How Does Subquery Support Differ Across Spark SQL Versions?

Subquery Support in Spark SQL

Spark SQL offers comprehensive subquery support. Here's an exploration of subqueries in Spark SQL.

Spark 2.0 and Later

Spark SQL in versions 2.0 and above boasts robust subquery capabilities, including:

• Correlated Subqueries: Allow subqueries to reference columns from the outer query.
• Uncorrelated Subqueries: Exist independently of the outer query.

Subquery Usage Examples

• select * from l where exists (select * from r where l.a = r.c)
• select * from l where a in (select c from r)

Note: DataFrame DSL is currently insufficient to express subquery logic in Spark versions prior to 2.0.

Spark Versions Prior to 2.0

In Spark versions below 2.0, subqueries are limited to the FROM clause:

• SELECT col FROM (SELECT * FROM t1 WHERE bar) t2

Subquery Limitations

Subqueries in the WHERE clause are not supported in Spark versions prior to 2.0. This is because arbitrary subqueries, particularly correlated ones, cannot be efficiently expressed using Spark's Cartesian join capabilities. Nonetheless, subqueries in the FROM clause provide an effective alternative.

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