[Backport 2.x] Fix back quoted alias of FROM subquery

docs/user/limitations/limitations.rst

@@ -34,23 +34,6 @@ Aggregation over expression is not supported for now. You can only apply aggrega
 Here's a link to the Github issue - [Issue #288](https://github.com/opendistro-for-elasticsearch/sql/issues/288).
 
 
-Limitations on Subqueries
-=========================
-
-Subqueries in the FROM clause
------------------------------
-
-Subquery in the `FROM` clause in this format: `SELECT outer FROM (SELECT inner)` is supported only when the query is merged into one query. For example, the following query is supported::
-
-    SELECT t.f, t.d
-    FROM (
-        SELECT FlightNum as f, DestCountry as d
-        FROM opensearch_dashboards_sample_data_flights
-        WHERE OriginCountry = 'US') t
-
-But, if the outer query has `GROUP BY` or `ORDER BY`, then it's not supported.
-
-
 Limitations on JOINs
 ====================
 

integ-test/src/test/resources/correctness/bugfixes/550.txt

@@ -0,0 +1,2 @@
+SELECT ABS(`flights`.`AvgTicketPrice`) FROM (SELECT `AvgTicketPrice` FROM `opensearch_dashboards_sample_data_flights`) AS `flights` GROUP BY ABS(`flights`.`AvgTicketPrice`)
+SELECT `b`.`Origin`, `b`.`avgPrice` FROM (SELECT `a`.`Origin` AS `Origin`, AVG(`AvgTicketPrice`) AS `avgPrice` FROM (SELECT `Origin`, `AvgTicketPrice` FROM `opensearch_dashboards_sample_data_flights` WHERE `FlightDelay` = True) AS `a` GROUP BY `a`.`Origin`) AS `b` ORDER BY `b`.`avgPrice` DESC

sql/src/main/java/org/opensearch/sql/sql/parser/AstBuilder.java

@@ -170,7 +170,8 @@ public UnresolvedPlan visitTableAsRelation(TableAsRelationContext ctx) {
 
   @Override
   public UnresolvedPlan visitSubqueryAsRelation(SubqueryAsRelationContext ctx) {
-    return new RelationSubquery(visit(ctx.subquery), ctx.alias().getText());
+    String subqueryAlias = StringUtils.unquoteIdentifier(ctx.alias().getText());
+    return new RelationSubquery(visit(ctx.subquery), subqueryAlias);
   }
 
   @Override

sql/src/test/java/org/opensearch/sql/sql/parser/AstBuilderTest.java

@@ -518,6 +518,27 @@ public void can_build_from_subquery() {
     );
   }
 
+  @Test
+  public void can_build_from_subquery_with_backquoted_alias() {
+    assertEquals(
+        project(
+            relationSubquery(
+                project(
+                    relation("test"),
+                    alias("firstname", qualifiedName("firstname"), "firstName")),
+                "a"),
+            alias("a.firstName", qualifiedName("a", "firstName"))
+        ),
+        buildAST(
+            "SELECT a.firstName "
+                + "FROM ( "
+                + " SELECT `firstname` AS `firstName` "
+                + " FROM `test` "
+                + ") AS `a`"
+        )
+    );
+  }
+
   @Test
   public void can_build_show_all_tables() {
     assertEquals(