value_exp_primary

value_exp_primary 
      ::= 
      unsigned_value_spec 
      | column_ref 
      | set_fct_spec 
      | scalar_subquery 
      | case_exp 
      | "(" value_exp ")" 
      | cast_spec 

This rule is called by

-> bit_primary
-> char_primary
-> datetime_primary
-> interval_primary
-> num_primary