问ORA-00904："TIMING"：无效的标识符

EN

create or replace function timing
    ( dat in date, bran in varchar2, audi in number )
    return number
as
    time number;
begin
    select s_end into time
    from   checking
    where  s_date = dat
    and    branch = bran
    and    a_id = audi;

    return time;
end;
/

Function created.

alter table checking add constraint chk check(s_start>timing(s_date,branch,a_id))
                                                      *
ERROR at line 1:
ORA-00904: "TIMING": invalid identifier

SQL> desc checking
 Name                                      Null?    Type
 ----------------------------------------- -------- -------------
 S_ID                                      NOT NULL NUMBER
 M_ID                                               NUMBER
 A_ID                                               NUMBER
 S_DATE                                             DATE
 S_START                                            NUMBER
 S_END                                              NUMBER
 BRANCH                                             VARCHAR2(30)