How do you compare multiple rows, within the same table, in SQL?

Question

I need to use a trigger to update the table when a new entry is inserted if it is the first time the ID has pledged. (their first pledge)

I need to search the table and see if the id already exists, and if it doesn't it should update firstpledge as 'Y' otherwise it should update firstpledge as 'N'.

Right now I'm receiving the error "exact fetch returns more than requested number of rows" at the select table in the trigger. How can I fix this? Any help is greatly appreciated.

Below shows the code I currently have:

CREATE OR REPLACE TRIGGER dd_firstpledge_trg
  AFTER INSERT ON dd_pledge
DECLARE
  lv_id dd_pledge.idpledge%TYPE;
BEGIN
  SELECT idpledge 
    INTO lv_id
    FROM dd_pledge;
  IF lv_id = NULL THEN
    UPDATE dd_pledge
      SET firstpledge = 'Y'
      WHERE firstpledge = NULL;
  ELSE
    UPDATE dd_pledge
      SET firstpledge = 'N'
      WHERE firstpledge = NULL;
  END IF;
END;
/
show errors

Here is how I'm trying to test it:

INSERT INTO dd_pledge
  VALUES(113, 303, '13-MAY-13', 1200, 503, 10, NULL, 0, 756, NULL);
SELECT * FROM dd_pledge;

Below is the table information for reference:

