sqlglot.transforms API documentation

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# sqlglot.transforms

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      1from __future__ import annotations
      2
      3import typing as t
      4
      5from sqlglot import expressions as exp
      6from sqlglot.helper import find_new_name, name_sequence
      7
      8if t.TYPE_CHECKING:
      9    from sqlglot.generator import Generator
     10
     11
     12def unalias_group(expression: exp.Expression) -> exp.Expression:
     13    """
     14    Replace references to select aliases in GROUP BY clauses.
     15
     16    Example:
     17        >>> import sqlglot
     18        >>> sqlglot.parse_one("SELECT a AS b FROM x GROUP BY b").transform(unalias_group).sql()
     19        'SELECT a AS b FROM x GROUP BY 1'
     20
     21    Args:
     22        expression: the expression that will be transformed.
     23
     24    Returns:
     25        The transformed expression.
     26    """
     27    if isinstance(expression, exp.Group) and isinstance(expression.parent, exp.Select):
     28        aliased_selects = {
     29            e.alias: i
     30            for i, e in enumerate(expression.parent.expressions, start=1)
     31            if isinstance(e, exp.Alias)
     32        }
     33
     34        for group_by in expression.expressions:
     35            if (
     36                isinstance(group_by, exp.Column)
     37                and not group_by.table
     38                and group_by.name in aliased_selects
     39            ):
     40                group_by.replace(exp.Literal.number(aliased_selects.get(group_by.name)))
     41
     42    return expression
     43
     44
     45def eliminate_distinct_on(expression: exp.Expression) -> exp.Expression:
     46    """
     47    Convert SELECT DISTINCT ON statements to a subquery with a window function.
     48
     49    This is useful for dialects that don't support SELECT DISTINCT ON but support window functions.
     50
     51    Args:
     52        expression: the expression that will be transformed.
     53
     54    Returns:
     55        The transformed expression.
     56    """
     57    if (
     58        isinstance(expression, exp.Select)
     59        and expression.args.get("distinct")
     60        and expression.args["distinct"].args.get("on")
     61        and isinstance(expression.args["distinct"].args["on"], exp.Tuple)
     62    ):
     63        distinct_cols = expression.args["distinct"].pop().args["on"].expressions
     64        outer_selects = expression.selects
     65        row_number = find_new_name(expression.named_selects, "_row_number")
     66        window = exp.Window(this=exp.RowNumber(), partition_by=distinct_cols)
     67        order = expression.args.get("order")
     68
     69        if order:
     70            window.set("order", order.pop().copy())
     71        else:
     72            window.set("order", exp.Order(expressions=[c.copy() for c in distinct_cols]))
     73
     74        window = exp.alias_(window, row_number)
     75        expression.select(window, copy=False)
     76
     77        return (
     78            exp.select(*outer_selects)
     79            .from_(expression.subquery())
     80            .where(exp.column(row_number).eq(1))
     81        )
     82
     83    return expression
     84
     85
     86def eliminate_qualify(expression: exp.Expression) -> exp.Expression:
     87    """
     88    Convert SELECT statements that contain the QUALIFY clause into subqueries, filtered equivalently.
     89
     90    The idea behind this transformation can be seen in Snowflake's documentation for QUALIFY:
     91    https://docs.snowflake.com/en/sql-reference/constructs/qualify
     92
     93    Some dialects don't support window functions in the WHERE clause, so we need to include them as
     94    projections in the subquery, in order to refer to them in the outer filter using aliases. Also,
     95    if a column is referenced in the QUALIFY clause but is not selected, we need to include it too,
     96    otherwise we won't be able to refer to it in the outer query's WHERE clause.
     97    """
     98    if isinstance(expression, exp.Select) and expression.args.get("qualify"):
     99        taken = set(expression.named_selects)
    100        for select in expression.selects:
    101            if not select.alias_or_name:
    102                alias = find_new_name(taken, "_c")
    103                select.replace(exp.alias_(select, alias))
    104                taken.add(alias)
    105
    106        outer_selects = exp.select(*[select.alias_or_name for select in expression.selects])
