Optional match does not support where clauses (#1896)

* Optional-match-does-not-support-where-clauses-

* Update optional-match.md

* Update optional-match.md

docs-2.0/2.quick-start/6.cheatsheet-for-ngql.md

@@ -213,7 +213,7 @@
 
  | Pattern | Example | Description |
  | --------------------------- | ------------------------------------------------------------ | ------------------------------------------------------------ |
- |Matches patterns against your graph database, just like `MATCH` does. | `MATCH (m)-[]->(n) WHERE id(m)=="player100" OPTIONAL MATCH (n)-[]->(l) WHERE id(n)=="player125" RETURN id(m),id(n),id(l)` | If no matches are found, `OPTIONAL MATCH` will use a null for missing parts of the pattern.|
+ |Matches patterns against your graph database, just like `MATCH` does. | `MATCH (m)-[]->(n) WHERE id(m)=="player100" OPTIONAL MATCH (n)-[]->(l) RETURN id(m),id(n),id(l)` | If no matches are found, `OPTIONAL MATCH` will use a null for missing parts of the pattern.|
 
 * [LOOKUP](../3.ngql-guide/7.general-query-statements/5.lookup.md)
 

docs-2.0/3.ngql-guide/7.general-query-statements/optional-match.md

@@ -10,19 +10,27 @@ The `OPTIONAL MATCH` clause is used to search for the pattern described in it. `
 
 This topic applies to the openCypher syntax in nGQL only.
 
+## Limitations
+
+The `WHERE` clause cannot be used in an `OPTIONAL MATCH` clause.
+
 ## Example
 
 The example of the use of `OPTIONAL MATCH` in the `MATCH` statement is as follows:
 
 ```ngql
 nebula> MATCH (m)-[]->(n) WHERE id(m)=="player100" \
- OPTIONAL MATCH (n)-[]->(l) WHERE id(n)=="player125" \
+ OPTIONAL MATCH (n)-[]->(l) \
  RETURN id(m),id(n),id(l);
 +-------------+-------------+-------------+
 | id(m) | id(n) | id(l) |
 +-------------+-------------+-------------+
 | "player100" | "team204" | __NULL__ |
-| "player100" | "player101" | __NULL__ |
+| "player100" | "player101" | "team204" |
+| "player100" | "player101" | "team215" |
+| "player100" | "player101" | "player100" |
+| "player100" | "player101" | "player102" |
+| "player100" | "player101" | "player125" |
 | "player100" | "player125" | "team204" |
 | "player100" | "player125" | "player100" |
 +-------------+-------------+-------------+
@@ -32,11 +40,16 @@ Using multiple `MATCH` instead of `OPTIONAL MATCH` returns rows that match the p
 
 ```ngql
 nebula> MATCH (m)-[]->(n) WHERE id(m)=="player100" \
- MATCH (n)-[]->(l) WHERE id(n)=="player125" \
+ MATCH (n)-[]->(l) \
  RETURN id(m),id(n),id(l);
 +-------------+-------------+-------------+
 | id(m) | id(n) | id(l) |
 +-------------+-------------+-------------+
+| "player100" | "player101" | "team204" |
+| "player100" | "player101" | "team215" |
+| "player100" | "player101" | "player100" |
+| "player100" | "player101" | "player102" |
+| "player100" | "player101" | "player125" |
 | "player100" | "player125" | "team204" |
 | "player100" | "player125" | "player100" |
 +-------------+-------------+-------------+