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