Problem

Currently HIVE doesn't support subqueries in SELECT statement e.g. following query will not run on HIVE

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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 assumption/limitations

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• Scalar subquery can only return at most one row. HIVE will check for this case at runtime and throw an error if not satisfied. e.g. following query is invalid

  Code Block
  language sql
  SELECT customer.customer_num, (SELECT ship_charge FROM orders WHERE customer.customer_num = orders.customer_num ) AS total_ship_chg FROM customer

• Scalar subquery can only have one column. HIVE will check for this case during compilation and throw an error. e.g following query is invalid

  Code Block
  language sql
  SELECT customer.customer_num, (SELECT ship_charge, customer_num FROM orders LIMIT 1 ) AS total_ship_chg FROM customer

Design

Given the assumptions above following kind of subqueries could be used in select 

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