fix: use adapter for identifier quoting in SQL generation

- heading.as_sql() now accepts optional adapter parameter
- Pass adapter from connection to all as_sql() calls in expression.py
- Changed fallback from MySQL backticks to ANSI double quotes

This ensures proper identifier quoting for PostgreSQL queries.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/expression.py

@@ -153,7 +153,7 @@ def make_sql(self, fields=None):
         """
         return "SELECT {distinct}{fields} FROM {from_}{where}{sorting}".format(
             distinct="DISTINCT " if self._distinct else "",
-            fields=self.heading.as_sql(fields or self.heading.names),
+            fields=self.heading.as_sql(fields or self.heading.names, adapter=self.connection.adapter),
             from_=self.from_clause(),
             where=self.where_clause(),
             sorting=self.sorting_clauses(),
@@ -881,17 +881,18 @@ def __len__(self):
         has_left_join = any(is_left for is_left, _ in result._joins)
 
         # Build COUNT query - PostgreSQL requires different syntax for multi-column DISTINCT
+        adapter = result.connection.adapter
         if has_left_join or len(result.primary_key) > 1:
             # Use subquery with DISTINCT for multi-column primary keys (backend-agnostic)
-            fields = result.heading.as_sql(result.primary_key, include_aliases=False)
+            fields = result.heading.as_sql(result.primary_key, include_aliases=False, adapter=adapter)
             query = (
                 f"SELECT count(*) FROM ("
                 f"SELECT DISTINCT {fields} FROM {result.from_clause()}{result.where_clause()}"
                 f") AS distinct_count"
             )
         else:
             # Single column - can use count(DISTINCT col) directly
-            fields = result.heading.as_sql(result.primary_key, include_aliases=False)
+            fields = result.heading.as_sql(result.primary_key, include_aliases=False, adapter=adapter)
             query = f"SELECT count(DISTINCT {fields}) FROM {result.from_clause()}{result.where_clause()}"
 
         return result.connection.query(query).fetchone()[0]
@@ -1018,7 +1019,7 @@ def where_clause(self):
         return "" if not self._left_restrict else " WHERE (%s)" % ")AND(".join(str(s) for s in self._left_restrict)
 
     def make_sql(self, fields=None):
-        fields = self.heading.as_sql(fields or self.heading.names)
+        fields = self.heading.as_sql(fields or self.heading.names, adapter=self.connection.adapter)
         assert self._grouping_attributes or not self.restriction
         distinct = set(self.heading.names) == set(self.primary_key)
         return "SELECT {distinct}{fields} FROM {from_}{where}{group_by}{sorting}".format(

src/datajoint/heading.py

@@ -322,7 +322,7 @@ def as_dtype(self) -> np.dtype:
         """
         return np.dtype(dict(names=self.names, formats=[v.dtype for v in self.attributes.values()]))
 
-    def as_sql(self, fields: list[str], include_aliases: bool = True) -> str:
+    def as_sql(self, fields: list[str], include_aliases: bool = True, adapter=None) -> str:
         """
         Generate SQL SELECT clause for specified fields.
 
@@ -332,19 +332,22 @@ def as_sql(self, fields: list[str], include_aliases: bool = True) -> str:
             Attribute names to include.
         include_aliases : bool, optional
             Include AS clauses for computed attributes. Default True.
+        adapter : DatabaseAdapter, optional
+            Database adapter for identifier quoting. If not provided, attempts
+            to get from table_info connection.
 
         Returns
         -------
         str
             Comma-separated SQL field list.
         """
         # Get adapter for proper identifier quoting
-        adapter = None
-        if self.table_info and "conn" in self.table_info and self.table_info["conn"]:
+        if adapter is None and self.table_info and "conn" in self.table_info and self.table_info["conn"]:
             adapter = self.table_info["conn"].adapter
 
         def quote(name):
-            return adapter.quote_identifier(name) if adapter else f"`{name}`"
+            # Use adapter if available, otherwise use ANSI SQL double quotes (not backticks)
+            return adapter.quote_identifier(name) if adapter else f'"{name}"'
 
         return ",".join(
             (