How to implement this trigger on Oracle SQL? - oracle

Question

I have found an exercise in the SQL book I study, which is not solved and I can not solve it.
The goal is to implement a trigger that avoids overlapping contracts. If a customer with a current contract signs a new one, the end date of the previous one will be one day before the new start date.
Tables given are:
CREATE TABLE CLIENTS (
clientId VARCHAR2(15),
DNI VARCHAR2(9),
name VARCHAR2(100) NOT NULL,
surname VARCHAR2(100) NOT NULL,
sec_surname VARCHAR2(100),
eMail VARCHAR2(100) NOT NULL,
phoneN NUMBER(12),
birthdate DATE,
CONSTRAINT PK_CLIENTS PRIMARY KEY (clientId),
CONSTRAINT UK1_CLIENTS UNIQUE (DNI),
CONSTRAINT UK2_CLIENTS UNIQUE (eMail),
CONSTRAINT UK3_CLIENTS UNIQUE (phoneN),
);
CREATE TABLE contracts(
contractId VARCHAR2(10),
clientId VARCHAR2(15),
startdate DATE NOT NULL,
enddate DATE,
contract_type VARCHAR2(50),
address VARCHAR2(100) NOT NULL,
town VARCHAR2(100) NOT NULL,
ZIPcode VARCHAR2(8) NOT NULL,
country VARCHAR2(100) NOT NULL,
CONSTRAINT PK_contracts PRIMARY KEY (contractId),
CONSTRAINT FK_contracts1 FOREIGN KEY (clientId) REFERENCES CLIENTS
);
Any suggestions?

