maropu commented on issue #20433: [SPARK-23264][SQL] Make INTERVAL keyword 
optional in INTERVAL clauses
URL: https://github.com/apache/spark/pull/20433#issuecomment-464451209
 
 
   I checked if the time unit keywords (YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, 
MILLISECOND, WEEK, MILLISECOND, MICROSECOND) are reserved in SQL-2011/database 
implementations:
   
   SQL-2011
    - reserved: YEAR, MONTH, DAY, HOUR, MINUTE, SECOND
    - non-reserved: WEEK, MILLISECOND, MICROSECOND
   
   PostgreSQL, MySQL, Oracle, and SQL Server
    - reserved:
    - non-reserved: YEAR, MONTH, WEEK, DAY, HOUR, MINUTE, SECOND, MILLISECOND, 
MICROSECOND
   
   Also, all the the plural forms (YEARS, MONTHS, ...) are not reserved in all 
the cases.
   I'm not sure about the historical reason why these databases don't handle 
them as reserved though, IMHO we would be better to follow SQL-2011; YEAR, 
MONTH, DAY, HOUR, MINUTE, and SECOND are reserved if ansi=true.
   
   As for SQL server, these time unit keywords are listed in future keywords:
   
https://docs.microsoft.com/ja-jp/sql/t-sql/language-elements/reserved-keywords-transact-sql?view=sql-server-2017#future-keywords

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