如何在内部联接中放入空集

Question

I am a complete beginner in MySql and I wanted you to help me. I want a DTR User Interface where there is time in and time out. So I have a fields like - id_biometrics,empno,datecreated,time_created,status,device the problem is if the employee forgot to time in or time out, I want to put the time in or time out to "NULL" here is what I coded.

SELECT start_log.empno AS "Employee Number", start_log.date_created, start_log.time_created AS "Time In", end_log.time_created AS "Time Out"    
FROM biometrics AS start_log
INNER JOIN biometrics AS end_log ON start_log.empno = end_log.empno
WHERE start_log.status =0
AND end_log.status =1
AND start_log.empno =2
GROUP BY start_log.date_created, start_log.empno

And what my output here if he completes his time in and time out for the day is.

Employee Number|date_created |Time_in   | Time_Out
             2 | 2013-07-15  | 11:08:07 | 15:00:00

And if he/she forgots to Time Out or time in it shows MySQL returned an empty result set (ie zero rows). ( Query took 0.0016 sec )

What I wanted is

Employee Number | date_created |Time_in   | Time_Out
         3      | 2013-07-15   | 11:50:00 | Null

Please help me with this. I needed it badly thanks.

Answer 1

Are you looking for this?

SELECT empno, date_created, 
       MIN(CASE WHEN status = 0 THEN time_created END) time_in,
       MIN(CASE WHEN status = 1 THEN time_created END) time_out
  FROM biometrics
 GROUP BY empno, date_created

Sample output:

+-------+--------------+----------+----------+
| empno | date_created | time_in  | time_out |
+-------+--------------+----------+----------+
|     2 | 2013-07-15   | 11:08:07 | 15:00:00 |
|     3 | 2013-07-15   | 11:50:00 | NULL     |
|     4 | 2013-07-15   | NULL     | 16:00:00 |
+-------+--------------+----------+----------+

Here is SQLFiddle demo

UPDATE If you may have multiple in or out records and you always want to fetch first in and last out then just use MAX() aggregate for time_out

SELECT empno, date_created, 
       MIN(CASE WHEN status = 0 THEN time_created END) time_in,
       MAX(CASE WHEN status = 1 THEN time_created END) time_out
  FROM biometrics
 GROUP BY empno, date_created

Here is SQLFiddle demo