?:: (question double colon sign) operator
Applies to: 
Databricks Runtime 15.3 and above
Casts the value expr to the target data type type with error toleration. This operator is a synonym for try_cast function.
Syntax
expr ?:: type
Arguments
expr: Any castable expression.
Returns
The result is type type.
Examples
SQL
> SELECT '20'?::INTEGER;
20
> SELECT 'twenty'?::INTEGER;
NULL
> SELECT typeof(NULL?::STRING);
string