operand:
          constant
     |    param_ref
     |    variable_ref
     |    function_ref
     |    sub_query
     |    ( value_expr )

param_ref:
          ? | : param_name

variable_ref:
          global_var | routine_var | column_var

global_var:
          [@] var_name

routine_var:
          [routine_name. | block_name.]var_name

column_var:
          [{table_name | correlation_name}.]column_name