module Sequel
  if RUBY_VERSION < '1.9.0'
    # If on Ruby 1.8, create a Sequel::BasicObject class that is similar to the
    # the Ruby 1.9 BasicObject class.  This is used in a few places where proxy
    # objects are needed that respond to any method call.
    class BasicObject
      (instance_methods - %w"__id__ __send__ instance_eval == equal?").each{|m| undef_method(m)}
    end
  else
    # If on 1.9, create a Sequel::BasicObject class that is just like the
    # default BasicObject class, except that missing constants are resolved in
    # Object.  This allows the virtual row support to work with classes
    # without prefixing them with ::, such as:
    #
    #   DB[:bonds].filter{maturity_date > Time.now}
    class BasicObject < ::BasicObject
      # Lookup missing constants in ::Object
      def self.const_missing(name)
        ::Object.const_get(name)
      end
    end
  end

  class LiteralString < ::String
  end

  # The SQL module holds classes whose instances represent SQL fragments.
  # It also holds modules that are included in core ruby classes that
  # make Sequel a friendly DSL.
  module SQL

    ### Parent Classes ###

    # Classes/Modules aren't an alphabetical order due to the fact that
    # some reference constants defined in others at load time.

    # Base class for all SQL fragments
    class Expression
      # Create a to_s instance method that takes a dataset, and calls
      # the method provided on the dataset with args as the argument (self by default).
      # Used to DRY up some code.
      def self.to_s_method(meth, args=:self) # :nodoc:
        class_eval("def to_s(ds); ds.#{meth}(#{args}) end", __FILE__, __LINE__)
      end
      private_class_method :to_s_method

      # Returns self, because SQL::Expression already acts like
      # LiteralString.
      def lit
        self
      end
      
      # Alias for to_s
      def sql_literal(ds)
        to_s(ds)
      end
    end

    # Represents a complex SQL expression, with a given operator and one
    # or more attributes (which may also be ComplexExpressions, forming
    # a tree).  This class is the backbone of the blockless filter support in
    # Sequel.
    #
    # This is an abstract class that is not that useful by itself.  The
    # subclasses BooleanExpression, NumericExpression, and StringExpression
    # define the behavior of the DSL via operators.
    class ComplexExpression < Expression
      # A hash of the opposite for each operator symbol, used for inverting
      # objects.
      OPERTATOR_INVERSIONS = {:AND => :OR, :OR => :AND, :< => :>=, :> => :<=,
        :<= => :>, :>= => :<, :'=' => :'!=' , :'!=' => :'=', :LIKE => :'NOT LIKE',
        :'NOT LIKE' => :LIKE, :~ => :'!~', :'!~' => :~, :IN => :'NOT IN',
        :'NOT IN' => :IN, :IS => :'IS NOT', :'IS NOT' => :IS, :'~*' => :'!~*',
        :'!~*' => :'~*', :NOT => :NOOP, :NOOP => :NOT, :ILIKE => :'NOT ILIKE',
        :'NOT ILIKE'=>:ILIKE}

      # Mathematical Operators used in NumericMethods
      MATHEMATICAL_OPERATORS = [:+, :-, :/, :*]

      # Mathematical Operators used in NumericMethods
      BITWISE_OPERATORS = [:&, :|, :^, :<<, :>>]

      # Inequality Operators used in InequalityMethods
      INEQUALITY_OPERATORS = [:<, :>, :<=, :>=]

      # Hash of ruby operator symbols to SQL operators, used in BooleanMethods
      BOOLEAN_OPERATOR_METHODS = {:& => :AND, :| =>:OR}

      # Operators that use IS, used for special casing to override literal true/false values
      IS_OPERATORS = [:IS, :'IS NOT']

      # Operator symbols that take exactly two arguments
      TWO_ARITY_OPERATORS = [:'=', :'!=', :LIKE, :'NOT LIKE', \
        :~, :'!~', :'~*', :'!~*', :IN, :'NOT IN', :ILIKE, :'NOT ILIKE'] + \
        INEQUALITY_OPERATORS + BITWISE_OPERATORS + IS_OPERATORS 

