text: from django.db.models.expressions import Func, Value
  from django.db.models.fields import IntegerField
  from django.db.models.functions import Coalesce
  from django.db.models.lookups import Transform
  
  
  class BytesToCharFieldConversionMixin:
      """
      Convert CharField results from bytes to str.
  
      MySQL returns long data types (bytes) instead of chars when it can't
      determine the length of the result string. For example:
          LPAD(column1, CHAR_LENGTH(column2), ' ')
      returns the LONGTEXT (bytes) instead of VARCHAR.
      """
      def convert_value(self, value, expression, connection):
          if connection.features.db_functions_convert_bytes_to_str:
              if self.output_field.get_internal_type() == 'CharField' and isinstance(value, bytes):
                  return value.decode()
          return super().convert_value(value, expression, connection)
  
  
  class Chr(Transform):
      function = 'CHR'
      lookup_name = 'chr'
  
      def as_mysql(self, compiler, connection, **extra_context):
          return super().as_sql(
              compiler, connection, function='CHAR',
              template='%(function)s(%(expressions)s USING utf16)',
              **extra_context
          )
  
      def as_oracle(self, compiler, connection, **extra_context):
          return super().as_sql(
              compiler, connection,
              template='%(function)s(%(expressions)s USING NCHAR_CS)',
              **extra_context
          )
  
      def as_sqlite(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='CHAR', **extra_context)
  
  
  class ConcatPair(Func):
      """
      Concatenate two arguments together. This is used by `Concat` because not
      all backend databases support more than two arguments.
      """
      function = 'CONCAT'
  
      def as_sqlite(self, compiler, connection, **extra_context):
          coalesced = self.coalesce()
          return super(ConcatPair, coalesced).as_sql(
              compiler, connection, template='%(expressions)s', arg_joiner=' || ',
              **extra_context
          )
  
      def as_mysql(self, compiler, connection, **extra_context):
          # Use CONCAT_WS with an empty separator so that NULLs are ignored.
          return super().as_sql(
              compiler, connection, function='CONCAT_WS',
              template="%(function)s('', %(expressions)s)",
              **extra_context
          )
  
      def coalesce(self):
          # null on either side results in null for expression, wrap with coalesce
          c = self.copy()
          expressions = [
              Coalesce(expression, Value('')) for expression in c.get_source_expressions()
          ]
          c.set_source_expressions(expressions)
          return c
  
  
  class Concat(Func):
      """
      Concatenate text fields together. Backends that result in an entire
      null expression when any arguments are null will wrap each argument in
      coalesce functions to ensure a non-null result.
      """
      function = None
      template = "%(expressions)s"
  
      def __init__(self, *expressions, **extra):
          if len(expressions) < 2:
              raise ValueError('Concat must take at least two expressions')
          paired = self._paired(expressions)
          super().__init__(paired, **extra)
  
      def _paired(self, expressions):
          # wrap pairs of expressions in successive concat functions
          # exp = [a, b, c, d]
          # -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
          if len(expressions) == 2:
              return ConcatPair(*expressions)
          return ConcatPair(expressions[0], self._paired(expressions[1:]))
  
  
  class Left(Func):
      function = 'LEFT'
      arity = 2
  
      def __init__(self, expression, length, **extra):
          """
          expression: the name of a field, or an expression returning a string
          length: the number of characters to return from the start of the string
          """
          if not hasattr(length, 'resolve_expression'):
              if length < 1:
                  raise ValueError("'length' must be greater than 0.")
          super().__init__(expression, length, **extra)
  
      def get_substr(self):
          return Substr(self.source_expressions[0], Value(1), self.source_expressions[1])
  
      def use_substr(self, compiler, connection, **extra_context):
          return self.get_substr().as_oracle(compiler, connection, **extra_context)
  
      as_oracle = use_substr
      as_sqlite = use_substr
  
  
  class Length(Transform):
      """Return the number of characters in the expression."""
      function = 'LENGTH'
      lookup_name = 'length'
      output_field = IntegerField()
  
