type query = Query of select_stmt
and select_stmt = 
  | Select of column list * table list
and column = 
  | Asterisk
  | Column of string
and table = 
  | Table of string
  | Join of table * join_type * table * condition option
and join_type =
  | Inner
  | Left
  | Right
  | Full
  | Cross
  | Union
  | Natural
and condition = 
  | Condition of string * predicate
  | And of condition * condition
  | Or of condition * condition
  | Not of condition
and predicate = 
  | Comparison of operator * string
  | Between of string * string
and operator =
  | Equals
  | NotEquals
  | LessThan
  | GreaterThan
  | LessEquals
  | GreaterEquals
and filter = 
  | Filter of string
and search_condition =
  | Search of string