Add nested JOIN example to README and tests

Add documentation and test example demonstrating how to use BuilderAs
to create nested JOIN queries. This is useful when joining with filtered
or transformed datasets.

fix #217

Author: huandu

README.md

@@ -15,6 +15,7 @@
   - [Build SQL for different systems](#build-sql-for-different-systems)
   - [Using `Struct` as a light weight ORM](#using-struct-as-a-light-weight-orm)
   - [Nested SQL](#nested-sql)
+  - [Nested `JOIN`](#nested-join)
   - [Use `sql.Named` in a builder](#use-sqlnamed-in-a-builder)
   - [Argument modifiers](#argument-modifiers)
   - [Freestyle builder](#freestyle-builder)
@@ -348,6 +349,38 @@ fmt.Println(args)
 // [4 1]
 ```
 
+### Nested `JOIN`
+
+In addition to nested subqueries, you can also use `BuilderAs` to create nested JOINs. This is particularly useful when you need to join with a filtered or transformed dataset.
+
+Here is an example showing how to join a table with a nested subquery:
+
+```go
+sb := sqlbuilder.NewSelectBuilder()
+nestedSb := sqlbuilder.NewSelectBuilder()
+
+// Build the nested subquery
+nestedSb.Select("b.id", "b.user_id")
+nestedSb.From("users2 AS b")
+nestedSb.Where(nestedSb.GreaterThan("b.age", 20))
+
+// Build the main query with nested join
+sb.Select("a.id", "a.user_id")
+sb.From("users AS a")
+sb.Join(
+    sb.BuilderAs(nestedSb, "b"),
+    "a.user_id = b.user_id",
+)
+
+sql, args := sb.Build()
+fmt.Println(sql)
+fmt.Println(args)
+
+// Output:
+// SELECT a.id, a.user_id FROM users AS a JOIN (SELECT b.id, b.user_id FROM users2 AS b WHERE b.age > ?) AS b ON a.user_id = b.user_id
+// [20]
+```
+
 ### Use `sql.Named` in a builder
 
 The `sql.Named` function, as defined in the `database/sql` package, facilitates the creation of named arguments within SQL statements. This feature is essential for scenarios where an argument needs to be reused multiple times within a single SQL statement. Incorporating named arguments into a builder is straightforward: treat them as regular arguments.

select_test.go

@@ -117,6 +117,32 @@ func ExampleSelectBuilder_join() {
 	// [1 2 5 %Du 86400]
 }
 
+func ExampleSelectBuilder_nestedJoin() {
+	sb := NewSelectBuilder()
+	nestedSb := NewSelectBuilder()
+
+	// Build the nested subquery
+	nestedSb.Select("b.id", "b.user_id")
+	nestedSb.From("users2 AS b")
+	nestedSb.Where(nestedSb.GreaterThan("b.age", 20))
+
+	// Build the main query with nested join
+	sb.Select("a.id", "a.user_id")
+	sb.From("users AS a")
+	sb.Join(
+		sb.BuilderAs(nestedSb, "b"),
+		"a.user_id = b.user_id",
+	)
+
+	sql, args := sb.Build()
+	fmt.Println(sql)
+	fmt.Println(args)
+
+	// Output:
+	// SELECT a.id, a.user_id FROM users AS a JOIN (SELECT b.id, b.user_id FROM users2 AS b WHERE b.age > ?) AS b ON a.user_id = b.user_id
+	// [20]
+}
+
 func ExampleSelectBuilder_limit_offset() {
 	flavors := []Flavor{MySQL, PostgreSQL, SQLite, SQLServer, CQL, ClickHouse, Presto, Oracle, Informix, Doris}
 	results := make([][]string, len(flavors))