support-set-operators-for-cypher (#2499)

* support-set-operators-for-cypher

* add note and define a different variable

docs-2.0/3.ngql-guide/5.operators/6.set.md

@@ -4,9 +4,9 @@
 
 所有集合运算符的优先级相同，如果一个 nGQL 语句中有多个集合运算符，NebulaGraph 会从左到右进行计算，除非用括号指定顺序。
 
-## openCypher 兼容性
+!!! caution
 
-集合运算符仅适用于原生 nGQL。
+    集合运算符前后的查询语句中定义的变量名及顺序必需保持一致，例如`RETURN a,b,c UNION RETURN a,b,c`中的`a,b,c`的名称及顺序需要保持一致。
 
 ## UNION、UNION DISTINCT、UNION ALL
 
@@ -35,6 +35,21 @@ nebula> GO FROM "player102" OVER follow YIELD dst(edge) \
 | "player125" |
 +-------------+
 
+nebula> MATCH (v:player) \
+        WITH v.player.name AS n \
+        RETURN n ORDER BY n LIMIT 3 \
+        UNION \
+        UNWIND ["Tony Parker", "Ben Simmons"] AS n \
+        RETURN n;
++---------------------+
+| n                   |
++---------------------+
+| "Amar'e Stoudemire" |
+| "Aron Baynes"       |
+| "Ben Simmons"       |
+| "Tony Parker"       |
++---------------------+
+
 # 返回两个查询结果的并集，包含重复的元素。
 nebula> GO FROM "player102" OVER follow YIELD dst(edge) \
         UNION ALL \
@@ -48,6 +63,22 @@ nebula> GO FROM "player102" OVER follow YIELD dst(edge) \
 | "player125" |
 +-------------+
 
+nebula> MATCH (v:player) \
+        WITH v.player.name AS n \
+        RETURN n ORDER BY n LIMIT 3 \
+        UNION ALL \
+        UNWIND ["Tony Parker", "Ben Simmons"] AS n \
+        RETURN n;
++---------------------+
+| n                   |
++---------------------+
+| "Amar'e Stoudemire" |
+| "Aron Baynes"       |
+| "Ben Simmons"       |
+| "Tony Parker"       |
+| "Ben Simmons"       |
++---------------------+
+
 # UNION 也可以和 YIELD 语句一起使用，去重时会检查每一行的所有列，每列都相同时才会去重。
 nebula> GO FROM "player102" OVER follow \
         YIELD dst(edge) AS id, properties(edge).degree AS Degree, properties($$).age AS Age \
@@ -77,6 +108,8 @@ nebula> GO FROM "player102" OVER follow \
 ### 示例
 
 ```ngql
+
+# 返回两个查询结果的交集。
 nebula> GO FROM "player102" OVER follow \
         YIELD dst(edge) AS id, properties(edge).degree AS Degree, properties($$).age AS Age \
         INTERSECT \
@@ -86,6 +119,17 @@ nebula> GO FROM "player102" OVER follow \
 | id | Degree | Age |
 +----+--------+-----+
 +----+--------+-----+
+
+nebula> UNWIND [1,2] AS a RETURN a \
+        INTERSECT \
+        UNWIND [1,2,3,4] AS a \
+        RETURN a;
++---+
+| a |
++---+
+| 1 |
+| 2 |
++---+
 ```
 
 ## MINUS
@@ -99,6 +143,7 @@ nebula> GO FROM "player102" OVER follow \
 ### 示例
 
 ```ngql
+# 返回在第一个查询结果中，但是不在第二个查询结果中的元素。
 nebula> GO FROM "player100" OVER follow YIELD dst(edge) \
         MINUS \
         GO FROM "player102" OVER follow YIELD dst(edge);
@@ -116,6 +161,18 @@ nebula> GO FROM "player102" OVER follow YIELD dst(edge) \
 +-------------+
 | "player100" |
 +-------------+
+
+nebula> UNWIND [1,2,3] AS a RETURN a \
+        MINUS \
+        WITH 4 AS a \
+        RETURN a;
++---+
+| a |
++---+
+| 1 |
+| 2 |
+| 3 |
++---+
 ```
 
 ## 集合运算符和管道符的优先级