## What changes were proposed in this pull request?
This pr targeted to define reserved/non-reserved keywords for Spark SQL based on the ANSI SQL standards and the other database-like systems (e.g., PostgreSQL). We assume that they basically follow the ANSI SQL-2011 standard, but it is slightly different between each other. Therefore, this pr documented all the keywords in `docs/sql-reserved-and-non-reserved-key-words.md`.
NOTE: This pr only added a small set of keywords as reserved ones and these keywords are reserved in all the ANSI SQL standards (SQL-92, SQL-99, SQL-2003, SQL-2008, SQL-2011, and SQL-2016) and PostgreSQL. This is because there is room to discuss which keyword should be reserved or not, .e.g., interval units (day, hour, minute, second, ...) are reserved in the ANSI SQL standards though, they are not reserved in PostgreSQL. Therefore, we need more researches about the other database-like systems (e.g., Oracle Databases, DB2, SQL server) in follow-up activities.
References:
- The reserved/non-reserved SQL keywords in the ANSI SQL standards: https://developer.mimer.com/wp-content/uploads/2018/05/Standard-SQL-Reserved-Words-Summary.pdf
- SQL Key Words in PostgreSQL: https://www.postgresql.org/docs/current/sql-keywords-appendix.html
## How was this patch tested?
Added tests in `TableIdentifierParserSuite`.
Closes#23259 from maropu/SPARK-26215-WIP.
Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
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