Understanding Someone Elses MYSQL

Working through someone elses lovely Legacy code I've come to this MySQL statement that seems a bit odd.  So this blog is about trying to understand it.  Which by the way ultimately I don't.  If you do and can explain it to me better than these notes then let me know :)  After this blog I will be going away and trying prove it is wrong - so please check future posts.

SELECT pay_rate, LEFT(TIMEDIFF('24:00:00',TIMEDIFF(start_time, end_time)),5) AS hours, hours_only FROM shifts WHERE (employee_id = '160' AND shift_date BETWEEN '2010-11-12' AND '2010-11-18') OR (employee_id = '160' AND postdate_staff_invoice = '2')



SELECT pay_rate,  // we only looking to return 1 field



LEFT     // check here for 'LEFT' explained  - http://www.roseindia.net/sql/mysql-example/mysql-left.shtml  -

take this example  mysql> SELECT left ('GirishTewari',6)as LeftString;

this means that in this example we're taking the first charcters of this result

TIMEDIFF('24:00:00',TIMEDIFF(start_time, end_time))

#########

So what does

TIMEDIFF('24:00:00',TIMEDIFF(start_time, end_time))

mean.  Well

TIMEDIFF() returns expr1 – expr2 expressed as a time value. expr1 and expr2 are time or date-and-time expressions, but both must be of the same type.

* in this example the second expression will be calculated from the row from the database we're in.

AS   - I normally use this after calling FROM as a form of shorthand ie
         
FROM tableOne AS t1


#################

WHERE (employee_id = '160' AND shift_date BETWEEN '2010-11-12' AND '2010-11-18') OR (employee_id = '160' AND postdate_staff_invoice = '2')

Well at least this makes more sense - except for what's after the 'OR'


To be honest this mysql looks all wrong.  I think the next stage is to find out what result we're expecting.  IS IT JUST TO FIND OUT THE PAY RATE !!


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Here's some searches I've made in looking for answers.


MYSQL LEFT EXPLAINED