      # Operator symbols that take one or more arguments
      N_ARITY_OPERATORS = [:AND, :OR, :'||'] + MATHEMATICAL_OPERATORS

      # Operator symbols that take one argument
      ONE_ARITY_OPERATORS = [:NOT, :NOOP, :'B~']

      # An array of args for this object
      attr_reader :args

      # The operator symbol for this object
      attr_reader :op
      
      # Set the operator symbol and arguments for this object to the ones given.
      # Convert all args that are hashes or arrays with all two pairs to BooleanExpressions.
      # Raise an error if the operator doesn't allow boolean input and a boolean argument is given.
      # Raise an error if the wrong number of arguments for a given operator is used.
      def initialize(op, *args)
        args.map!{|a| Sequel.condition_specifier?(a) ? SQL::BooleanExpression.from_value_pairs(a) : a}
        case op
        when *N_ARITY_OPERATORS
          raise(Error, "The #{op} operator requires at least 1 argument") unless args.length >= 1
        when *TWO_ARITY_OPERATORS
          raise(Error, "The #{op} operator requires precisely 2 arguments") unless args.length == 2
        when *ONE_ARITY_OPERATORS
          raise(Error, "The #{op} operator requires a single argument") unless args.length == 1
        else
          raise(Error, "Invalid operator #{op}")
        end
        @op = op
        @args = args
      end

      # Returns true if the receiver is the same expression as the
      # the +other+ expression.
      def eql?(other)
        other.is_a?(self.class) && @op.eql?(other.op) && @args.eql?(other.args)
      end
      alias == eql?
      
      to_s_method :complex_expression_sql, '@op, @args'
    end

    # The base class for expressions that can be used in multiple places in
    # the SQL query.  
    class GenericExpression < Expression
    end
    
    ### Modules ###

    # Methods that create aliased identifiers
    module AliasMethods
      # Create an SQL column alias of the receiving column or expression to the given alias.
      def as(aliaz)
        AliasedExpression.new(self, aliaz)
      end
    end

    # This defines the bitwise methods: &, |, ^, ~, <<, and >>.  Because these
    # methods overlap with the standard BooleanMethods methods, and they only
    # make sense for numbers, they are only included in NumericExpression.
    module BitwiseMethods
      ComplexExpression::BITWISE_OPERATORS.each do |o|
        define_method(o) do |ce|
          case ce
          when BooleanExpression, StringExpression
            raise(Sequel::Error, "cannot apply #{o} to a non-numeric expression")
          else  
            NumericExpression.new(o, self, ce)
          end
        end
      end

      # Do the bitwise compliment of the self
      def ~
        NumericExpression.new(:'B~', self)
      end
    end

    # This module includes the methods that are defined on objects that can be 
    # used in a boolean context in SQL (Symbol, LiteralString, SQL::Function,
    # and SQL::BooleanExpression).
    #
    # This defines the ~ (NOT), & (AND), and | (OR) methods.
    module BooleanMethods
      # Create a new BooleanExpression with NOT, representing the inversion of whatever self represents.
      def ~
        BooleanExpression.invert(self)
      end
      
      ComplexExpression::BOOLEAN_OPERATOR_METHODS.each do |m, o|
        define_method(m) do |ce|
          case ce
          when NumericExpression, StringExpression
            raise(Sequel::Error, "cannot apply #{o} to a non-boolean expression")
          else  
            BooleanExpression.new(o, self, ce)
          end
        end
      end
    end

    # Holds methods that are used to cast objects to differen SQL types.
    module CastMethods 
      # Cast the reciever to the given SQL type.  You can specify a ruby class as a type,
      # and it is handled similarly to using a database independent type in the schema methods.
      def cast(sql_type)
        Cast.new(self, sql_type)
      end

      # Cast the reciever to the given SQL type (or the database's default integer type if none given),
      # and return the result as a NumericExpression. 
      def cast_numeric(sql_type = nil)
        cast(sql_type || Integer).sql_number
      end