      def as_mysql(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='CHAR_LENGTH', **extra_context)
  
  
  class Lower(Transform):
      function = 'LOWER'
      lookup_name = 'lower'
  
  
  class LPad(BytesToCharFieldConversionMixin, Func):
      function = 'LPAD'
  
      def __init__(self, expression, length, fill_text=Value(' '), **extra):
          if not hasattr(length, 'resolve_expression') and length is not None and length < 0:
              raise ValueError("'length' must be greater or equal to 0.")
          super().__init__(expression, length, fill_text, **extra)
  
  
  class LTrim(Transform):
      function = 'LTRIM'
      lookup_name = 'ltrim'
  
  
  class Ord(Transform):
      function = 'ASCII'
      lookup_name = 'ord'
      output_field = IntegerField()
  
      def as_mysql(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='ORD', **extra_context)
  
      def as_sqlite(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='UNICODE', **extra_context)
  
  
  class Repeat(BytesToCharFieldConversionMixin, Func):
      function = 'REPEAT'
  
      def __init__(self, expression, number, **extra):
          if not hasattr(number, 'resolve_expression') and number is not None and number < 0:
              raise ValueError("'number' must be greater or equal to 0.")
          super().__init__(expression, number, **extra)
  
      def as_oracle(self, compiler, connection, **extra_context):
          expression, number = self.source_expressions
          length = None if number is None else Length(expression) * number
          rpad = RPad(expression, length, expression)
          return rpad.as_sql(compiler, connection, **extra_context)
  
  
  class Replace(Func):
      function = 'REPLACE'
  
      def __init__(self, expression, text, replacement=Value(''), **extra):
          super().__init__(expression, text, replacement, **extra)
  
  
  class Reverse(Transform):
      function = 'REVERSE'
      lookup_name = 'reverse'
  
      def as_oracle(self, compiler, connection, **extra_context):
          # REVERSE in Oracle is undocumented and doesn't support multi-byte
          # strings. Use a special subquery instead.
          return super().as_sql(
              compiler, connection,
              template=(
                  '(SELECT LISTAGG(s) WITHIN GROUP (ORDER BY n DESC) FROM '
                  '(SELECT LEVEL n, SUBSTR(%(expressions)s, LEVEL, 1) s '
                  'FROM DUAL CONNECT BY LEVEL <= LENGTH(%(expressions)s)) '
                  'GROUP BY %(expressions)s)'
              ),
              **extra_context
          )
  
  
  class Right(Left):
      function = 'RIGHT'
  
      def get_substr(self):
          return Substr(self.source_expressions[0], self.source_expressions[1] * Value(-1))
  
  
  class RPad(LPad):
      function = 'RPAD'
  
  
  class RTrim(Transform):
      function = 'RTRIM'
      lookup_name = 'rtrim'
  
  
  class StrIndex(Func):
      """
      Return a positive integer corresponding to the 1-indexed position of the
      first occurrence of a substring inside another string, or 0 if the
      substring is not found.
      """
      function = 'INSTR'
      arity = 2
      output_field = IntegerField()
  
      def as_postgresql(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='STRPOS', **extra_context)
  
  
  class Substr(Func):
      function = 'SUBSTRING'
  
      def __init__(self, expression, pos, length=None, **extra):
          """
          expression: the name of a field, or an expression returning a string
          pos: an integer > 0, or an expression returning an integer
          length: an optional number of characters to return
          """
          if not hasattr(pos, 'resolve_expression'):
              if pos < 1:
                  raise ValueError("'pos' must be greater than 0")
          expressions = [expression, pos]
          if length is not None:
              expressions.append(length)
          super().__init__(*expressions, **extra)
  
      def as_sqlite(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='SUBSTR', **extra_context)
  
      def as_oracle(self, compiler, connection, **extra_context):
          return super().as_sql(compiler, connection, function='SUBSTR', **extra_context)
  
  
  class Trim(Transform):
      function = 'TRIM'
      lookup_name = 'trim'
  
  
  class Upper(Transform):
      function = 'UPPER'
      lookup_name = 'upper'
  