[SPARK-4226] [SQL] Support IN/EXISTS Subqueries

### What changes were proposed in this pull request?
This PR adds support for in/exists predicate subqueries to Spark. Predicate sub-queries are used as a filtering condition in a query (this is the only supported use case). A predicate sub-query comes in two forms:

- `[NOT] EXISTS(subquery)`
- `[NOT] IN (subquery)`

This PR is (loosely) based on the work of davies (https://github.com/apache/spark/pull/10706) and chenghao-intel (https://github.com/apache/spark/pull/9055). They should be credited for the work they did.

### How was this patch tested?
Modified parsing unit tests.
Added tests to `org.apache.spark.sql.SQLQuerySuite`

cc rxin, davies & chenghao-intel

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #12306 from hvanhovell/SPARK-4226.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala
index b3e8b37a2e..71b6a97852 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala
@@ -18,8 +18,9 @@
 package org.apache.spark.sql.execution
 
 import org.apache.spark.sql.SQLContext
-import org.apache.spark.sql.catalyst.{expressions, InternalRow}
-import org.apache.spark.sql.catalyst.expressions.{ExprId, Literal, SubqueryExpression}
+import org.apache.spark.sql.catalyst.expressions
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.{Expression, ExprId, Literal, SubqueryExpression}
 import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, ExprCode}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.Rule
@@ -42,6 +43,7 @@ case class ScalarSubquery(
   override def plan: SparkPlan = Subquery(simpleString, executedPlan)
 
   override def dataType: DataType = executedPlan.schema.fields.head.dataType
+  override def children: Seq[Expression] = Nil
   override def nullable: Boolean = true
   override def toString: String = s"subquery#${exprId.id}"