[docs] add 'create tag/edgetype' statement and 'show syntax' docs (ve…

…soft-inc#797)

docs/get-started.md

@@ -235,7 +235,7 @@ Make sure all the services are working
 - -u is to set user name, `user` is the default Nebula user account
 - -p is to set password, `password` is the default password for account `user`
 
-Add host to make console connect to the Nebula service:
+`Add HOSTS` to register the storage hosts:
 
 ```
 > ADD HOSTS $storage_ip:$storage_port

docs/manual-doc/statement-syntax/data-definition-statements/create-tag-edge-syntax.md

@@ -0,0 +1,87 @@
+### Create TAG / EDGE Syntax
+
+```
+CREATE {TAG | EDGE} tag_name|edge_name
+    (create_definition, ...)
+    [tag_edge_options]
+  
+create_definition:
+    prop_name data_type
+    
+tag_edge_options:
+    option [, option ...]
+
+option:
+    TTL_DURATION [=] ttl_duration
+    | TTL_COL [=] prop_name
+```
+
+Nebula's graph schema is composed of tags and edges, either of which may have properties. `CREATE TAG` statement defines a tag with the given name. `CREATE EDGE` statement is to define an edge type.
+
+There are several aspects to this syntax, described under the following topics in this section:
+
+* [Tag Name and Edge Type Name](#tag-name-and-edgetype-name)
+
+* [Property Name and Data Type](#property-name-and-data-type)
+
+### Tag Name and EdgeType Name
+
+* **tag_name and edge_name**
+
+    The name of tags and edgeTypes should be **unique** within the space. Once the name is defined, it can not be altered. The rules of tag and edgeType names are the same as those for names of spaces. See [Schema Object Name](../../language-structure/schema-object-names.md)
+
+### Property Name and Data Type
+
+* **prop_name**
+
+    prop_name indicates the name of properties. It must be unique for each tag or edgeType.
+
+* **data_type**
+
+    data_type represents the data type of each property. For more information about data types that Nebula Graph supports, see data types section.
+
+    > NULL and NOT NULL constrain are not supported yet when creating tags/edges (comparing with relational databases).
+
+### Time-to-Live (TTL) syntax
+
+* TTL_DURATION
+
+    ttl_duration specifies the life cycle of vertices (or edges). TTL expires the vertexes or edges after the specified number of seconds has passed since the TTL_COL configured value; i.e. the expiration threshold is TTL_COL configured property's value plus the specified number of seconds.
+
+    > If the value for ttl_duration is zero or negative, the vertices or edges will not expire.
+
+* TTL_COL
+
+    The data type of prop_name must be either int64 or timestamp.
+
+* multiple TTL definition
+
+    If TTL_COL is a list of prop_name, and there are multiple ttl_duration, **Nebula Graph** uses the lowest(i.e. earliest) expiration threshold to expire data.
+
+### Examples
+
+```
+CREATE TAG course(name string, credits int) 
+CREATE TAG notag()  -- empty properties
+
+CREATE EDGE follow(start_time timestamp, likeness double)
+CREATE EDGE noedge()  -- empty properties
+
+CREATE TAG woman(name string, age int, 
+   married bool, salary double, create_time timestamp)
+   TTL_DRUATION = 100, TTL_COL = create_time -- expired when now is later than create_time + 100
+   
+CREATE EDGE marriage(location string, since timestamp)
+    TTL_DURATION = 0, TTL_COL = since -- negative or zero, not expire
+   
+CREATE TAG icecream(made timestamp, temprature int)
+   TTL_DURATION = 100, TTL_COL = made,
+   TTL_DRUATION = 10, TTL_COL = temprature 
+   --  no matter which comes first: made + 100 or temprature + 10
+ 
+CREATE EDGE garbge (thrown timestamp, temprature int)
+   TTL_DURATION = -2, TTL_COL = thrown, 
+   TTL_DRUATION = 10, TTL_COL = thrown 
+   --  legal, but not recommended. expired at thrown + 10
+```
+

docs/manual-doc/statement-syntax/data-definition-statements/show-syntax.md

@@ -0,0 +1,29 @@
+```
+SHOW {SPACES | TAGS | EDGES | HOSTS}
+SHOW VARIABLES [graph|meta|storage] 
+```
+
+`SHOW SPACES` lists the SPACES on the Nebula Graph cluster. 
+
+`SHOW TAGS` and `SHOW EDGES` return the defined tags and edge types in the space, respectively. 
+
+`SHOW HOSTS` is to list storage hosts registered by the meta server.
+
+For more information about `SHOW VARIABLES [graph|meta|storage]`, please refer to [variable syntax](../data-administration-statements/configuration-statements/variables-syntax.md).
+
+### Example
+
+```
+nebula> SHOW SPACES;
+========
+| Name |
+========
+| test |
+--------
+
+nebula> USE test;
+
+nebula> SHOW TAGS;
+
+nebula> SHOW EDGES;
+```