Answer 1

I agree with the posted comments that it helps to have some specifics about what is failing in the prior attempts, and I would also recommend not using a TRIGGER at all for this kind of thing.
But as this is for a study exercise, here are some examples that might be a starting place.
I've modified your tables to disallow NULL PRIMARY KEYs in these examples.
To get started, create the tables:
CREATE TABLE CLIENTS (
CLIENTID VARCHAR2(15) NOT NULL,
DNI VARCHAR2(9),
NAME VARCHAR2(100) NOT NULL,
SURNAME VARCHAR2(100) NOT NULL,
SEC_SURNAME VARCHAR2(100),
EMAIL VARCHAR2(100) NOT NULL,
PHONEN NUMBER(12),
BIRTHDATE DATE,
CONSTRAINT PK_CLIENTS PRIMARY KEY (CLIENTID),
CONSTRAINT UK1_CLIENTS UNIQUE (DNI),
CONSTRAINT UK2_CLIENTS UNIQUE (EMAIL),
CONSTRAINT UK3_CLIENTS UNIQUE (PHONEN)
);
CREATE TABLE CONTRACTS (
CONTRACTID VARCHAR2(10) NOT NULL,
CLIENTID VARCHAR2(15) NOT NULL,
STARTDATE DATE NOT NULL,
ENDDATE DATE,
CONTRACT_TYPE VARCHAR2(50),
ADDRESS VARCHAR2(100) NOT NULL,
TOWN VARCHAR2(100) NOT NULL,
ZIPCODE VARCHAR2(8) NOT NULL,
COUNTRY VARCHAR2(100) NOT NULL,
CONSTRAINT PK_CONTRACTS PRIMARY KEY (CONTRACTID),
CONSTRAINT FK_CONTRACTS1 FOREIGN KEY (CLIENTID) REFERENCES CLIENTS
);
Then, create the first CLIENTs:
INSERT INTO CLIENTS VALUES (1,NULL,'Frodo','Baggins',NULL,'the.real.frodo#adventure.com',NULL,NULL);
INSERT INTO CLIENTS VALUES (2,NULL,'Chewbacca','UNKNOWN',NULL,'chewio.#kashyyyk.org',NULL,NULL);
COMMIT;
Then create a TRIGGER. In this first example, the TRIGGER is an AFTER STATEMENT type.
It is simple but inefficient since it evaluates every CLIENT after each INSERT statement.
Against a large data set, or in the face of multiple TRIGGERs, this could be a problem.
This TRIGGER will check for the prior contract and will set its ENDDATE to one day before the new contract, if it is null or after the start of the new contract.
CREATE OR REPLACE TRIGGER CONTRACT_ENDDATE_ADJUSTER
AFTER INSERT ON CONTRACTS
BEGIN
MERGE INTO CONTRACTS
USING (
SELECT CONTRACTID,
CANDIDATE_ENDDATE AS ENDDATE
FROM
(SELECT CONTRACTS.CONTRACTID,
(TRUNC(LEAD(STARTDATE) OVER (PARTITION BY CLIENTID ORDER BY STARTDATE ASC) - 1)) AS CANDIDATE_ENDDATE,
DENSE_RANK() OVER (PARTITION BY CLIENTID ORDER BY STARTDATE DESC) AS CONTRACT_ORDER
FROM CONTRACTS)
WHERE CONTRACT_ORDER = 2) CANDIDATE_CONTRACT
ON (CONTRACTS.CONTRACTID = CANDIDATE_CONTRACT.CONTRACTID)
WHEN MATCHED THEN UPDATE SET CONTRACTS.ENDDATE = CANDIDATE_CONTRACT.ENDDATE
WHERE CONTRACTS.ENDDATE IS NULL OR CONTRACTS.ENDDATE > CANDIDATE_CONTRACT.ENDDATE;
END;
/
Then, test it out.
Add the initial contracts. No enddate changes expected, as these are the first. Frodo's contract here has an end-date already set.
INSERT INTO CONTRACTS VALUES('Break-Ring',1,TO_DATE('19560511','YYYYMMDD'), TO_DATE('19851014','YYYYMMDD'), NULL, 'No 1', 'Doom Mountain', 'MORD', 'Middle-Earth');
INSERT INTO CONTRACTS VALUES('SaveGalaxy',2,TO_DATE('19770615','YYYYMMDD'), NULL, NULL, 'No 75', 'Rwookrrorro', 'RWKR', 'Kashyyyk');
SELECT CONTRACTID, CLIENTID, STARTDATE, ENDDATE FROM CONTRACTS ORDER BY CLIENTID ASC, STARTDATE ASC;
CONTRACTID CLIENTID STARTDATE ENDDATE
Break-Ring 1 11-MAY-56 14-OCT-85
SaveGalaxy 2 15-JUN-77
Then add new contracts.
Frodo's new contract starts before the end of his existing contract, so the enddate will be adjusted.
Chewie's initial contract had no ENDDATE, so it will be adjusted as well.
INSERT INTO CONTRACTS VALUES('GoBackHome',1,TO_DATE('19570219','YYYYMMDD'), NULL, NULL, 'No 13', 'Hobbiton', 'HBTN', 'Middle-Earth');
INSERT INTO CONTRACTS VALUES('DefendHoth',2,TO_DATE('19801115','YYYYMMDD'), NULL, NULL, 'Meteor Crater', 'Ice Ridge', 'METEO', 'Hoth');
SELECT CONTRACTID, CLIENTID, STARTDATE, ENDDATE FROM CONTRACTS ORDER BY CLIENTID ASC, STARTDATE ASC;
CONTRACTID CLIENTID STARTDATE ENDDATE
Break-Ring 1 11-MAY-56 18-FEB-57
GoBackHome 1 19-FEB-57
SaveGalaxy 2 15-JUN-77 14-NOV-80
DefendHoth 2 15-NOV-80
And as other contracts are signed, the pattern continues:
INSERT INTO CONTRACTS VALUES('GoWedding',2,TO_DATE('19830309','YYYYMMDD'), NULL, NULL, 'Main Hall', 'Grand Palace', 'ALLNC', 'Coruscant');
INSERT INTO CONTRACTS VALUES('Gardening',1,TO_DATE('19570503','YYYYMMDD'), NULL, NULL, 'No 13', 'Hobbiton', 'HBTN', 'Middle-Earth');
SELECT CONTRACTID, CLIENTID, STARTDATE, ENDDATE FROM CONTRACTS ORDER BY CLIENTID ASC, STARTDATE ASC;
CONTRACTID CLIENTID STARTDATE ENDDATE
Break-Ring 1 11-MAY-56 18-FEB-57
GoBackHome 1 19-FEB-57 02-MAY-57
Gardening 1 03-MAY-57
SaveGalaxy 2 15-JUN-77 14-NOV-80
DefendHoth 2 15-NOV-80 08-MAR-83
GoWedding 2 09-MAR-83
To stabilize the workload on this query, a COMPOUND TRIGGER may be used instead. This second example achieves the same result as the first, but only interrogates the CONTRACTs of CLIENTs that have changed:
First, ROLLBACK;
Then, create a type to be used by the TRIGGER:
CREATE OR REPLACE TYPE NUMBER_LIST IS TABLE OF NUMBER;
/
Then create the COMPOUND TRIGGER:
CREATE OR REPLACE TRIGGER CONTRACT_ENDDATE_ADJUSTER
FOR INSERT ON CONTRACTS
COMPOUND TRIGGER
V_CLIENTS NUMBER_LIST;
BEFORE STATEMENT
IS
BEGIN
V_CLIENTS:= NUMBER_LIST();
END BEFORE STATEMENT;
AFTER EACH ROW
IS
BEGIN
V_CLIENTS.EXTEND();
V_CLIENTS(V_CLIENTS.COUNT) := :NEW.CLIENTID;
END AFTER EACH ROW;
AFTER STATEMENT IS
BEGIN
MERGE INTO CONTRACTS
USING (
SELECT CONTRACTID,
CANDIDATE_ENDDATE AS ENDDATE
FROM
(SELECT CONTRACTS.CONTRACTID,
(TRUNC(LEAD(STARTDATE) OVER (PARTITION BY CLIENTID ORDER BY STARTDATE ASC) - 1)) AS CANDIDATE_ENDDATE,
DENSE_RANK() OVER (PARTITION BY CLIENTID ORDER BY STARTDATE DESC) AS CONTRACT_ORDER
FROM CONTRACTS
WHERE CONTRACTS.CLIENTID IN (SELECT * FROM TABLE(V_CLIENTS)))
WHERE CONTRACT_ORDER = 2) CANDIDATE_CONTRACT
ON (CONTRACTS.CONTRACTID = CANDIDATE_CONTRACT.CONTRACTID)
WHEN MATCHED THEN UPDATE SET CONTRACTS.ENDDATE = CANDIDATE_CONTRACT.ENDDATE
WHERE CONTRACTS.ENDDATE IS NULL OR CONTRACTS.ENDDATE > CANDIDATE_CONTRACT.ENDDATE;
END AFTER STATEMENT;
END CONTRACT_ENDDATE_ADJUSTER;
/
After repeating the above inserts, the result is the same:
CONTRACTID CLIENTID STARTDATE ENDDATE
Break-Ring 1 11-MAY-56 18-FEB-57
GoBackHome 1 19-FEB-57 02-MAY-57
Gardening 1 03-MAY-57
SaveGalaxy 2 15-JUN-77 14-NOV-80
DefendHoth 2 15-NOV-80 08-MAR-83
GoWedding 2 09-MAR-83