      # Cast the reciever to the given SQL type (or the database's default string type if none given),
      # and return the result as a StringExpression, so you can use +
      # directly on the result for SQL string concatenation.
      def cast_string(sql_type = nil)
        cast(sql_type || String).sql_string
      end
    end
    
    # Adds methods that allow you to treat an object as an instance of a specific
    # ComplexExpression subclass.  This is useful if another library
    # overrides the methods defined by Sequel.
    #
    # For example, if Symbol#/ is overridden to produce a string (for
    # example, to make file system path creation easier), the
    # following code will not do what you want:
    #
    #   :price/10 > 100
    #
    # In that case, you need to do the following:
    #
    #   :price.sql_number/10 > 100
    module ComplexExpressionMethods
      # Extract a datetime_part (e.g. year, month) from self:
      #
      #   :date.extract(:year) # SQL:  extract(year FROM "date")
      #
      # Also has the benefit of returning the result as a
      # NumericExpression instead of a generic ComplexExpression.
      #
      # The extract function is in the SQL standard, but it doesn't
      # doesn't use the standard function calling convention.
      def extract(datetime_part)
        Function.new(:extract, PlaceholderLiteralString.new("#{datetime_part} FROM ?", [self])).sql_number
      end

      # Return a BooleanExpression representation of self.
      def sql_boolean
        BooleanExpression.new(:NOOP, self)
      end

      # Return a NumericExpression representation of self.
      def sql_number
        NumericExpression.new(:NOOP, self)
      end

      # Return a StringExpression representation of self.
      def sql_string
        StringExpression.new(:NOOP, self)
      end
    end

    # Includes a method that returns Identifiers.
    module IdentifierMethods
      # Return self wrapped as an identifier.
      def identifier
        Identifier.new(self)
      end
    end

    # This module includes the methods that are defined on objects that can be 
    # used in a numeric or string context in SQL (Symbol (except on ruby 1.9), LiteralString, 
    # SQL::Function, and SQL::StringExpression).
    #
    # This defines the >, <, >=, and <= methods.
    module InequalityMethods
      ComplexExpression::INEQUALITY_OPERATORS.each do |o|
        define_method(o) do |ce|
          case ce
          when BooleanExpression, TrueClass, FalseClass, NilClass, Hash, Array
            raise(Error, "cannot apply #{o} to a boolean expression")
          else  
            BooleanExpression.new(o, self, ce)
          end
        end
      end
    end

    # This module augments the default initalize method for the 
    # ComplexExpression subclass it is included in, so that
    # attempting to use boolean input when initializing a NumericExpression
    # or StringExpression results in an error.
    module NoBooleanInputMethods
      # Raise an Error if one of the args would be boolean in an SQL
      # context, otherwise call super.
      def initialize(op, *args)
        args.each do |a|
          case a
          when BooleanExpression, TrueClass, FalseClass, NilClass, Hash, Array
            raise(Error, "cannot apply #{op} to a boolean expression")
          end
        end
        super
      end
    end

    # This module includes the methods that are defined on objects that can be 
    # used in a numeric context in SQL (Symbol, LiteralString, SQL::Function,
    # and SQL::NumericExpression).
    #
    # This defines the +, -, *, and / methods.
    module NumericMethods
      ComplexExpression::MATHEMATICAL_OPERATORS.each do |o|
        define_method(o) do |ce|
          case ce
          when BooleanExpression, StringExpression
            raise(Sequel::Error, "cannot apply #{o} to a non-numeric expression")
          else  
            NumericExpression.new(o, self, ce)
          end
        end
      end
    end

    # Methods that create OrderedExpressions, used for sorting by columns
    # or more complex expressions.
    module OrderMethods
      # Mark the receiving SQL column as sorting in a descending fashion.
      def desc
        OrderedExpression.new(self)
      end
      
      # Mark the receiving SQL column as sorting in an ascending fashion (generally a no-op).
      def asc
        OrderedExpression.new(self, false)
      end
    end

    # Methods that created QualifiedIdentifiers, used for qualifying column
    # names with a table or table names with a schema.
    module QualifyingMethods
      # Qualify the current object with the given table/schema.
      def qualify(ts)
        QualifiedIdentifier.new(ts, self)
      end
    end

