Like MySQL/Impala group_concat and PostgreSQL string_agg. Takes a string array and a separator (possibly optional?) and returns one scalar string (one per group in the case of group aggregation) representing all the string values in the array concatenated together, with the separator added between each pair of concatenated values.
For example, in the case of no grouping and using separator "-", this would take input:
and return the following string scalar as output: