Thank you all. This kind of data structure in J has a lot of power and potential. Especially when the data is big. Jd database supports this. Bill mentions an article with a set of definitions. I got from Chris a bit different definition set. It would be nice to have a summary of this technology and further standardize inverted tables in J library. These definitions are not so easy to find by yourself as it shows below:
a +--------+--------+------+------------------+--------+--------+ |20160104|1003005 |SEY |300511379 |20140314|20150401| |20160104|1003006 |SEY |300533247 |20140429|20151112| |20160104|1003012 |VF |6002894281 |20140401|20150904| +--------+--------+------+------------------+--------+--------+ b +--------+--------+------+------------------+--------+--------+ |20160104|1013327 |VF |6002924680 |20140403|20160102| |20160104|1015127 |EONS |146149 |20090514|20150801| |20160104|1042009 |ISS |201509245169 |20151010|20151201| |20160104|1046107 |FE |14545413 |20150527|20160101| |20160104|1103401 |FE |14419077 |20140206|20160101| +--------+--------+------+------------------+--------+--------+ a,&.>b NB. This is Roger's solution and does what I want +--------+--------+------+------------------+--------+--------+ |20160104|1003005 |SEY |300511379 |20140314|20150401| |20160104|1003006 |SEY |300533247 |20140429|20151112| |20160104|1003012 |VF |6002894281 |20140401|20150904| |20160104|1013327 |VF |6002924680 |20140403|20160102| |20160104|1015127 |EONS |146149 |20090514|20150801| |20160104|1042009 |ISS |201509245169 |20151010|20151201| |20160104|1046107 |FE |14545413 |20150527|20160101| |20160104|1103401 |FE |14419077 |20140206|20160101| +--------+--------+------+------------------+--------+--------+ a,:b NB. I unsuccessfully tried this and many variations +--------+--------+------+------------------+--------+--------+ |20160104|1003005 |SEY |300511379 |20140314|20150401| |20160104|1003006 |SEY |300533247 |20140429|20151112| |20160104|1003012 |VF |6002894281 |20140401|20150904| +--------+--------+------+------------------+--------+--------+ |20160104|1013327 |VF |6002924680 |20140403|20160102| |20160104|1015127 |EONS |146149 |20090514|20150801| |20160104|1042009 |ISS |201509245169 |20151010|20151201| |20160104|1046107 |FE |14545413 |20150527|20160101| |20160104|1103401 |FE |14419077 |20140206|20160101| +--------+--------+------+------------------+--------+--------+ ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
