Question

I am trying to write a PLSQL function that implements a constraint that an employee cannot be both a driver and a mechanic at the same time, in other words, the same E# from TRKEMPLOYEE cannot be in TRKDRIVER and TRKMECHANIC at the same time. If the contents of the DB violate that constraint, list the E# and NAME of all employees that are both mechanics and drivers. The abovementioned tables are something like as follows:

TRKEMPLOYEE(E#              NUMBER(12)      NOT NULL
    NAME            VARCHAR(50)     NOT NULL,
    DOB             DATE                    ,
    ADDRESS         VARCHAR(300)    NOT NULL,
    HIREDATE        DATE            NOT NULL,
CONSTRAINT TRKEMPLOYEE_PKEY PRIMARY KEY(E#))

TRKDRIVER(E#              NUMBER(12)      NOT NULL
L#              NUMBER(8)       NOT NULL,
STATUS          VARCHAR(10)     NOT NULL,
CONSTRAINT TRKDRIVER_PKEY PRIMARY KEY(E#),
CONSTRAINT TRKDRIVER_FKEY FOREIGN KEY(E#) REFERENCES TRKEMPLOYEE(E#))

TRKMECHANIC(E#              NUMBER(12)      NOT NULL
L#              NUMBER(8)       NOT NULL,
STATUS          VARCHAR(10)     NOT NULL,
EXPERIENCE      VARCHAR(10)     NOT NULL,
CONSTRAINT TRKMECHANIC_PKEY PRIMARY KEY(E#),
CONSTRAINT TRKMECHANIC_FKEY FOREIGN KEY(E#) REFERENCES TRKEMPLOYEE(E#))

I have attempted to write a function but keep getting a compile error in line 1 column 7. Can someone tell me why my code doesn’t work? My code is as follows

CREATE OR REPLACE FUNCTION Verify()
IS DECLARE
  E# TRKEMPLOYEE.E#%TYPE;
  CURSOR C1 IS SELECT E# FROM TRKEMPLOYEE;
BEGIN
  OPEN C1;
  LOOP
   FETCH C1 INTO EMPNUM;
   IF(EMPNUM IN(SELECT E# FROM TRKMECHANIC )AND EMPNUM IN(SELECT E# FROM TRKDRIVER))
     SELECT E#, NAME FROM TRKEMPLOYEE WHERE E#=EMPNUM;
   ELSE
     dbms_output.put_line(“OK”);
   ENDIF
   EXIT WHEN C1%NOTFOUND;
  END LOOP;
  CLOSE C1;
END;
/

Any help would be appreciated. Thanks.

Answer 1

You are creating a function but missing the RETURN declaration. If you don’t want to return a result, the use create or replace procedure.

Additionally, if you don’t have any parameters, remove the brackets () and the DECLARE keyword is not correct either.

---

The the way you check the tables is not really efficient. You can get the count of all employees violating your business constraint with a single query:

select emp.e#, 
       emp.name,
       count(drv.e#) + count(mec.e#) as cnt
from trkemployee emp
   left join trkdriver drv on drv.e# = emp.e#
   left join trkmechanic mec on mec.e# = emp.e#
group by emp.e#, emp.name
having count(drv.e#) + count(mec.e#) > 1;

Additionally the EXIT WHEN C1%NOTFOUND; should be right after the FETCH statement. Otherwise you are staying in the loop even though the cursor didn’t fetch anything.

If you take my simplified statement, you can loop once through all employees and print OK, nor Not OK depending on the count:

CREATE OR REPLACE procedure Verify
IS 
  empnum number(12);
  cnt    integer;
  empname varchar(50);

  CURSOR C1 IS 
      select emp.e#,
             emp.name,
             count(drv.e#) + count(mec.e#) as cnt  
      from trkemployee emp
         left join trkdriver drv on drv.e# = emp.e#
         left join trkmechanic mec on mec.e# = emp.e#
      group by emp.e#, emp.name;
BEGIN
  OPEN C1;
  LOOP
    FETCH C1 INTO empnum, empname, cnt;
    EXIT WHEN C1%NOTFOUND;

    if cnt > 1 then 
       dbms_output.put_line(to_char(empnum)||' NOK');
    else
       dbms_output.put_line(to_char(empnum)||' OK');
    end if;

  END LOOP;
CLOSE C1;
END;
/

---

Solving the problem without a stored procedure

I was thinking about a way how this business rule can be enforced with only constraints in the database:

create table trkemployee
(
    E#              NUMBER(12)      NOT NULL,
    NAME            VARCHAR(50)     NOT NULL,
    DOB             DATE                    ,
    ADDRESS         VARCHAR(300)    NOT NULL,
    HIREDATE        DATE            NOT NULL,
    emp_type        char(1)         NOT NULL,
    constraint trkemployee_pkey primary key(e#, emp_type),
    constraint chk_emp_type check (emp_type in ('d','m'))
);

create table TRKDRIVER
(
  e#              number(12)      not null,
  emp_type        char(1)         default 'd' not null,
  l#              number(8)       not null,
  status          varchar(10)     not null,
  constraint trkdriver_pkey primary key(e#),
  constraint trkdriver_fkey foreign key(e#,emp_type) references trkemployee(e#, emp_type),
  constraint check_drv_type check (emp_type = 'd')
);

create table trkmechanic
(
  e#              number(12)      not null,
  emp_type        char(1)         default 'm' not null,
  l#              number(8)       not null,
  status          varchar(10)     not null,
  experience      varchar(10)     not null,
  constraint trkmechanic_pkey primary key(e#),
  constraint trkmechanic_fkey foreign key(e#,emp_type) references trkemployee(e#, emp_type),
  constraint check_mec_type check (emp_type = 'm')
);

The idea is to include the employee type in its foreign key, and to make sure that the dependent tables cannot use the wrong type. When inserting a new trkemployee the type must be specified. The check constraint on the detail tables will reject any row with the wrong type.

To “move” an employee from one type to the other (if this is possible at all), the old detail row must be deleted first. Only then the employee’s type can be changed to the other type. And finally the new detail could be inserted.