fix(mssql): map bare Stddev to STDEV, not STDEVP

ibis/backends/sql/dialects.py

@@ -360,7 +360,7 @@ class MSSQL(TSQL):
     class Generator(TSQL.Generator):
         TRANSFORMS = TSQL.Generator.TRANSFORMS.copy() | {
             sge.ApproxDistinct: rename_func("approx_count_distinct"),
-            sge.Stddev: rename_func("stdevp"),
+            sge.Stddev: rename_func("stdev"),
             sge.StddevPop: rename_func("stdevp"),
             sge.StddevSamp: rename_func("stdev"),
             sge.Variance: rename_func("var"),

ibis/backends/sql/tests/test_compiler.py

@@ -4,7 +4,7 @@
 
 import ibis
 from ibis import _
-from ibis.backends.sql.dialects import Trino
+from ibis.backends.sql.dialects import MSSQL, Trino
 
 
 def test_window_with_row_number_compiles():
@@ -26,3 +26,24 @@ def test_transpile_join():
         "SELECT * FROM t1 JOIN t2 ON x = y", read="duckdb", write=Trino
     )
     assert "CROSS JOIN" not in result
+
+
+def test_mssql_stdev_roundtrips_as_sample():
+    # GH #12057: sqlglot parses T-SQL STDEV (sample) into sge.Stddev. The MSSQL
+    # dialect used to render that node as STDEVP (population), so a same-
+    # dialect round trip through Table.sql() / add_query_to_expr silently
+    # changed the statistic. Bare Stddev must render as STDEV; StddevPop
+    # already covers STDEVP.
+    (result,) = sg.transpile(
+        "SELECT STDEV([x]) AS [s] FROM [t]", read=MSSQL, write=MSSQL
+    )
+    assert result == "SELECT STDEV([x]) AS [s] FROM [t]"
+
+
+def test_mssql_std_how_emits_distinct_functions():
+    t = ibis.table({"x": "float"}, name="t")
+    sample_sql = ibis.to_sql(t.x.std(how="sample"), dialect="mssql")
+    pop_sql = ibis.to_sql(t.x.std(how="pop"), dialect="mssql")
+    assert "STDEV(" in sample_sql
+    assert "STDEVP(" not in sample_sql
+    assert "STDEVP(" in pop_sql