Code:
-sh-4.1$ join -1 1 -2 1 -t, a1 b1
1,ABC,GGG,20160401,01-APR-16 07.11.43.380674 PM,TGG,1
2,XYZ,KKK,20160401,01-APR-16 07.40.44.223631 PM,YYH,2
3,LMN,TTT,20160401,04-APR-16 12.45.08.531580 AM,JUI,6
4,OPQ,QQQ,20160401,02-APR-16 01.36.40.508209 AM,TSH,3
5,STV,III,20160401,02-APR-16 03.48.06.361722 AM,OPP,5
6,WXY,PPP,20160401,04-APR-16 03.03.02.873126 AM,RRT,13
join: file 2 is not in sorted order
8,TTT,ARR,20160401,02-APR-16 01.54.24.063732 AM,QWERT,4
join: file 1 is not in sorted order