Question

I was under the impression that argument-less functions can be called with empty parentheses after the function name, i.e. what some other databases allow to do:

current_timestamp()

Whereas in Oracle, I have to write

current_timestamp

With user-defined functions, this rule doesn’t apply (in 11g). I can write both

my_function
my_function()

My question is: Is CURRENT_TIMESTAMP really a true function or should I consider it to be a language construct / pseudo-column of the Oracle SQL dialect (compatible with the SQL standard)? Is there any formal definition about when I can (optionally, mandatorily) add the () and when I have to omit them?

Background-info:

• SQL 1992 defines:

  <current timestamp value function> ::=
    CURRENT_TIMESTAMP [ <left paren> <timestamp precision> <right paren> ]
  

• Derby, HSQLDB, Ingres, Postgres, SQLite, SQL Server behave like Oracle, where there are no parentheses allowed for CURRENT_TIMESTAMP

• Sybase SQL Anywhere knows a CURRENT TIMESTAMP function (without parentheses, without underscore)
• CUBRID, MySQL, Sybase ASE allow for using CURRENT_TIMESTAMP()

Answer 1

SQL standards back to 1992 refer to CURRENT_TIMESTAMP as both a “time-varying system variable” and a “datetime value function”. See, for example, Database Language SQL.

But AFAIK the standards always use CURRENT_TIMESTAMP, never CURRENT_TIMESTAMP(). Using CURRENT_TIMESTAMP() on a compliant dbms should fail with a syntax error.

I’m not sure what the standards have to say about user-defined functions.