example of scope and order of variable and set operator (vesoft-inc#2280

)

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

@@ -48,3 +48,18 @@ nebula> $var = GO FROM "player100" OVER follow YIELD dst(edge) AS id; \
 | "Spurs" | "Manu Ginobili" |
 +-----------+-----------------+
 ```
+
+## Set operations and scope of user-defined variables
+
+When assigning variables within a compound statement involving set operations, it is important to enclose the scope of the variable assignment in parentheses. In the example below, the source of the `$var` assignment is the results of the output of two `INTERSECT` statements. 
+
+```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
+```