example of scope and order of variable and set operator (#3024)

* example of scope and order of variable and set operator

* Update docs-2.0/3.ngql-guide/4.variable-and-composite-queries/2.user-defined-variables.md

---------

Co-authored-by: abby.huang <78209557+abby-cyber@users.noreply.github.com>

docs-2.0/3.ngql-guide/4.variable-and-composite-queries/2.user-defined-variables.md

@@ -48,3 +48,18 @@ nebula> $var = GO FROM "player100" OVER follow YIELD dst(edge) AS id; \
 | "Spurs"   | "Manu Ginobili" |
 +-----------+-----------------+
 ```
+
+## 集合运算与变量语句的范围与运算顺序
+
+当我们需要对集合运算的复合语句进行变量赋值时，需要注意用括号包裹语句的范围，比如下边的例子里`$var`赋值的来源是两个语句`INTERSECT`之后的输出。
+
+```ngql
+$var = ( \
+    GO FROM "player100" OVER follow \
+      YIELD dst(edge) AS id \
+    INTERSECT \
+    GO FROM "player100" OVER follow \
+      YIELD dst(edge) AS id \
+       ); \
+    GO FROM $var.id OVER follow YIELD follow.degree AS degree
+```