    107        qualify_filters = expression.args["qualify"].pop().this
    108
    109        select_candidates = exp.Window if expression.is_star else (exp.Window, exp.Column)
    110        for expr in qualify_filters.find_all(select_candidates):
    111            if isinstance(expr, exp.Window):
    112                alias = find_new_name(expression.named_selects, "_w")
    113                expression.select(exp.alias_(expr, alias), copy=False)
    114                column = exp.column(alias)
    115
    116                if isinstance(expr.parent, exp.Qualify):
    117                    qualify_filters = column
    118                else:
    119                    expr.replace(column)
    120            elif expr.name not in expression.named_selects:
    121                expression.select(expr.copy(), copy=False)
    122
    123        return outer_selects.from_(expression.subquery(alias="_t")).where(qualify_filters)
    124
    125    return expression
    126
    127
    128def remove_precision_parameterized_types(expression: exp.Expression) -> exp.Expression:
    129    """
    130    Some dialects only allow the precision for parameterized types to be defined in the DDL and not in
    131    other expressions. This transforms removes the precision from parameterized types in expressions.
    132    """
    133    for node in expression.find_all(exp.DataType):
    134        node.set(
    135            "expressions", [e for e in node.expressions if not isinstance(e, exp.DataTypeParam)]
    136        )
    137
    138    return expression
    139
    140
    141def unnest_to_explode(expression: exp.Expression) -> exp.Expression:
    142    """Convert cross join unnest into lateral view explode (used in presto -> hive)."""
    143    if isinstance(expression, exp.Select):
    144        for join in expression.args.get("joins") or []:
    145            unnest = join.this
    146
    147            if isinstance(unnest, exp.Unnest):
    148                alias = unnest.args.get("alias")
    149                udtf = exp.Posexplode if unnest.args.get("ordinality") else exp.Explode
    150
    151                expression.args["joins"].remove(join)
    152
    153                for e, column in zip(unnest.expressions, alias.columns if alias else []):
    154                    expression.append(
    155                        "laterals",
    156                        exp.Lateral(
    157                            this=udtf(this=e),
    158                            view=True,
    159                            alias=exp.TableAlias(this=alias.this, columns=[column]),  # type: ignore
    160                        ),
    161                    )
    162
    163    return expression
    164
    165
    166def explode_to_unnest(expression: exp.Expression) -> exp.Expression:
    167    """Convert explode/posexplode into unnest (used in hive -> presto)."""
    168    if isinstance(expression, exp.Select):
    169        from sqlglot.optimizer.scope import Scope
    170
    171        taken_select_names = set(expression.named_selects)
    172        taken_source_names = {name for name, _ in Scope(expression).references}
    173
    174        for select in expression.selects:
    175            to_replace = select
    176
    177            pos_alias = ""
    178            explode_alias = ""
    179
    180            if isinstance(select, exp.Alias):
    181                explode_alias = select.alias
    182                select = select.this
    183            elif isinstance(select, exp.Aliases):
    184                pos_alias = select.aliases[0].name
    185                explode_alias = select.aliases[1].name
    186                select = select.this
    187
    188            if isinstance(select, (exp.Explode, exp.Posexplode)):
    189                is_posexplode = isinstance(select, exp.Posexplode)
    190
    191                explode_arg = select.this
    192                unnest = exp.Unnest(expressions=[explode_arg.copy()], ordinality=is_posexplode)
    193
    194                # This ensures that we won't use [POS]EXPLODE's argument as a new selection
    195                if isinstance(explode_arg, exp.Column):
    196                    taken_select_names.add(explode_arg.output_name)
    197
    198                unnest_source_alias = find_new_name(taken_source_names, "_u")
    199                taken_source_names.add(unnest_source_alias)
    200
    201                if not explode_alias:
    202                    explode_alias = find_new_name(taken_select_names, "col")
    203                    taken_select_names.add(explode_alias)
    204
    205                    if is_posexplode:
