fix AVG over nullable int?/long? window

src/ExpressiveSharp.EntityFrameworkCore.RelationalExtensions/Infrastructure/Internal/WindowFunctionMethodCallTranslator.cs

@@ -81,7 +81,7 @@ [new OrderingExpression(
                     // cast the argument so SQL computes a floating-point AVG, not integer division.
                     [NeedsFloatCast(expr, method.ReturnType)
                         ? _sqlExpressionFactory.ApplyDefaultTypeMapping(
-                            _sqlExpressionFactory.Convert(expr, method.ReturnType))
+                            _sqlExpressionFactory.Convert(expr, typeof(double)))
                         : expr],
                     spec, method.ReturnType),
 
@@ -209,7 +209,7 @@ private WindowFunctionSqlExpression MakeNavigation(
     // SQL Server performs integer division for AVG(int) — cast the argument when the
     // CLR return type is floating-point but the expression is an integer type.
     private static bool NeedsFloatCast(SqlExpression expr, Type returnType) =>
-        returnType == typeof(double)
+        (returnType == typeof(double) || returnType == typeof(double?))
         && expr.Type is var t
         && (t == typeof(int) || t == typeof(long) || t == typeof(int?) || t == typeof(long?));
 

tests/ExpressiveSharp.EntityFrameworkCore.IntegrationTests/Infrastructure/WindowFunctionTestBase.cs

@@ -798,6 +798,31 @@ public async Task Average_IntColumn_ReturnsDouble()
         Assert.AreEqual(2.1, results[^1].AvgQty, 0.01);
     }
 
+    [TestMethod]
+    public async Task Average_NullableIntColumn_CastsToFloat()
+    {
+        var query = Context.Orders
+            .Select(o => new
+            {
+                o.Id,
+                AvgCustomer = WindowFunction.Average(o.CustomerId,
+                    Window.OrderBy(o.Id)
+                          .RowsBetween(WindowFrameBound.UnboundedPreceding, WindowFrameBound.UnboundedFollowing)),
+            })
+            .OrderBy(x => x.Id);
+
+        var sql = query.ToQueryString();
+        StringAssert.Contains(sql, "AVG");
+        // Float cast renders per-provider: CAST(... AS ...) on most, ::double precision on PostgreSQL.
+        Assert.IsTrue(sql.Contains("CAST(") || sql.Contains("::"),
+            $"AVG argument should be cast to floating point, but no cast was found in: {sql}");
+
+        var results = await query.ToListAsync();
+        Assert.AreEqual(10, results.Count);
+        // AVG of CustomerId (five 1s + five 2s = 15/10) must be 1.5, not integer-divided to 1.
+        Assert.AreEqual(1.5, results[0].AvgCustomer!.Value, 0.01);
+    }
+
     [TestMethod]
     public async Task Sum_WithoutFrame_UsesDefaultFrame()
     {