    # This module includes the methods that are defined on objects that can be 
    # used in a string context in SQL (Symbol, LiteralString, SQL::Function,
    # and SQL::StringExpression).
    #
    # This defines the like (LIKE) and ilike methods, used for pattern matching.
    # like is case sensitive, ilike is case insensitive.
    module StringMethods
      # Create a BooleanExpression case insensitive pattern match of self
      # with the given patterns.  See StringExpression.like.
      def ilike(*ces)
        StringExpression.like(self, *(ces << {:case_insensitive=>true}))
      end

      # Create a BooleanExpression case sensitive pattern match of self with
      # the given patterns.  See StringExpression.like.
      def like(*ces)
        StringExpression.like(self, *ces)
      end
    end

    # This module is included in StringExpression and can be included elsewhere
    # to allow the use of the + operator to represent concatenation of SQL
    # Strings:
    #
    #   :x.sql_string + :y => # SQL: x || y
    module StringConcatenationMethods
      def +(ce)
        StringExpression.new(:'||', self, ce)
      end
    end

    # Methods that create Subscripts (SQL array accesses).
    module SubscriptMethods
      # Return an SQL array subscript with the given arguments.
      #
      #   :array.sql_subscript(1) # SQL: array[1]
      #   :array.sql_subscript(1, 2) # SQL: array[1, 2]
      def sql_subscript(*sub)
        Subscript.new(self, sub.flatten)
      end
    end

    ### Classes ###

    # Represents an aliasing of an expression/column to a given name.
    class AliasedExpression < Expression
      # The expression to alias
      attr_reader :expression

      # The alias to use for the expression, not alias since that is
      # a keyword in ruby.
      attr_reader :aliaz

      # Create an object with the given expression and alias.
      def initialize(expression, aliaz)
        @expression, @aliaz = expression, aliaz
      end

      to_s_method :aliased_expression_sql
    end

    # Blob is used to represent binary data in the Ruby environment that is
    # stored as a blob type in the database. Sequel represents binary data as a Blob object because 
    # certain database engines require binary data to be escaped.
    class Blob < ::String
      # Returns self
      def to_sequel_blob
        self
      end
    end

    # Subclass of ComplexExpression where the expression results
    # in a boolean value in SQL.
    class BooleanExpression < ComplexExpression
      include BooleanMethods
      
      # Take pairs of values (e.g. a hash or array of arrays of two pairs)
      # and converts it to a BooleanExpression.  The operator and args
      # used depends on the case of the right (2nd) argument:
      #
      # * 0..10 - left >= 0 AND left <= 10
      # * [1,2] - left IN (1,2)
      # * nil - left IS NULL
      # * /as/ - left ~ 'as'
      # * :blah - left = blah
      # * 'blah' - left = 'blah'
      #
      # If multiple arguments are given, they are joined with the op given (AND
      # by default, OR possible).  If negate is set to true,
      # all subexpressions are inverted before used.  Therefore, the following
      # expressions are equivalent:
      #
      #   ~from_value_pairs(hash)
      #   from_value_pairs(hash, :OR, true)
      def self.from_value_pairs(pairs, op=:AND, negate=false)
        pairs = pairs.collect do |l,r|
          ce = case r
          when Range
            new(:AND, new(:>=, l, r.begin), new(r.exclude_end? ? :< : :<=, l, r.end))
          when Array, ::Sequel::Dataset, SQLArray
            new(:IN, l, r)
          when NilClass, TrueClass, FalseClass
            new(:IS, l, r)
          when Regexp
            StringExpression.like(l, r)
          else
            new(:'=', l, r)
          end
          negate ? invert(ce) : ce
        end
        pairs.length == 1 ? pairs.at(0) : new(op, *pairs)
      end
      
