查询语句校验边或点的字段是否存在

[GitLLTT]

Is your feature request related to a problem? Please describe.

问题：目前查询语句没有校验字段是否存在，字段名是否正确的功能
Describe the solution you'd like

建议：希望增加校验字段名是否存在的功能
Describe alternatives you've considered

Additional context

[xtcyclist]

你好，这个问题在当前版本中还是比较复杂的，因为当前版本同时支持 nGQL 和 openCypher，同时支持强 schema 和弱 schema 两种情形，有的字段缺失会报错，有的并不会，体现为 NULL 或者 UNKOWN_PROP 等 NULL 值。

请问具体到你的需求，你是希望只要有字段不存在的记录存在，查询就直接报错吗？

下面简单介绍一下现状：

对于点来说：

1. 如果一个 tag 不存在，match (v:test) where id(v) == "Tim Duncan" return v;，目前会报错：[ERROR (-1009)]: SemanticError: `test': Unknown tag。
2. 如果一个 tag 存在，但是相应的点上没有这个 tag，当前版本不会报错，比如 match (v:team) where id(v) == "Tim Duncan" return v;，返回是空的 result set。
3. 如果查询一个不存在的属性，比如match (v) where id(v) == "Tim Duncan" return v.team.name; ，不会报错，但是结果可能有不同情形，有的 query 下是__NULL__，有时是UNKNOWN_PROP，这是因为当前版本同时支持 nGQL 和 openCyper 造成的。

对于边来说：

如下语句为例，如果有相应属性，能返回；没有的话，返回__NULL__。

(root@nebula) [nba]> match (v:player)-[e]-() where id(v) == "Tim Duncan" return e.likeness, e;
+------------+-------------------------------------------------------------------------------------+
| e.likeness | e                                                                                   |
+------------+-------------------------------------------------------------------------------------+
| __NULL__   | [:teammate "Manu Ginobili"->"Tim Duncan" @0 {end_year: 2016, start_year: 2002}]     |
| __NULL__   | [:teammate "Tony Parker"->"Tim Duncan" @0 {end_year: 2016, start_year: 2001}]       |
| __NULL__   | [:teammate "Tim Duncan"->"Danny Green" @0 {end_year: 2016, start_year: 2010}]       |
| __NULL__   | [:teammate "Tim Duncan"->"LaMarcus Aldridge" @0 {end_year: 2016, start_year: 2015}] |
| __NULL__   | [:teammate "Tim Duncan"->"Manu Ginobili" @0 {end_year: 2016, start_year: 2002}]     |
| __NULL__   | [:teammate "Tim Duncan"->"Tony Parker" @0 {end_year: 2016, start_year: 2001}]       |
| __NULL__   | [:serve "Tim Duncan"->"Spurs" @0 {end_year: 2016, start_year: 1997}]                |
| 80         | [:like "Aron Baynes"->"Tim Duncan" @0 {likeness: 80}]                               |
| 80         | [:like "Boris Diaw"->"Tim Duncan" @0 {likeness: 80}]                                |
| 70         | [:like "Danny Green"->"Tim Duncan" @0 {likeness: 70}]                               |
| 99         | [:like "Dejounte Murray"->"Tim Duncan" @0 {likeness: 99}]                           |
| 75         | [:like "LaMarcus Aldridge"->"Tim Duncan" @0 {likeness: 75}]                         |
| 90         | [:like "Manu Ginobili"->"Tim Duncan" @0 {likeness: 90}]                             |
| 55         | [:like "Marco Belinelli"->"Tim Duncan" @0 {likeness: 55}]                           |
| 80         | [:like "Shaquille O'Neal"->"Tim Duncan" @0 {likeness: 80}]                          |
| 80         | [:like "Tiago Splitter"->"Tim Duncan" @0 {likeness: 80}]                            |
| 95         | [:like "Tony Parker"->"Tim Duncan" @0 {likeness: 95}]                               |
| 95         | [:like "Tim Duncan"->"Manu Ginobili" @0 {likeness: 95}]                             |
| 95         | [:like "Tim Duncan"->"Tony Parker" @0 {likeness: 95}]                               |
+------------+-------------------------------------------------------------------------------------+