DROP TABLE dd_donor CASCADE CONSTRAINTS;
DROP TABLE dd_project CASCADE CONSTRAINTS;
DROP TABLE dd_status CASCADE CONSTRAINTS;
DROP TABLE dd_pledge CASCADE CONSTRAINTS;
DROP TABLE dd_payment CASCADE CONSTRAINTS;
CREATE TABLE DD_Donor (
    idDonor number(4),
    Firstname varchar2(15),
    Lastname varchar2(30),
                  Typecode CHAR(1),
    Street varchar2(40),
    City varchar2(20),
    State char(2),
    Zip varchar2(9),
    Phone varchar2(10),
    Fax varchar2(10),
    Email varchar2(25),
                  News char(1),
    dtentered date DEFAULT SYSDATE,
    CONSTRAINT donor_id_pk PRIMARY KEY(idDonor) );
CREATE TABLE DD_Project (
                   idProj number(6),
                   Projname varchar2(60),
                   Projstartdate DATE,
                   Projenddate DATE,
                   Projfundgoal number(12,2),
                   ProjCoord varchar2(20),
                   CONSTRAINT project_id_pk PRIMARY KEY(idProj),
                   CONSTRAINT project_name_uk  UNIQUE (Projname)  );      
CREATE TABLE DD_Status (
                   idStatus number(2),
                   Statusdesc varchar2(15),
                   CONSTRAINT status_id_pk PRIMARY KEY(idStatus) );     
CREATE TABLE DD_Pledge (
                   idPledge number(5),
                   idDonor number(4),
                   Pledgedate DATE,
                   Pledgeamt number(8,2),
                   idProj number(5),
                   idStatus number(2),
                   Writeoff number(8,2),
                   paymonths number(3),
                   Campaign number(4),
                   Firstpledge char(1),
                   CONSTRAINT pledge_id_pk PRIMARY KEY(idPledge),
                   CONSTRAINT pledge_idDonor_fk FOREIGN KEY (idDonor)
                           REFERENCES dd_donor (idDonor), 
                   CONSTRAINT pledge_idProj_fk FOREIGN KEY (idProj)
                           REFERENCES dd_project (idProj),
                   CONSTRAINT pledge_idStatus_fk FOREIGN KEY (idStatus)
                           REFERENCES dd_status (idStatus));                   
CREATE TABLE DD_Payment (
                   idPay number(6),
                   idPledge number(5),
                   Payamt number(8,2),
                   Paydate DATE,
                   Paymethod char(2),
                   CONSTRAINT payment_id_pk PRIMARY KEY(idPay),
                   CONSTRAINT pay_idpledge_fk FOREIGN KEY (idPledge)
                           REFERENCES dd_pledge (idPledge) );                   
INSERT INTO dd_donor  
  VALUES (301, 'Mary', 'Treanor', 'I','243 main St.', 'Norfolk', 'VA','23510',NULL,NULL,'mtrea492@mdv.com','Y','01-SEP-2012');
INSERT INTO dd_donor  
  VALUES (302, 'Patrick', 'Lee', 'I','11 Hooper St.', 'Norfolk', 'VA','23510','7572115445',NULL,'pleeNorf@gmail.com','N','09-SEP-2012');
INSERT INTO dd_donor  
  VALUES (303, 'Terry', 'Venor', 'I','556 Loop Lane.', 'Chesapeake', 'VA','23320',NULL,NULL,'tervenr@drw.edu','Y','18-SEP-2012');
INSERT INTO dd_donor  
  VALUES (304, 'Sherry', 'Pane', 'I','Center Blvd.', 'Virginia Beach', 'VA','23455',NULL,NULL,'toppane@yahoo.com','Y','21-SEP-2012');
INSERT INTO dd_donor  
  VALUES (305, 'Thomas', 'Sheer', 'I','66 Train St.', 'Chesapeake', 'VA','23322','7579390022',NULL,'tls3488@sheer.com','Y','01-MAR-2013');
INSERT INTO dd_donor  
  VALUES (306, NULL, 'Coastal Developers', 'B','3667 Shore Dr.', 'Virginia Beach', 'VA','23450','8889220004',NULL,'coastVA@cdev.com','Y','30-SEP-2012');
INSERT INTO dd_donor  
  VALUES (307, NULL, 'VA Community Org', 'G','689 Bush Dr.', 'Norfolk', 'VA','23513','7578337467','7578337468','vacmorg@biz.com','Y','03-OCT-2012');
INSERT INTO dd_donor  
  VALUES (308, 'Betty', 'Konklin', 'I','11 Shark Ln.', 'Virginia Beach', 'VA','23455','7574550087',NULL,'shark11@cox.net','N','04-OCT-2012');
INSERT INTO dd_donor  
  VALUES (309, 'Jim', 'Tapp', 'I','200 Pine Tree Blvd.', 'Chesapeake', 'VA','23320','',NULL,'','N','08-OCT-2012');
INSERT INTO dd_donor  
  VALUES (310, NULL, 'Unique Dezigns', 'B','Connect Circle Unit 12', 'Chesapeake', 'VA','23320','7574442121',NULL,'UDezigns@cox.net','Y','11-SEP-2012');
INSERT INTO dd_project
  VALUES (500,'Elders Assistance League', '01-SEP-2012','31-OCT-2012',15000,'Shawn Hasee');
INSERT INTO dd_project
  VALUES (501,'Community food pantry #21 freezer equipment', '01-OCT-2012','31-DEC-2012',65000,'Shawn Hasee');
INSERT INTO dd_project
  VALUES (502,'Lang Scholarship Fund', '01-JAN-2013','01-NOV-2013',100000,'Traci Brown');
INSERT INTO dd_project
  VALUES (503,'Animal shelter Vet Connect Program', '01-DEC-2012','30-MAR-2013',25000,'Traci Brown');
INSERT INTO dd_project
  VALUES (504,'Shelter Share Project 2013', '01-FEB-2013','31-JUL-2013',35000,'Traci Brown');
INSERT INTO dd_status
  VALUES (10,'Open');
