Repository: tobymao/sqlglot

Issue:
Support `with arrayJoin .. as ..` expression (ClickHouse)
**Code snippet**
```python
from sqlglot import dialects, parse_one

sql = """
with arrayJoin([(1, [2, 3])]) as arr
select arr
"""

parse_one(sql=sql, dialect=dialects.ClickHouse)
```
**Python** 3.11
**sqlglot** 26.0.1

**Expected**: successful parsing
**Actual**: 
```python
ParseError: Required keyword: 'this' missing for <class 'sqlglot.expressions.Alias'>. Line 3, Col: 6.
  
with arrayJoin([(1, [2,3])]) as arr
select arr
```

**Official Documentation**
[WITH](https://clickhouse.com/docs/en/sql-reference/statements/select/with#syntax)
[arrayJoin](https://clickhouse.com/docs/en/sql-reference/functions/array-join)

**Check the correctness of the expression**
[Tap](https://play.clickhouse.com/play?user=play#d2l0aCBhcnJheUpvaW4oWygxLCBbMiwzXSldKSBhcyBhcnIKc2VsZWN0IGFyci4x)

Discussion:

Solution patch:
diff --git a/sqlglot/dialects/clickhouse.py b/sqlglot/dialects/clickhouse.py
index 911e8c51d2..cc85de01a2 100644
--- a/sqlglot/dialects/clickhouse.py
+++ b/sqlglot/dialects/clickhouse.py
@@ -633,7 +633,7 @@ def _parse_position(self, haystack_first: bool = False) -> exp.StrPosition:
             return super()._parse_position(haystack_first=True)
 
         # https://clickhouse.com/docs/en/sql-reference/statements/select/with/
-        def _parse_cte(self) -> exp.CTE:
+        def _parse_cte(self) -> t.Optional[exp.CTE]:
             # WITH <identifier> AS <subquery expression>
             cte: t.Optional[exp.CTE] = self._try_parse(super()._parse_cte)
 
diff --git a/sqlglot/dialects/spark.py b/sqlglot/dialects/spark.py
index 08d99e4e51..c84632f28f 100644
--- a/sqlglot/dialects/spark.py
+++ b/sqlglot/dialects/spark.py
@@ -102,6 +102,8 @@ class Tokenizer(Spark2.Tokenizer):
         ]
 
     class Parser(Spark2.Parser):
+        OPTIONAL_ALIAS_TOKEN_CTE = True
+
         FUNCTIONS = {
             **Spark2.Parser.FUNCTIONS,
             "ANY_VALUE": _build_with_ignore_nulls(exp.AnyValue),
diff --git a/sqlglot/parser.py b/sqlglot/parser.py
index 20ca97c4bd..2fa9ab448e 100644
--- a/sqlglot/parser.py
+++ b/sqlglot/parser.py
@@ -1379,6 +1379,9 @@ class Parser(metaclass=_Parser):
     # Whether the `name AS expr` schema/column constraint requires parentheses around `expr`
     WRAPPED_TRANSFORM_COLUMN_CONSTRAINT = True
 
+    # Whether the 'AS' keyword is optional in the CTE definition syntax
+    OPTIONAL_ALIAS_TOKEN_CTE = False
+
     __slots__ = (
         "error_level",
         "error_message_context",
@@ -3131,12 +3134,17 @@ def _parse_with(self, skip_with_token: bool = False) -> t.Optional[exp.With]:
             exp.With, comments=comments, expressions=expressions, recursive=recursive
         )
 
-    def _parse_cte(self) -> exp.CTE:
+    def _parse_cte(self) -> t.Optional[exp.CTE]:
+        index = self._index
+
         alias = self._parse_table_alias(self.ID_VAR_TOKENS)
         if not alias or not alias.this:
             self.raise_error("Expected CTE to have alias")
 
-        self._match(TokenType.ALIAS)
+        if not self._match(TokenType.ALIAS) and not self.OPTIONAL_ALIAS_TOKEN_CTE:
+            self._retreat(index)
+            return None
+
         comments = self._prev_comments
 
         if self._match_text_seq("NOT", "MATERIALIZED"):


Test patch:
diff --git a/tests/dialects/test_clickhouse.py b/tests/dialects/test_clickhouse.py
index 49705d51fa..3bff6e2279 100644
--- a/tests/dialects/test_clickhouse.py
+++ b/tests/dialects/test_clickhouse.py
@@ -29,6 +29,7 @@ def test_clickhouse(self):
         self.assertEqual(expr.sql(dialect="clickhouse"), "COUNT(x)")
         self.assertIsNone(expr._meta)
 
+        self.validate_identity("WITH arrayJoin([(1, [2, 3])]) AS arr SELECT arr")
         self.validate_identity("CAST(1 AS Bool)")
         self.validate_identity("SELECT toString(CHAR(104.1, 101, 108.9, 108.9, 111, 32))")
         self.validate_identity("@macro").assert_is(exp.Parameter).this.assert_is(exp.Var)