      # Invert the expression, if possible.  If the expression cannot
      # be inverted, raise an error.  An inverted expression should match everything that the
      # uninverted expression did not match, and vice-versa, except for possible issues with
      # SQL NULL (i.e. 1 == NULL is NULL and 1 != NULL is also NULL).
      def self.invert(ce)
        case ce
        when BooleanExpression
          case op = ce.op
          when :AND, :OR
            BooleanExpression.new(OPERTATOR_INVERSIONS[op], *ce.args.collect{|a| BooleanExpression.invert(a)})
          else
            BooleanExpression.new(OPERTATOR_INVERSIONS[op], *ce.args.dup)
          end
        when StringExpression, NumericExpression
          raise(Sequel::Error, "cannot invert #{ce.inspect}")
        else
          BooleanExpression.new(:NOT, ce)
        end
      end
    end

    # Represents an SQL CASE expression, used for conditions.
    class CaseExpression < GenericExpression
      # An array of all two pairs with the first element specifying the
      # condition and the second element specifying the result.
      attr_reader :conditions

      # The default value if no conditions are true
      attr_reader :default

      # The expression to test the conditions against
      attr_reader :expression

      # Create an object with the given conditions and
      # default value.
      def initialize(conditions, default, expression = nil)
        raise(Sequel::Error, 'CaseExpression conditions must be a hash or array of all two pairs') unless Sequel.condition_specifier?(conditions)
        @conditions, @default, @expression = conditions.to_a, default, expression
      end

      to_s_method :case_expression_sql
    end

    # Represents a cast of an SQL expression to a specific type.
    class Cast < GenericExpression
      # The expression to cast
      attr_reader :expr

      # The type to which to cast the expression
      attr_reader :type
      
      # Set the attributes to the given arguments
      def initialize(expr, type)
        @expr = expr
        @type = type
      end

      to_s_method :cast_sql, '@expr, @type'
    end

    # Represents all columns in a given table, table.* in SQL
    class ColumnAll < Expression
      # The table containing the columns being selected
      attr_reader :table

      # Create an object with the given table
      def initialize(table)
        @table = table
      end

      # ColumnAll expressions are considered equivalent if they
      # have the same class and string representation
      def ==(x)
        x.class == self.class and @table == x.table
      end

      to_s_method :column_all_sql
    end
    
    class ComplexExpression
      include AliasMethods
      include CastMethods
      include OrderMethods
      include SubscriptMethods
    end

    # Represents constants or psuedo-constants (e.g. CURRENT_DATE) in SQL
    class Constant < GenericExpression
      # Create an object with the given table
      def initialize(constant)
        @constant = constant
      end
      
      to_s_method :constant_sql, '@constant'
    end
    
    # Holds default generic constants that can be referenced.  These
    # are included in the Sequel top level module and are also available
    # in this module which can be required at the top level to get
    # direct access to the constants.
    module Constants
      CURRENT_DATE = Constant.new(:CURRENT_DATE)
      CURRENT_TIME = Constant.new(:CURRENT_TIME)
      CURRENT_TIMESTAMP = Constant.new(:CURRENT_TIMESTAMP)
    end

    # Represents an SQL function call.
    class Function < GenericExpression
      # The array of arguments to pass to the function (may be blank)
      attr_reader :args

      # The SQL function to call
      attr_reader :f
      
      # Set the attributes to the given arguments
      def initialize(f, *args)
        @f, @args = f, args
      end

      # Functions are considered equivalent if they
      # have the same class, function, and arguments.
      def ==(x)
         x.class == self.class && @f == x.f && @args == x.args
      end

      to_s_method :function_sql
    end
    
    class GenericExpression
      include AliasMethods
      include BooleanMethods
      include CastMethods
      include ComplexExpressionMethods
      include InequalityMethods
      include NumericMethods
      include OrderMethods
      include StringMethods
      include SubscriptMethods
    end

    # Represents an identifier (column or table). Can be used
    # to specify a Symbol with multiple underscores should not be
    # split, or for creating an identifier without using a symbol.
    class Identifier < GenericExpression
      include QualifyingMethods

      # The table and column to reference
      attr_reader :value

      # Set the value to the given argument
      def initialize(value)
        @value = value
      end
      
      to_s_method :quote_identifier, '@value'
    end
    
    # Represents an SQL JOIN clause, used for joining tables.
    class JoinClause < Expression
      # The type of join to do
      attr_reader :join_type

