:: (Scope Resolution) (Transact-SQL)
Applies to: 
SQL Server
Azure SQL Managed Instance
The scope resolution operator :: provides access to static members of a compound data type. A compound data type is one that contains multiple simple data types and methods. Compound data types include the built-in CLR types and custom SQLCLR User-Defined Types (UDTs).
Examples
The following example shows how to use the scope resolution operator to access the GetRoot() member of the hierarchyid type.
DECLARE @hid hierarchyid;
SELECT @hid = hierarchyid::GetRoot();
PRINT @hid.ToString();
Here's the result set.
/