firstsortedvalue ([ distinct ] expression [, sort-weight [, n ]])
Returns the first value of expression sorted by corresponding sort-weight when expression is iterated over a number of records as defined by a group by clause. Sort-weight should return a numeric value where the lowest value will render the corresponding value of expression to be sorted first. By preceding the sort-value expression with a minus sign, the function will return the last value instead. If more than one value of expression share the same lowest sort-order, the function will return NULL. By stating an n larger than 1, the nth value in order will be returned. If the word distinct occurs before the expression, all duplicates will be disregarded.