Add distinct (#1928)

docs-2.0/3.ngql-guide/7.general-query-statements/4.fetch.md

@@ -15,7 +15,7 @@ This topic applies to native nGQL only.
 ```ngql
 FETCH PROP ON {<tag_name>[, tag_name ...] | *}
 <vid> [, vid ...]
-YIELD <return_list> [AS <alias>];
+YIELD [DISTINCT] <return_list> [AS <alias>];
 ```
 
 | Parameter  | Description                                                                                                                                                                                                                                                                  |

docs-2.0/3.ngql-guide/7.general-query-statements/5.lookup.md

@@ -37,7 +37,7 @@ Before using the `LOOKUP` statement, make sure that at least one index is create
 ```ngql
 LOOKUP ON {<vertex_tag> | <edge_type>}
 [WHERE <expression> [AND <expression> ...]]
-YIELD <return_list> [AS <alias>];
+YIELD [DISTINCT] <return_list> [AS <alias>];
 
 <return_list>
     <prop_name> [AS <col_alias>] [, <prop_name> [AS <prop_alias>] ...];
@@ -47,6 +47,8 @@ YIELD <return_list> [AS <alias>];
 
 - `YIELD`: Define the output to be returned. For details, see [`YIELD`](../8.clauses-and-options/yield.md).
 
+- `DISTINCT`: Aggregate the output results and return the de-duplicated result set.
+
 - `AS`: Set an alias.
 
 ## Limitations of using `WHERE` in `LOOKUP`

docs-2.0/3.ngql-guide/8.clauses-and-options/yield.md

@@ -127,6 +127,19 @@ nebula> $var1 = GO FROM "player101" OVER follow \
 +-------------+
 ```
 
+The following query finds the vertices in the player that are older than 30 and younger than 32, and returns the de-duplicate results.
+
+```ngql
+nebula> LOOKUP ON player  \
+        WHERE player.age < 32 and player.age >30  \
+        YIELD DISTINCT properties(vertex).age as v;
++--------+
+| v      |
++--------+
+| 31     |
++--------+
+```
+
 ### Use a standalone YIELD statement
 
 A `YIELD` statement can calculate a valid expression and output the result.