Problem
Currently HIVE doesn't support subqueries in SELECT statement e.g. following query will not run on HIVE
SELECT customer.customer_num, (SELECT SUM(ship_charge) FROM orders WHERE customer.customer_num = orders.customer_num ) AS total_ship_chg FROM customer
Recently a lot of work has been done to extend support for subqueries(HIVE-15456). But this work primarily targeted to extend subquery support in WHERE and HAVING clause. We plan to continue the work done in HIVE-15456 to support subqueries in SELECT list.
Assumptions
We plan to limit the scope with following assumptions
Subqueries could only be top level expressions in select i.e. subqueries in complex expressions, aggregates, UDFs etc will not be supported for now. e.g. following queries will not run on HIVE
-- subquery in non-simple expression SELECT 1 + (SELECT SUM(ship_charge) FROM orders), customer.customer_num FROM customer -- subquery in CASE SELECT CASE WHEN (select count(*) from store_sales where ss_quantity between 1 and 20) > 409437 THEN (select avg(ss_ext_list_price) from store_sales where ss_quantity between 1 and 20) ELSE (select avg(ss_net_paid_inc_tax) from store_sales where ss_quantity between 1 and 20) end bucket1 FROM reason WHERE r_reason_sk = 1