mysql - How to make multiple LEFT JOINs with OR fully use a composite index? (part 2) -
    it system calculates how users scan fingerprints when enter/leave workplace. don't know how called in english. need determine if user late in morning, , if user leaves work early.   this tb_scan  table contains date , time user scans fingerprint.   create table `tb_scan` (   `scpercode` varchar(6) default null,   `scyear` varchar(4) default null,   `scmonth` varchar(2) default null,   `scday` varchar(2) default null,   `scscantime` datetime,   key `all` (`scyear`,`scmonth`,`scday`) ) engine=myisam default charset=latin1   it has 100,000+ rows, this   scpercode scyear scmonth scday     scdateandtime 000001    2010      10     10      2016-01-10 08:02:00 000001    2010      10     10      2016-01-02 17:33:00 000001    2010      10     11      2016-01-11 07:48:00 000001    2010      10     11      2016-01-11 17:29:00 000002    2010      10     10      2016-01-10 17:31:00 000002    2010      10     10      2016-01-02 17:28:00 000002    2010      10     11      2016-01-11 05:35:00 0...