      # The actual table to join
      attr_reader :table

      # The table alias to use for the join, if any
      attr_reader :table_alias

      # Create an object with the given join_type, table, and table alias
      def initialize(join_type, table, table_alias = nil)
        @join_type, @table, @table_alias = join_type, table, table_alias
      end

      to_s_method :join_clause_sql
    end

    # Represents an SQL JOIN table ON conditions clause.
    class JoinOnClause < JoinClause
      # The conditions for the join
      attr_reader :on

      # Create an object with the ON conditions and call super with the
      # remaining args.
      def initialize(on, *args)
        @on = on
        super(*args)
      end

      to_s_method :join_on_clause_sql
    end

    # Represents an SQL JOIN table USING (columns) clause.
    class JoinUsingClause < JoinClause
      # The columns that appear in both tables that should be equal 
      # for the conditions to match.
      attr_reader :using

      # Create an object with the given USING conditions and call super
      # with the remaining args.
      def initialize(using, *args)
        @using = using
        super(*args)
      end

      to_s_method :join_using_clause_sql
    end

    # Represents a literal string with placeholders and arguments.
    # This is necessary to ensure delayed literalization of the arguments
    # required for the prepared statement support
    class PlaceholderLiteralString < Expression
      # The arguments that will be subsituted into the placeholders.
      # Either an array of unnamed placeholders (which will be substituted in
      # order for ? characters), or a hash of named placeholders (which will be
      # substituted for :key phrases).
      attr_reader :args

      # The literal string containing placeholders
      attr_reader :str

      # Whether to surround the expression with parantheses
      attr_reader :parens

      # Create an object with the given string, placeholder arguments, and parens flag.
      def initialize(str, args, parens=false)
        @str = str
        @args = args.is_a?(Array) && args.length == 1 && (v = args.at(0)).is_a?(Hash) ? v : args
        @parens = parens
      end

      to_s_method :placeholder_literal_string_sql
    end

    # Subclass of ComplexExpression where the expression results
    # in a numeric value in SQL.
    class NumericExpression < ComplexExpression
      include BitwiseMethods 
      include NumericMethods
      include InequalityMethods
      include NoBooleanInputMethods
    end

    # Represents a column/expression to order the result set by.
    class OrderedExpression < Expression
      # The expression to order the result set by.
      attr_reader :expression

      # Whether the expression should order the result set in a descending manner
      attr_reader :descending

      # Set the expression and descending attributes to the given values.
      def initialize(expression, descending = true)
        @expression, @descending = expression, descending
      end

      # Return a copy that is ASC
      def asc
        OrderedExpression.new(@expression, false)
      end

      # Return a copy that is DESC
      def desc
        OrderedExpression.new(@expression)
      end

      # Return an inverted expression, changing ASC to DESC and vice versa
      def invert
        OrderedExpression.new(@expression, !@descending)
      end

      to_s_method :ordered_expression_sql
    end

    # Represents a qualified (column with table or table with schema) reference. 
    class QualifiedIdentifier < GenericExpression
      include QualifyingMethods

      # The column to reference
      attr_reader :column

      # The table to reference
      attr_reader :table

      # Set the table and column to the given arguments
      def initialize(table, column)
        @table, @column = table, column
      end
      
      to_s_method :qualified_identifier_sql
    end
    
    # Subclass of ComplexExpression where the expression results
    # in a text/string/varchar value in SQL.
    class StringExpression < ComplexExpression
      include StringMethods
      include StringConcatenationMethods
      include InequalityMethods
      include NoBooleanInputMethods

      # Map of [regexp, case_insenstive] to ComplexExpression operator
      LIKE_MAP = {[true, true]=>:'~*', [true, false]=>:~, [false, true]=>:ILIKE, [false, false]=>:LIKE}
      
