Wrap nullable hoisted filter in IsTrue

Author: angelamayxie

sql/analyzer/hoist_filters.go

@@ -131,7 +131,13 @@ func recurseSubqueryForOuterFilters(n sql.Node, a *Analyzer, corr sql.ColSet) (s
 				newCorr = newCorr.Union(outOfScope)
 				keepFilters = append(keepFilters, e)
 			} else {
-				// nothing tethers the subquery to this scope
+				// nothing tethers the subquery or filter expression to this scope
+				if sq == nil && e.IsNullable() {
+					// If a filter expression has been hoisted out of a subquery, it needs to be made not nullable. This
+					// is because filters that evaluate to null in an Exists or In subquery are treated the same as
+					// false.
+					e = expression.NewIsTrue(e)
+				}
 				hoistFilters = append(hoistFilters, e)
 			}
 		}