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We plan to limit the scope with following assumption
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
Code Block language sql -- subquery in non-simple expression SELECT 1 + (SELECT SUM(ship_charge) FROM orders), customer.customer_num FROM customer
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-- 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
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