diff --git a/lib/ast.ml b/lib/ast.ml
index 4a000be..8e43286 100644
--- a/lib/ast.ml
+++ b/lib/ast.ml
@@ -1,9 +1,12 @@
 type query = Query of select_stmt
 and select_stmt = 
-  | Select of column list * table list * filter option
+  | Select of column list * table list option * filter option
 and column = 
   | Asterisk
-  | Column of string
+(*  | Column of string *)
+  | Column of expression * as_clause option
+and as_clause =
+  | As of string
 and table = 
   | Table of string
   | Join of table * join_type * table * condition option
@@ -20,8 +23,7 @@ and condition =
   | And of condition * condition
   | Or of condition * condition
   | Not of condition
-and predicand = 
-  | Ref of string
+and predicand = expression
 and predicate = 
   | Comparison of operator * predicand
   | Between of predicand * predicand
@@ -39,8 +41,24 @@ and operator =
   | GreaterEquals
 and filter = 
   | Filter of condition
-and search_condition =
-  | Search of string
+and aggregate = 
+  | Aggregate of func * filter option
+and func =
+  | Function of function_type * quantifier option * expression
+and function_type = 
+  | Avg
+  | Max
+  | Min
+  | Sum
+  | Count
+and quantifier = 
+  | All
+  | Distinct
+and expression = 
+  | Ref of string
+  | StringLiteral of string
+  | DateLiteral of string
+  | TimeLiteral of string
 
 let rec pp_query fmt ast = 
   match ast with
@@ -58,8 +76,9 @@ and pp_columns cols =
 
 and pp_column col = 
   match col with
-  | Column(name) -> name
+  | Column(Ref(name),_) -> name
   | Asterisk -> "*"
+  | _ -> failwith "not supported"
 
 and pp_tables tables =
   match tables with
diff --git a/lib/logical_plan.ml b/lib/logical_plan.ml
index 8ef8dca..9fe1f4d 100644
--- a/lib/logical_plan.ml
+++ b/lib/logical_plan.ml
@@ -19,9 +19,10 @@ let rec generate_from_clause tables =
 
 let generate_logical_plan ast =
   match ast with 
-  | Ast.Query(Select(_, tables, _)) -> 
+  | Ast.Query(Select(_, Some(tables), _)) -> 
       let base_plan = generate_from_clause tables in
       base_plan
+  | Ast.Query(Select(_, None, _)) -> failwith "not supported"
 
 (*let evaluate_plan plan =
   match plan with