    206                        pos_alias = find_new_name(taken_select_names, "pos")
    207                        taken_select_names.add(pos_alias)
    208
    209                if is_posexplode:
    210                    column_names = [explode_alias, pos_alias]
    211                    to_replace.pop()
    212                    expression.select(pos_alias, explode_alias, copy=False)
    213                else:
    214                    column_names = [explode_alias]
    215                    to_replace.replace(exp.column(explode_alias))
    216
    217                unnest = exp.alias_(unnest, unnest_source_alias, table=column_names)
    218
    219                if not expression.args.get("from"):
    220                    expression.from_(unnest, copy=False)
    221                else:
    222                    expression.join(unnest, join_type="CROSS", copy=False)
    223
    224    return expression
    225
    226
    227def remove_within_group_for_percentiles(expression: exp.Expression) -> exp.Expression:
    228    if (
    229        isinstance(expression, exp.WithinGroup)
    230        and isinstance(expression.this, (exp.PercentileCont, exp.PercentileDisc))
    231        and isinstance(expression.expression, exp.Order)
    232    ):
    233        quantile = expression.this.this
    234        input_value = t.cast(exp.Ordered, expression.find(exp.Ordered)).this
    235        return expression.replace(exp.ApproxQuantile(this=input_value, quantile=quantile))
    236
    237    return expression
    238
    239
    240def add_recursive_cte_column_names(expression: exp.Expression) -> exp.Expression:
    241    if isinstance(expression, exp.With) and expression.recursive:
    242        next_name = name_sequence("_c_")
    243
    244        for cte in expression.expressions:
    245            if not cte.args["alias"].columns:
    246                query = cte.this
    247                if isinstance(query, exp.Union):
    248                    query = query.this
    249
    250                cte.args["alias"].set(
    251                    "columns",
    252                    [exp.to_identifier(s.alias_or_name or next_name()) for s in query.selects],
    253                )
    254
    255    return expression
    256
    257
    258def epoch_cast_to_ts(expression: exp.Expression) -> exp.Expression:
    259    if (
    260        isinstance(expression, (exp.Cast, exp.TryCast))
    261        and expression.name.lower() == "epoch"
    262        and expression.to.this in exp.DataType.TEMPORAL_TYPES
    263    ):
    264        expression.this.replace(exp.Literal.string("1970-01-01 00:00:00"))
    265
    266    return expression
    267
    268
    269def preprocess(
    270    transforms: t.List[t.Callable[[exp.Expression], exp.Expression]],
    271) -> t.Callable[[Generator, exp.Expression], str]:
    272    """
    273    Creates a new transform by chaining a sequence of transformations and converts the resulting
    274    expression to SQL, using either the "_sql" method corresponding to the resulting expression,
    275    or the appropriate `Generator.TRANSFORMS` function (when applicable -- see below).
    276
    277    Args:
    278        transforms: sequence of transform functions. These will be called in order.
    279
    280    Returns:
    281        Function that can be used as a generator transform.
    282    """
    283
    284    def _to_sql(self, expression: exp.Expression) -> str:
    285        expression_type = type(expression)
    286
    287        expression = transforms[0](expression.copy())
    288        for t in transforms[1:]:
    289            expression = t(expression)
    290
    291        _sql_handler = getattr(self, expression.key + "_sql", None)
    292        if _sql_handler:
    293            return _sql_handler(expression)
    294
    295        transforms_handler = self.TRANSFORMS.get(type(expression))
    296        if transforms_handler:
    297            # Ensures we don't enter an infinite loop. This can happen when the original expression
    298            # has the same type as the final expression and there's no _sql method available for it,
    299            # because then it'd re-enter _to_sql.
    300            if expression_type is type(expression):
    301                raise ValueError(
    302                    f"Expression type {expression.__class__.__name__} requires a _sql method in order to be transformed."
    303                )
    304
    305            return transforms_handler(self, expression)
    306
    307        raise ValueError(f"Unsupported expression type {expression.__class__.__name__}.")
    308
    309    return _to_sql

