Contents
The subquery is correlated because the number that it produces depends on main. ship_date, a value that the outer SELECT produces. Thus, the subquery must be re-executed for every row that the outer query considers. The query uses the COUNT function to return a value to the main query.
What is subquery in hive?
Hive supports subqueries in FROM clauses and in WHERE clauses of SQL statements. A subquery is a SQL expression that is evaluated and returns a result set. Then that result set is used to evaluate the parent query. The parent query is the outer query that contains the child subquery.
How many times correlated subquery will get executed?
Working. A non-correlated subquery is executed only once and its result can be swapped back for a query, on the other hand, a correlated subquery is executed multiple times, precisely once for each row returned by the outer query. SELECT MAX(Salary) from Employee where Salary NOT IN (10000).
What is faster a correlated subquery or an inner join?
The advantage of a join includes that it executes faster. The retrieval time of the query using joins almost always will be faster than that of a subquery. By using joins, you can maximize the calculation burden on the database i.e., instead of multiple queries using one join query.
Does subquery run first?
With a normal nested subquery, the inner SELECT query runs first and executes once, returning values to be used by the main query. A correlated subquery, however, executes once for each candidate row considered by the outer query. In other words, the inner query is driven by the outer query.
Can we use subquery in Hive?
Hive supports subqueries only in the FROM clause (through Hive 0.12). The subquery has to be given a name because every table in a FROM clause must have a name. The columns in the subquery select list are available in the outer query just like columns of a table. The subquery can also be a query expression with UNION.
Does Hive support not in?
Unfortunately, Hive doesn’t support in, exists or subqueries.
How to use subqueries in select statement in hive?
Currently Hive doesn’t support subqueries in a SELECT statement, for example, the 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
Which is an invalid query in Apache Hive?
For example the following query is invalid: SELECT customer.customer_num, (SELECT ship_charge FROM orders WHERE customer.customer_num = orders.customer_num ) AS total_ship_chg FROM customer Scalar subqueries can only have one column. Hive will check for this case during compilation and throw an error.
Are there subqueries in select-Apache Software Foundation?
Recently a lot of work has been done to extend support for subqueries ( HIVE-15456 ). But this work primarily targeted extending subquery support in WHERE and HAVING clauses. We plan to continue the work done in HIVE-15456 to support subqueries in a select list (see HIVE-16091 ).
Can a subquery be a complex expression in select?
We plan to limit the scope with the following assumptions and limitations. Subqueries could only be top-level expressions in SELECT. That is, subqueries in complex expressions, aggregates, UDFs, etc. will not be supported for now.