update

分组聚合查询.md

@@ -61,6 +61,19 @@ var list = fsql.Select<Topic>()
     .ToList(a => new { a.Key.Area.Name });
 ```
 
+## 多表分组
+
+```c#
+var list = fsql.Select<Topic, Category, Area>()
+    .GroupBy((a, b, c) => new { a.Title, c.Name })
+    .Having(g => g.Count() < 300 || g.Avg(g.Value.Item1.Clicks) < 100)
+    .ToList(g => new { count = g.Count(), Name = g.Key.Name });
+```
+
+- g.Value.Item1 对应 Topic
+- g.Value.Item2 对应 Category
+- g.Value.Item3 对应 Area
+
 ## Aggregate
 
 ```c#