def unalias_group( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    13def unalias_group(expression: exp.Expression) -> exp.Expression:
    14    """
    15    Replace references to select aliases in GROUP BY clauses.
    16
    17    Example:
    18        >>> import sqlglot
    19        >>> sqlglot.parse_one("SELECT a AS b FROM x GROUP BY b").transform(unalias_group).sql()
    20        'SELECT a AS b FROM x GROUP BY 1'
    21
    22    Args:
    23        expression: the expression that will be transformed.
    24
    25    Returns:
    26        The transformed expression.
    27    """
    28    if isinstance(expression, exp.Group) and isinstance(expression.parent, exp.Select):
    29        aliased_selects = {
    30            e.alias: i
    31            for i, e in enumerate(expression.parent.expressions, start=1)
    32            if isinstance(e, exp.Alias)
    33        }
    34
    35        for group_by in expression.expressions:
    36            if (
    37                isinstance(group_by, exp.Column)
    38                and not group_by.table
    39                and group_by.name in aliased_selects
    40            ):
    41                group_by.replace(exp.Literal.number(aliased_selects.get(group_by.name)))
    42
    43    return expression

Replace references to select aliases in GROUP BY clauses.

###### Example:

>     >>> import sqlglot
>     >>> sqlglot.parse_one("SELECT a AS b FROM x GROUP BY b").transform(unalias_group).sql()
>     'SELECT a AS b FROM x GROUP BY 1'

###### Arguments:

- expression: the expression that will be transformed.

###### Returns:

> The transformed expression.

def eliminate_distinct_on( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    46def eliminate_distinct_on(expression: exp.Expression) -> exp.Expression:
    47    """
    48    Convert SELECT DISTINCT ON statements to a subquery with a window function.
    49
    50    This is useful for dialects that don't support SELECT DISTINCT ON but support window functions.
    51
    52    Args:
    53        expression: the expression that will be transformed.
    54
    55    Returns:
    56        The transformed expression.
    57    """
    58    if (
    59        isinstance(expression, exp.Select)
    60        and expression.args.get("distinct")
    61        and expression.args["distinct"].args.get("on")
    62        and isinstance(expression.args["distinct"].args["on"], exp.Tuple)
    63    ):
    64        distinct_cols = expression.args["distinct"].pop().args["on"].expressions
    65        outer_selects = expression.selects
    66        row_number = find_new_name(expression.named_selects, "_row_number")
    67        window = exp.Window(this=exp.RowNumber(), partition_by=distinct_cols)
    68        order = expression.args.get("order")
    69
    70        if order:
    71            window.set("order", order.pop().copy())
    72        else:
    73            window.set("order", exp.Order(expressions=[c.copy() for c in distinct_cols]))
    74
    75        window = exp.alias_(window, row_number)
    76        expression.select(window, copy=False)
    77
    78        return (
    79            exp.select(*outer_selects)
    80            .from_(expression.subquery())
    81            .where(exp.column(row_number).eq(1))
    82        )
    83
    84    return expression

Convert SELECT DISTINCT ON statements to a subquery with a window function.

This is useful for dialects that don't support SELECT DISTINCT ON but support window functions.

###### Arguments:

- expression: the expression that will be transformed.

###### Returns:

> The transformed expression.

def eliminate_qualify( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

     87def eliminate_qualify(expression: exp.Expression) -> exp.Expression:
     88    """
     89    Convert SELECT statements that contain the QUALIFY clause into subqueries, filtered equivalently.
     90
     91    The idea behind this transformation can be seen in Snowflake's documentation for QUALIFY:
     92    https://docs.snowflake.com/en/sql-reference/constructs/qualify
     93
     94    Some dialects don't support window functions in the WHERE clause, so we need to include them as
     95    projections in the subquery, in order to refer to them in the outer filter using aliases. Also,
     96    if a column is referenced in the QUALIFY clause but is not selected, we need to include it too,
     97    otherwise we won't be able to refer to it in the outer query's WHERE clause.
     98    """
     99    if isinstance(expression, exp.Select) and expression.args.get("qualify"):
    100        taken = set(expression.named_selects)
    101        for select in expression.selects:
    102            if not select.alias_or_name:
    103                alias = find_new_name(taken, "_c")
    104                select.replace(exp.alias_(select, alias))
    105                taken.add(alias)
    106
    107        outer_selects = exp.select(*[select.alias_or_name for select in expression.selects])
    108        qualify_filters = expression.args["qualify"].pop().this
    109
    110        select_candidates = exp.Window if expression.is_star else (exp.Window, exp.Column)
    111        for expr in qualify_filters.find_all(select_candidates):
    112            if isinstance(expr, exp.Window):
    113                alias = find_new_name(expression.named_selects, "_w")
    114                expression.select(exp.alias_(expr, alias), copy=False)
    115                column = exp.column(alias)
    116
    117                if isinstance(expr.parent, exp.Qualify):
    118                    qualify_filters = column
    119                else:
    120                    expr.replace(column)
    121            elif expr.name not in expression.named_selects:
    122                expression.select(expr.copy(), copy=False)
    123
    124        return outer_selects.from_(expression.subquery(alias="_t")).where(qualify_filters)
    125
    126    return expression

Convert SELECT statements that contain the QUALIFY clause into subqueries, filtered equivalently.