      # Creates a SQL pattern match exprssion. left (l) is the SQL string we
      # are matching against, and ces are the patterns we are matching.
      # The match succeeds if any of the patterns match (SQL OR).  Patterns
      # can be given as strings or regular expressions.  Strings will cause
      # the SQL LIKE operator to be used, and should be supported by most
      # databases.  Regular expressions will probably only work on MySQL
      # and PostgreSQL, and SQL regular expression syntax is not fully compatible
      # with ruby regular expression syntax, so be careful if using regular
      # expressions.
      # 
      # The pattern match will be case insensitive if the last argument is a hash
      # with a key of :case_insensitive that is not false or nil. Also,
      # if a case insensitive regular expression is used (//i), that particular
      # pattern which will always be case insensitive.
      def self.like(l, *ces)
        l, lre, lci = like_element(l)
        lci = (ces.last.is_a?(Hash) ? ces.pop : {})[:case_insensitive] ? true : lci
        ces.collect! do |ce|
          r, rre, rci = like_element(ce)
          BooleanExpression.new(LIKE_MAP[[lre||rre, lci||rci]], l, r)
        end
        ces.length == 1 ? ces.at(0) : BooleanExpression.new(:OR, *ces)
      end
      
      # An array of three parts:
      # * The object to use
      # * Whether it is a regular expression
      # * Whether it is case insensitive
      def self.like_element(re) # :nodoc:
        if re.is_a?(Regexp)
          [re.source, true, re.casefold?]
        else
          [re, false, false]
        end
      end
      private_class_method :like_element
    end

    # Represents an SQL array.  Added so it is possible to deal with a
    # ruby array of all two pairs as an SQL array instead of an ordered
    # hash-like conditions specifier.
    class SQLArray < Expression
      # The array of objects this SQLArray wraps
      attr_reader :array
      alias to_a array

      # Create an object with the given array.
      def initialize(array)
        @array = array
      end

      to_s_method :array_sql, '@array'
    end

    # Represents an SQL array access, with multiple possible arguments.
    class Subscript < GenericExpression
      # The SQL array column
      attr_reader :f

      # The array of subscripts to use (should be an array of numbers)
      attr_reader :sub

      # Set the array column and subscripts to the given arguments
      def initialize(f, sub)
        @f, @sub = f, sub
      end

      # Create a new subscript appending the given subscript(s)
      # the the current array of subscripts.
      def |(sub)
        Subscript.new(@f, @sub + Array(sub))
      end
      
      to_s_method :subscript_sql
    end

    # The purpose of this class is to allow the easy creation of SQL identifiers and functions
    # without relying on methods defined on Symbol.  This is useful if another library defines
    # the methods defined by Sequel, or if you are running on ruby 1.9.
    #
    # An instance of this class is yielded to the block supplied to filter, order, and select.
    # If the block doesn't take an argument, the block is instance_evaled in the context of
    # a new instance of this class.
    #
    # VirtualRow uses method_missing to return Identifiers, QualifiedIdentifiers, Functions, or WindowFunctions, 
    # depending on how it is called.  If a block is not given, creates one of the following objects:
    # * Function - returned if any arguments are supplied, using the method name
    #   as the function name, and the arguments as the function arguments.
    # * QualifiedIdentifier - returned if the method name contains __, with the
    #   table being the part before __, and the column being the part after.
    # * Identifier - returned otherwise, using the method name.
    # If a block is given, it returns either a Function or WindowFunction, depending on the first
    # argument to the method.  Note that the block is currently not called by the code, though
    # this may change in a future version.  If the first argument is:
    # * no arguments given - uses a Function with no arguments.
    # * :* - uses a Function with a literal wildcard argument (*), mostly useful for COUNT.
    # * :distinct - uses a Function that prepends DISTINCT to the rest of the arguments, mostly
    #   useful for aggregate functions.
    # * :over - uses a WindowFunction.  If a second argument is provided, it should be a hash
    #   of options which are passed to Window (e.g. :window, :partition, :order, :frame).  The
    #   arguments to the function itself should be specified as :*=>true for a wildcard, or via
    #   the :args option.
    #
    # Examples:
    #
    #   ds = DB[:t]
    #   # Argument yielded to block
    #   ds.filter{|r| r.name < 2} # SELECT * FROM t WHERE (name < 2)
    #   # Block without argument (instance_eval)
    #   ds.filter{name < 2} # SELECT * FROM t WHERE (name < 2)
    #   # Qualified identifiers
    #   ds.filter{table__column + 1 < 2} # SELECT * FROM t WHERE ((table.column + 1) < 2)
    #   # Functions
    #   ds.filter{is_active(1, 'arg2')} # SELECT * FROM t WHERE is_active(1, 'arg2')
    #   ds.select{version{}} # SELECT version() FROM t
    #   ds.select{count(:*){}} # SELECT count(*) FROM t
    #   ds.select{count(:distinct, col1){}} # SELECT count(DISTINCT col1) FROM t
    #   # Window Functions
    #   ds.select{rank(:over){}} # SELECT rank() OVER () FROM t
    #   ds.select{count(:over, :*=>true){}} # SELECT count(*) OVER () FROM t
    #   ds.select{sum(:over, :args=>col1, :partition=>col2, :order=>col3){}} # SELECT sum(col1) OVER (PARTITION BY col2 ORDER BY col3) FROM t
    class VirtualRow < BasicObject
      WILDCARD = LiteralString.new('*').freeze
      QUESTION_MARK = LiteralString.new('?').freeze
      COMMA_SEPARATOR = LiteralString.new(', ').freeze
      DOUBLE_UNDERSCORE = '__'.freeze

