SQL alternative to sub-query in SELECT Item list

Question

I have RDBMS table and Queries which are working perfectly. I have offloaded data from RDBMS to HIVE table.To run the existing queries on HIVE, we need first to make them compatible to HIVE.

Let's take below example with sub-query in select item list. It is syntactically valid and working fine on RDBMS system. But It Will not work on HIVE As per HIVE manual , Hive supports subqueries only in the FROM and WHERE clause.

Example 1 :

SELECT t1.one
    ,(SELECT t2.two 
        FROM TEST2 t2 
        WHERE t1.one=t2.two) t21
    ,(SELECT t3.three 
        FROM TEST3 t3 
        WHERE t1.one=t3.three) t31
FROM TEST1 t1 ;

Example 2:

SELECT a.*
   , CASE
          WHEN EXISTS
                 (SELECT 1
                    FROM tblOrder O
                      INNER JOIN tblProduct P
                              ON O.Product_id = P.Product_id
                   WHERE O.customer_id        = C.customer_id
                     AND P.Product_Type IN (2, 5, 6, 9)
                 )
          THEN 1
          ELSE 0
   END AS My_Custom_Indicator
   FROM tblCustomer C
   INNER JOIN tblOtherStuff S
       ON C.CustomerID = S.CustomerID ;

Example 3 :

Select component_location_id, component_type_code,
  ( select clv.LOCATION_VALUE 
  from stg_dev.component_location_values clv 
  where  identifier_code = 'AXLE' 
       and component_location_id = cl.component_location_id ) as AXLE,
  ( select clv.LOCATION_VALUE 
  from stg_dev.component_location_values clv 
  where identifier_code = 'SIDE' 
       and component_location_id = cl.component_location_id ) as SIDE
  from stg_dev.component_locations cl ;

I want to know the possible alternative of sub-queries in select item list to make it compatible to hive. Apparently I will be able to transform existing queries in HIVE format.

Any help and guidance is highly appreciated !

Answer 1

The query you provided could be transformed to a simple query with LEFT JOIN s.

SELECT
  t1.one, t2.two AS t21, t3.three AS t31
FROM
  TEST1 t1
  LEFT JOIN TEST2 t2
    ON t1.one = t2.two
  LEFT JOIN TEST3 t3
    ON t1.one = t3.three

Since there is no limitation in the subqueries, the joins will return the same data. (The subqueries should return only one or no row for each row in TEST1.)

Please note, that your original query could not handle 1..n connections. In most DBMS, subqueries in the SELECT list should return only with a resultset with one columns and one or no row.

Answer 2

Based on HIVE manual :

SELECT t1.one, t2.two, t3.three 
FROM TEST1 t1,TEST2 t2, TEST3 t3
WHERE t1.one=t2.two AND t1.one=t3.three;

Answer 3

    SELECT t1.one,t2.two,t3.three FROM TEST1 t1 INNER 
   JOIN TEST2 t2 ON t1.one=t2.two INNER JOIN TEST3 t3 
   ON t1.one=t3.three WHERE t1.one=t2.two AND t1.one=t3.three;

Answer 4

SELECT t1.one,t2.two as t21,t3.three as t31 FROM TEST1 t1 
INNER JOIN TEST2 t2 ON t1.one=t2.two
INNER JOIN TEST3 t3 ON t1.one=t3.three