The idea behind this transformation can be seen in Snowflake's documentation for QUALIFY: https://docs.snowflake.com/en/sql-reference/constructs/qualify

Some dialects don't support window functions in the WHERE clause, so we need to include them as projections in the subquery, in order to refer to them in the outer filter using aliases. Also, if a column is referenced in the QUALIFY clause but is not selected, we need to include it too, otherwise we won't be able to refer to it in the outer query's WHERE clause.

def remove_precision_parameterized_types( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    129def remove_precision_parameterized_types(expression: exp.Expression) -> exp.Expression:
    130    """
    131    Some dialects only allow the precision for parameterized types to be defined in the DDL and not in
    132    other expressions. This transforms removes the precision from parameterized types in expressions.
    133    """
    134    for node in expression.find_all(exp.DataType):
    135        node.set(
    136            "expressions", [e for e in node.expressions if not isinstance(e, exp.DataTypeParam)]
    137        )
    138
    139    return expression

Some dialects only allow the precision for parameterized types to be defined in the DDL and not in other expressions. This transforms removes the precision from parameterized types in expressions.

def unnest_to_explode( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    142def unnest_to_explode(expression: exp.Expression) -> exp.Expression:
    143    """Convert cross join unnest into lateral view explode (used in presto -> hive)."""
    144    if isinstance(expression, exp.Select):
    145        for join in expression.args.get("joins") or []:
    146            unnest = join.this
    147
    148            if isinstance(unnest, exp.Unnest):
    149                alias = unnest.args.get("alias")
    150                udtf = exp.Posexplode if unnest.args.get("ordinality") else exp.Explode
    151
    152                expression.args["joins"].remove(join)
    153
    154                for e, column in zip(unnest.expressions, alias.columns if alias else []):
    155                    expression.append(
    156                        "laterals",
    157                        exp.Lateral(
    158                            this=udtf(this=e),
    159                            view=True,
    160                            alias=exp.TableAlias(this=alias.this, columns=[column]),  # type: ignore
    161                        ),
    162                    )
    163
    164    return expression

Convert cross join unnest into lateral view explode (used in presto -> hive).

def explode_to_unnest( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    167def explode_to_unnest(expression: exp.Expression) -> exp.Expression:
    168    """Convert explode/posexplode into unnest (used in hive -> presto)."""
    169    if isinstance(expression, exp.Select):
    170        from sqlglot.optimizer.scope import Scope
    171
    172        taken_select_names = set(expression.named_selects)
    173        taken_source_names = {name for name, _ in Scope(expression).references}
    174
    175        for select in expression.selects:
    176            to_replace = select
    177
    178            pos_alias = ""
    179            explode_alias = ""
    180
    181            if isinstance(select, exp.Alias):
    182                explode_alias = select.alias
    183                select = select.this
    184            elif isinstance(select, exp.Aliases):
    185                pos_alias = select.aliases[0].name
    186                explode_alias = select.aliases[1].name
    187                select = select.this
    188
    189            if isinstance(select, (exp.Explode, exp.Posexplode)):
    190                is_posexplode = isinstance(select, exp.Posexplode)
    191
    192                explode_arg = select.this
    193                unnest = exp.Unnest(expressions=[explode_arg.copy()], ordinality=is_posexplode)
    194
    195                # This ensures that we won't use [POS]EXPLODE's argument as a new selection
    196                if isinstance(explode_arg, exp.Column):
    197                    taken_select_names.add(explode_arg.output_name)
    198
    199                unnest_source_alias = find_new_name(taken_source_names, "_u")
    200                taken_source_names.add(unnest_source_alias)
    201
    202                if not explode_alias:
    203                    explode_alias = find_new_name(taken_select_names, "col")
    204                    taken_select_names.add(explode_alias)
    205
    206                    if is_posexplode:
    207                        pos_alias = find_new_name(taken_select_names, "pos")
    208                        taken_select_names.add(pos_alias)
    209
    210                if is_posexplode:
    211                    column_names = [explode_alias, pos_alias]
    212                    to_replace.pop()
    213                    expression.select(pos_alias, explode_alias, copy=False)
    214                else:
    215                    column_names = [explode_alias]
    216                    to_replace.replace(exp.column(explode_alias))
    217
    218                unnest = exp.alias_(unnest, unnest_source_alias, table=column_names)
    219
    220                if not expression.args.get("from"):
    221                    expression.from_(unnest, copy=False)
    222                else:
    223                    expression.join(unnest, join_type="CROSS", copy=False)
    224
    225    return expression

