and who is absent. To do this I am checking the clock_log table for all records from today's date, like so:

SELECT `Clock_Log`.Clock_Number, `Clock_Log`.Date, `Clock_Log`.Time_In,
`Clock_Log`.Time_Out, IF(`Clock_Log`.Shift_Ended = 0, 'On Site', 'Shift Ended') 
AS Status FROM `Clock_Log` WHERE Date = CURDATE();

Which outputs something like:

Clock_Number | Date       | Time_In  | Time_Out | Status
123          | 2019-02-14 | 07:00:00 | 00:00:00 | On Site
456          | 2019-02-14 | 07:00:00 | 15:00:00 | Shift Ended

789          | 2019-02-14 | 00:00:00 | 00:00:00 | Absent

What I now need to do is select all records from the list of all staff, who have not clocked in for today, and append those to the results with the status set to Absent. As the third entry above.

Is this possible? If not could an additional column be added to the end which simply lists the numbers who aren't clocked in?

Answer 1

Use your staff table as your FROM, then left join this query to it

Also, aliases

select t1.*, t2.*
from all_staff t1
left join 
    (
    SELECT t3.Clock_Number, 
           t3.Date, 
           t3.Time_In,
           t3.Time_Out, 
           IF(t3.Shift_Ended = 0, 'On Site', 'Shift Ended') AS Status 
    FROM `Clock_Log` t3 WHERE Date = CURDATE() 
    ) t2
on t1.clock_number = t2.clock_number