1# ruff: noqa: F401
  2"""
  3## Dialects
  4
  5While there is a SQL standard, most SQL engines support a variation of that standard. This makes it difficult
  6to write portable SQL code. SQLGlot bridges all the different variations, called "dialects", with an extensible
  7SQL transpilation framework.
  8
  9The base `sqlglot.dialects.dialect.Dialect` class implements a generic dialect that aims to be as universal as possible.
 10
 11Each SQL variation has its own `Dialect` subclass, extending the corresponding `Tokenizer`, `Parser` and `Generator`
 12classes as needed.
 13
 14### Implementing a custom Dialect
 15
 16Creating a new SQL dialect may seem complicated at first, but it is actually quite simple in SQLGlot:
 17
 18```python
 19from sqlglot import exp
 20from sqlglot.dialects.dialect import Dialect
 21from sqlglot.generator import Generator
 22from sqlglot.tokens import Tokenizer, TokenType
 23
 24
 25class Custom(Dialect):
 26    class Tokenizer(Tokenizer):
 27        QUOTES = ["'", '"']  # Strings can be delimited by either single or double quotes
 28        IDENTIFIERS = ["`"]  # Identifiers can be delimited by backticks
 29
 30        # Associates certain meaningful words with tokens that capture their intent
 31        KEYWORDS = {
 32            **Tokenizer.KEYWORDS,
 33            "INT64": TokenType.BIGINT,
 34            "FLOAT64": TokenType.DOUBLE,
 35        }
 36
 37    class Generator(Generator):
 38        # Specifies how AST nodes, i.e. subclasses of exp.Expression, should be converted into SQL
 39        TRANSFORMS = {
 40            exp.Array: lambda self, e: f"[{self.expressions(e)}]",
 41        }
 42
 43        # Specifies how AST nodes representing data types should be converted into SQL
 44        TYPE_MAPPING = {
 45            exp.DataType.Type.TINYINT: "INT64",
 46            exp.DataType.Type.SMALLINT: "INT64",
 47            exp.DataType.Type.INT: "INT64",
 48            exp.DataType.Type.BIGINT: "INT64",
 49            exp.DataType.Type.DECIMAL: "NUMERIC",
 50            exp.DataType.Type.FLOAT: "FLOAT64",
 51            exp.DataType.Type.DOUBLE: "FLOAT64",
 52            exp.DataType.Type.BOOLEAN: "BOOL",
 53            exp.DataType.Type.TEXT: "STRING",
 54        }
 55```
 56
 57The above example demonstrates how certain parts of the base `Dialect` class can be overridden to match a different
 58specification. Even though it is a fairly realistic starting point, we strongly encourage the reader to study existing
 59dialect implementations in order to understand how their various components can be modified, depending on the use-case.
 60
 61----
 62"""
 63
 64import importlib
 65import threading
 66
 67DIALECTS = [
 68    "Athena",
 69    "BigQuery",
 70    "ClickHouse",
 71    "Databricks",
 72    "Doris",
 73    "Drill",
 74    "Druid",
 75    "DuckDB",
 76    "Dune",
 77    "Hive",
 78    "Materialize",
 79    "MySQL",
 80    "Oracle",
 81    "Postgres",
 82    "Presto",
 83    "PRQL",
 84    "Redshift",
 85    "RisingWave",
 86    "Snowflake",
 87    "Spark",
 88    "Spark2",
 89    "SQLite",
 90    "StarRocks",
 91    "Tableau",
 92    "Teradata",
 93    "Trino",
 94    "TSQL",
 95]
 96
 97MODULE_BY_DIALECT = {name: name.lower() for name in DIALECTS}
 98DIALECT_MODULE_NAMES = MODULE_BY_DIALECT.values()
 99
100MODULE_BY_ATTRIBUTE = {
101    **MODULE_BY_DIALECT,
102    "Dialect": "dialect",
103    "Dialects": "dialect",
104}
105
106__all__ = list(MODULE_BY_ATTRIBUTE)
107
108_import_lock = threading.Lock()
109
110
111def __getattr__(name):
112    module_name = MODULE_BY_ATTRIBUTE.get(name)
113    if module_name:
114        with _import_lock:
115            module = importlib.import_module(f"sqlglot.dialects.{module_name}")
116        return getattr(module, name)
117
118    raise AttributeError(f"module {__name__} has no attribute {name}")