      # Return Identifiers, QualifiedIdentifiers, Functions, or WindowFunctions, depending
      # on arguments and whether a block is provided.  Does not currently call the block.
      # See the class level documentation.
      def method_missing(m, *args, &block)
        if block
          if args.empty?
            Function.new(m)
          else
            case arg = args.shift
            when :*
              Function.new(m, WILDCARD)
            when :distinct
              Function.new(m, PlaceholderLiteralString.new("DISTINCT #{args.map{QUESTION_MARK}.join(COMMA_SEPARATOR)}", args))
            when :over
              opts = args.shift || {}
              fun_args = ::Kernel.Array(opts[:*] ? WILDCARD : opts[:args])
              WindowFunction.new(Function.new(m, *fun_args), Window.new(opts))
            else
              raise Error, 'unsupported VirtualRow method argument used with block'
            end
          end
        elsif args.empty?
          table, column = m.to_s.split(DOUBLE_UNDERSCORE, 2)
          column ? QualifiedIdentifier.new(table, column) : Identifier.new(m)
        else
          Function.new(m, *args)
        end
      end
    end

    # A window is part of a window function specifying the window over which the function operates.
    # It is separated from the WindowFunction class because it also can be used separately on
    # some databases.
    class Window < Expression
      # The options for this window.  Options currently used are:
      # * :frame - if specified, should be :all or :rows.  :all always operates over all rows in the
      #   partition, while :rows excludes the current row's later peers.  The default is to include
      #   all previous rows in the partition up to the current row's last peer.
      # * :order - order on the column(s) given
      # * :partition - partition/group on the column(s) given
      # * :window - base results on a previously specified named window
      attr_reader :opts

      # Set the options to the options given
      def initialize(opts={})
        @opts = opts
      end

      to_s_method :window_sql, '@opts'
    end

    # A WindowFunction is a grouping of a function with a window over which it operates.
    class WindowFunction < GenericExpression
      # The function to use, should be an SQL::Function.
      attr_reader :function

      # The window to use, should be an SQL::Window.
      attr_reader :window

      # Set the function and window.
      def initialize(function, window)
        @function, @window = function, window
      end

      to_s_method :window_function_sql, '@function, @window'
    end
  end

  # LiteralString is used to represent literal SQL expressions. A 
  # LiteralString is copied verbatim into an SQL statement. Instances of
  # LiteralString can be created by calling String#lit.
  class LiteralString
    include SQL::OrderMethods
    include SQL::ComplexExpressionMethods
    include SQL::BooleanMethods
    include SQL::NumericMethods
    include SQL::StringMethods
    include SQL::InequalityMethods
  end
  
  include SQL::Constants
end
