postgresql - Double colon (::) notation in SQL

Question

Have picked up someone's code and this is a part of a where clause, anyone know what the double colon indicates?

b.date_completed >  a.dc::date + INTERVAL '1 DAY 7:20:00'

Answer 1

It varies based on RDBMS, but if I guess right, that's PostgreSQL, in which case the :: converts a.dc to a date type of date.

In other flavors...

In MS SQL Server 2000:

For built-in user-defined functions that return a table, the function name must be specified with a leading double colon (::) to distinguish it from user-defined functions that are not built-in. It also must be specified as a one-part name with no database or owner qualifications. For example: SELECT * FROM ::fn_helpcollations() b.. For built-in user-defined functions that return a scalar value, the function name must be specified as a one-part name (do not specify database or owner). Do not specify a leading double colon (::).

In MS SQL Server 2005:

Double-colons are no longer required for UDFs that return a table.

However...

Double-colons are required in SQL Server 2005 when granting permissions on schemas, certificates, endpoints, and a few other securables.

As well as...

When using User-Defined Types, static methods of the type must be called using the double-colon syntax.

Sources: BOL and Kalen Delaney's Blog