From c864418c1bb20fe92dcd5c705d0ed1c2d1e407df Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Furga=C5=82a?=
 <83299832+00200200@users.noreply.github.com>
Date: Tue, 29 Sep 2026 12:14:16 +0200
Subject: [PATCH] fix(mssql): map bare Stddev to STDEV, not STDEVP

sqlglot parses T-SQL STDEV (sample) into sge.Stddev, but the MSSQL
generator rendered that node as STDEVP (population). Same-dialect
round-trips through Table.sql() therefore changed the statistic.
StddevPop already maps to STDEVP; Variance already maps to the sample
function, so this matches that pattern.

Closes #12057
---
 ibis/backends/sql/dialects.py            |  2 +-
 ibis/backends/sql/tests/test_compiler.py | 23 ++++++++++++++++++++++-
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/ibis/backends/sql/dialects.py b/ibis/backends/sql/dialects.py
index 41e416c9a001..be8be5ac6a39 100644
--- a/ibis/backends/sql/dialects.py
+++ b/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"),
diff --git a/ibis/backends/sql/tests/test_compiler.py b/ibis/backends/sql/tests/test_compiler.py
index 3596a7287b8c..f3966ced8517 100644
--- a/ibis/backends/sql/tests/test_compiler.py
+++ b/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