Convert explode/posexplode into unnest (used in hive -> presto).

def remove_within_group_for_percentiles( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    228def remove_within_group_for_percentiles(expression: exp.Expression) -> exp.Expression:
    229    if (
    230        isinstance(expression, exp.WithinGroup)
    231        and isinstance(expression.this, (exp.PercentileCont, exp.PercentileDisc))
    232        and isinstance(expression.expression, exp.Order)
    233    ):
    234        quantile = expression.this.this
    235        input_value = t.cast(exp.Ordered, expression.find(exp.Ordered)).this
    236        return expression.replace(exp.ApproxQuantile(this=input_value, quantile=quantile))
    237
    238    return expression

def add_recursive_cte_column_names( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    241def add_recursive_cte_column_names(expression: exp.Expression) -> exp.Expression:
    242    if isinstance(expression, exp.With) and expression.recursive:
    243        next_name = name_sequence("_c_")
    244
    245        for cte in expression.expressions:
    246            if not cte.args["alias"].columns:
    247                query = cte.this
    248                if isinstance(query, exp.Union):
    249                    query = query.this
    250
    251                cte.args["alias"].set(
    252                    "columns",
    253                    [exp.to_identifier(s.alias_or_name or next_name()) for s in query.selects],
    254                )
    255
    256    return expression

def epoch_cast_to_ts( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    259def epoch_cast_to_ts(expression: exp.Expression) -> exp.Expression:
    260    if (
    261        isinstance(expression, (exp.Cast, exp.TryCast))
    262        and expression.name.lower() == "epoch"
    263        and expression.to.this in exp.DataType.TEMPORAL_TYPES
    264    ):
    265        expression.this.replace(exp.Literal.string("1970-01-01 00:00:00"))
    266
    267    return expression

def preprocess( transforms: List[Callable[[sqlglot.expressions.Expression], sqlglot.expressions.Expression]]) -> Callable[[sqlglot.generator.Generator, sqlglot.expressions.Expression], str]: View Source

    270def preprocess(
    271    transforms: t.List[t.Callable[[exp.Expression], exp.Expression]],
    272) -> t.Callable[[Generator, exp.Expression], str]:
    273    """
    274    Creates a new transform by chaining a sequence of transformations and converts the resulting
    275    expression to SQL, using either the "_sql" method corresponding to the resulting expression,
    276    or the appropriate `Generator.TRANSFORMS` function (when applicable -- see below).
    277
    278    Args:
    279        transforms: sequence of transform functions. These will be called in order.
    280
    281    Returns:
    282        Function that can be used as a generator transform.
    283    """
    284
    285    def _to_sql(self, expression: exp.Expression) -> str:
    286        expression_type = type(expression)
    287
    288        expression = transforms[0](expression.copy())
    289        for t in transforms[1:]:
    290            expression = t(expression)
    291
    292        _sql_handler = getattr(self, expression.key + "_sql", None)
    293        if _sql_handler:
    294            return _sql_handler(expression)
    295
    296        transforms_handler = self.TRANSFORMS.get(type(expression))
    297        if transforms_handler:
    298            # Ensures we don't enter an infinite loop. This can happen when the original expression
    299            # has the same type as the final expression and there's no _sql method available for it,
    300            # because then it'd re-enter _to_sql.
    301            if expression_type is type(expression):
    302                raise ValueError(
    303                    f"Expression type {expression.__class__.__name__} requires a _sql method in order to be transformed."
    304                )
    305
    306            return transforms_handler(self, expression)
    307
    308        raise ValueError(f"Unsupported expression type {expression.__class__.__name__}.")
    309
    310    return _to_sql

Creates a new transform by chaining a sequence of transformations and converts the resulting expression to SQL, using either the "_sql" method corresponding to the resulting expression, or the appropriate `Generator.TRANSFORMS` function (when applicable -- see below).

###### Arguments:

- transforms: sequence of transform functions. These will be called in order.

###### Returns:

> Function that can be used as a generator transform.