fix(tsql): ALTER COLUMN with a nullability clause is parsed as a Command

sqlmesh/core/dialect.py

@@ -551,6 +551,36 @@ def _parse_table_parts(
     return table
 
 
+# Only needed for T-SQL: it spells a column's nullability right after its type, e.g.
+# ALTER TABLE t ALTER COLUMN c INT NOT NULL. Without this the trailing clause is left
+# over, so the whole statement falls back to a Command and any macros it contains (such as
+# @this_model) are no longer resolved, which means they reach the engine verbatim.
+#
+# See: https://learn.microsoft.com/en-us/sql/t-sql/statements/alter-table-transact-sql
+def _parse_alter_table_alter(self: Parser) -> t.Optional[exp.Expr]:
+    alter_column = self.__parse_alter_table_alter()  # type: ignore
+
+    if isinstance(alter_column, exp.AlterColumn) and alter_column.args.get("dtype"):
+        if self._match_pair(TokenType.NOT, TokenType.NULL):
+            alter_column.set("allow_null", False)
+        elif self._match(TokenType.NULL):
+            alter_column.set("allow_null", True)
+
+    return alter_column
+
+
+def altercolumn_sql(self: Generator, expression: exp.AlterColumn) -> str:
+    sql = self._altercolumn_sql(expression)  # type: ignore
+
+    # sqlglot's generator returns as soon as it renders the type, so the nullability parsed
+    # above has to be appended here
+    allow_null = expression.args.get("allow_null")
+    if expression.args.get("dtype") and allow_null is not None:
+        sql = f"{sql} NULL" if allow_null else f"{sql} NOT NULL"
+
+    return sql
+
+
 def _parse_if(self: Parser) -> t.Optional[exp.Expr]:
     # If we fail to parse an IF function with expressions as arguments, we then try
     # to parse a statement / command to support the macro @IF(condition, statement)
@@ -780,7 +810,7 @@ def _parse_interval_span(self: Parser, this: exp.Expr) -> exp.Interval:
     return interval
 
 
-def _override(klass: t.Type[Tokenizer | Parser], func: t.Callable) -> None:
+def _override(klass: t.Type[Tokenizer | Parser | Generator], func: t.Callable) -> None:
     name = func.__name__
     setattr(klass, f"_{name}", getattr(klass, name))
     setattr(klass, name, func)
@@ -1194,6 +1224,8 @@ def extend_sqlglot() -> None:
     _override(Parser, _parse_interval_span)
     _override(Parser, _warn_unsupported)
     _override(Snowflake.Parser, _parse_table_parts)
+    _override(TSQL.Parser, _parse_alter_table_alter)
+    _override(TSQL.Generator, altercolumn_sql)
 
     # DuckDB's prefix absolute power operator `@` clashes with the macro syntax
     DuckDB.Parser.NO_PAREN_FUNCTION_PARSERS.pop("@", None)

tests/core/test_dialect.py

@@ -995,6 +995,38 @@ def test_conditional_statement():
     assert q.sql(dialect="tsql") == "@IF(@runtime_stage = 'evaluating', SELECT 1)"
 
 
+def test_tsql_alter_column_nullability():
+    # Issue #5932: T-SQL spells nullability right after the type in ALTER COLUMN. Without support
+    # for it the statement falls back to a Command, so any macros it contains go unresolved.
+    for sql, expected in [
+        (
+            "ALTER TABLE x ALTER COLUMN y INT NOT NULL",
+            "ALTER TABLE x ALTER COLUMN y INTEGER NOT NULL",
+        ),
+        ("ALTER TABLE x ALTER COLUMN y INT NULL", "ALTER TABLE x ALTER COLUMN y INTEGER NULL"),
+        ("ALTER TABLE x ALTER COLUMN y INT", "ALTER TABLE x ALTER COLUMN y INTEGER"),
+    ]:
+        e = parse_one(sql, read="tsql")
+        assert isinstance(e, exp.Alter)
+        assert e.sql(dialect="tsql") == expected
+
+    # The macro must survive parsing so that it can be resolved later
+    e = parse_one(
+        "@IF(@runtime_stage = 'creating', ALTER TABLE @SQL('@this_model') ALTER COLUMN id INT NOT NULL);",
+        read="tsql",
+    )
+    assert (
+        e.sql(dialect="tsql")
+        == "@IF(@runtime_stage = 'creating', ALTER TABLE @SQL('@this_model') ALTER COLUMN id INTEGER NOT NULL)"
+    )
+
+    # Statements that don't carry a type are unaffected
+    assert (
+        parse_one("ALTER TABLE x ALTER COLUMN y DROP NOT NULL", read="tsql").sql(dialect="tsql")
+        == "ALTER TABLE x ALTER COLUMN y DROP NOT NULL"
+    )
+
+
 def test_model_name_cannot_be_string():
     with pytest.raises(ParseError) as parse_error:
         parse(

tests/core/test_model.py

@@ -1958,6 +1958,45 @@ def test_render_definition():
     assert "def test_macro(evaluator, v):" in d.format_model_expressions(model.render_definition())
 
 
+def test_tsql_alter_column_post_statement(make_snapshot: t.Callable) -> None:
+    # Issue #5932: the trailing NOT NULL made this parse as a Command, which left @this_model
+    # unresolved and sent the macro to the engine verbatim.
+    expressions = d.parse(
+        """
+        MODEL (
+            name test.test_model,
+            dialect tsql,
+        );
+
+        SELECT 1 AS id;
+
+        @IF(@runtime_stage = 'creating', ALTER TABLE @SQL('@this_model') ALTER COLUMN id INT NOT NULL);
+        """
+    )
+
+    model = load_sql_based_model(expressions, default_catalog="catalog")
+
+    snapshot = make_snapshot(model)
+    snapshot.categorize_as(SnapshotChangeCategory.BREAKING)
+
+    post_statements = model.render_post_statements(
+        snapshots={model.fqn: snapshot},
+        runtime_stage=RuntimeStage.CREATING,
+    )
+
+    assert len(post_statements) == 1
+    assert (
+        post_statements[0].sql(dialect="tsql")
+        == f"ALTER TABLE [catalog].[sqlmesh__test].[test__test_model__{snapshot.version}] /* catalog.test.test_model */ ALTER COLUMN [id] INTEGER NOT NULL"
+    )
+
+    # The statement is skipped outside of the creating stage
+    assert not model.render_post_statements(
+        snapshots={model.fqn: snapshot},
+        runtime_stage=RuntimeStage.EVALUATING,
+    )
+
+
 def test_render_definition_with_defaults():
     query = """
         SELECT