How to run a select without table?

Sometimes you want to get some system variable like CURRENT_TIMESTAMP or result of CAST or GEN_ID function, and you don't have a table to select from. Some other database systems support selecting without a table:

select current_timestamp;

and other's have a dummy 1-row table, like Oracle's DUAL:

select current_timestamp from DUAL;

Firebird uses the same approach. You can use the RDB$DATABASE which is a single-row table, and part of each database's metadata:

select current_timestamp from RDB$DATABASE;

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