Simple expression can be either a column, a constant, or NULL.
A column expression refers to a column in a table by using its name, which can be either a fully qualified name or a simple name.
column_expression ::= [keyspace_name.][table_name.][column_name]
A constant expression represents a simple value by using literals.
constant_expression ::= string | number
When an expression, typically a column, does not have a value, it is represented as NULL.
null_expression ::= NULL