Suppose I have the following table where there are two sets of observation dates (2015-01-01, 2016-01-01) in the first column. For each observation date, there are associated item_date and item_value. observation_date item_date item_value 2015-01-01 2012-12-31 0 2015-01-01 2013-03-31 1 2015-01-01 2013-06-30 2 2015-01-01 2013-09-30 3 2015-01-01 2013-12-31 4 2015-01-01 2014-03-31 5 2015-01-01 2014-06-30 6 2015-01-01 2014-09-30 7 2016-01-01 2013-09-30 8