INSERT INTO dd_status
  VALUES (20,'Complete');
INSERT INTO dd_status
  VALUES (30,'Overdue');
INSERT INTO dd_status
  VALUES (40,'Closed');
INSERT INTO dd_status
  VALUES (50,'Hold');
INSERT INTO dd_pledge
   VALUES (100,303,'18-SEP-2012',80,500,20,NULL,0,738,'Y');
INSERT INTO dd_pledge
   VALUES (101,304,'21-SEP-2012',35,500,20,NULL,0,738,'Y');
INSERT INTO dd_pledge
   VALUES (102,310,'01-OCT-2012',500,501,20,NULL,0,749,'Y');
INSERT INTO dd_pledge
   VALUES (103,307,'03-OCT-2012',2000,501,20,NULL,0,749,'N');
INSERT INTO dd_pledge
   VALUES (104,308,'04-OCT-2012',240,501,10,NULL,12,749,'Y');
INSERT INTO dd_pledge
   VALUES (105,309,'08-OCT-2012',120,501,10,NULL,12,749,'Y');
INSERT INTO dd_pledge
   VALUES (106,301,'12-OCT-2012',75,500,20,NULL,0,738,'N');
INSERT INTO dd_pledge
   VALUES (107,302,'15-OCT-2012',1200,501,10,NULL,24,749,'Y');
INSERT INTO dd_pledge
   VALUES (108,308,'20-JAN-2013',480,503,10,NULL,24,790,'N');
INSERT INTO dd_pledge
   VALUES (109,301,'01-FEB-2013',360,503,10,NULL,12,790,'N');
INSERT INTO dd_pledge
   VALUES (110,303,'01-MAR-2013',300,504,10,NULL,12,756,'N');
INSERT INTO dd_pledge
   VALUES (111,306,'01-MAR-2013',1500,504,20,NULL,0,756,'Y');
INSERT INTO dd_pledge
   VALUES (112,309,'16-MAR-2013',240,504,10,NULL,12,756,'N');
INSERT INTO dd_payment
   VALUES (1425,100,80,'18-SEP-2012','CC');
INSERT INTO dd_payment
   VALUES (1426,101,35,'21-SEP-2012','DC');
INSERT INTO dd_payment
   VALUES (1427,102,500,'01-OCT-2012','CH');
INSERT INTO dd_payment
   VALUES (1428,103,2000,'03-OCT-2012','CH');
INSERT INTO dd_payment
   VALUES (1429,106,75,'12-OCT-2012','CC');
INSERT INTO dd_payment
   VALUES (1430,104,20,'01-NOV-2012','CC');
INSERT INTO dd_payment
   VALUES (1431,105,10,'01-NOV-2012','CC');
INSERT INTO dd_payment
   VALUES (1432,107,50,'01-NOV-2012','CC');
INSERT INTO dd_payment
   VALUES (1433,104,20,'01-DEC-2012','CC');
INSERT INTO dd_payment
   VALUES (1434,105,10,'01-DEC-2012','CC');
INSERT INTO dd_payment
  VALUES (1435,107,50,'01-DEC-2012','CC');
INSERT INTO dd_payment
   VALUES (1436,104,20,'01-JAN-2013','CC');
INSERT INTO dd_payment
   VALUES (1437,105,10,'01-JAN-2013','CC');
INSERT INTO dd_payment
   VALUES (1438,107,50,'01-JAN-2013','CC');
INSERT INTO dd_payment
   VALUES (1439,104,20,'01-FEB-2013','CC');
INSERT INTO dd_payment
   VALUES (1440,105,10,'01-FEB-2013','CC');
INSERT INTO dd_payment
   VALUES (1441,107,50,'01-FEB-2013','CC');
INSERT INTO dd_payment
   VALUES (1442,108,20,'01-FEB-2013','CC');
INSERT INTO dd_payment
   VALUES (1443,109,30,'01-FEB-2013','CC');
INSERT INTO dd_payment
   VALUES (1444,104,20,'01-MAR-2013','CC');
INSERT INTO dd_payment
   VALUES (1445,105,10,'01-MAR-2013','CC');
INSERT INTO dd_payment
   VALUES (1446,107,50,'01-MAR-2013','CC');
INSERT INTO dd_payment
   VALUES (1447,108,20,'01-MAR-2013','CC');
INSERT INTO dd_payment
   VALUES (1448,109,30,'01-MAR-2013','CC');
INSERT INTO dd_payment
   VALUES (1449,110,25,'01-MAR-2013','CC');
INSERT INTO dd_payment
   VALUES (1450,111,1500,'01-MAR-2013','CH');
INSERT INTO dd_payment
   VALUES (1451,104,20,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1452,105,10,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1453,107,50,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1454,108,20,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1455,109,30,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1456,110,25,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1457,112,20,'01-APR-2013','CC');
INSERT INTO dd_payment
   VALUES (1458,104,20,'01-MAY-2013','CC');
INSERT INTO dd_payment
   VALUES (1459,105,10,'01-MAY-2013','CC');
INSERT INTO dd_payment
   VALUES (1460,107,50,'01-MAY-2013','CC');
INSERT INTO dd_payment
   VALUES (1461,108,20,'01-MAY-2013','CC');
INSERT INTO dd_payment
   VALUES (1462,109,30,'01-MAY-2013','CC');
INSERT INTO dd_payment
   VALUES (1463,110,25,'01-MAY-2013','CC');
INSERT INTO dd_payment
   VALUES (1464,112,20,'01-MAY-2013','CC');
COMMIT;

Answer 1

My understanding is that when you insert the first row you want to have the flag equal to Y otherwise N .

Your code

SELECT idpledge 
    INTO lv_id
    FROM dd_pledge;

returns an error because when you have more than one row Oracle doesn't know which value to assign to lv_id.

A solution can be to use

SELECT count(1) INTO lv_id FROM dd_pledge WHERE idProj = :new.idProj; -- you need this otherwise you will get the pledges for all projects

If the table is empty lv_id will be 0, otherwise whatever number of rows you have in the table.

Of course also the IF statement will become:

IF lv_id = 0 THEN

Also I would remove the ELSE branch and put the flag as 'N' by default.

Answer 2

Even if you use count(1), it will insert into NUll into the newly inserted pledge id, the reason being you are using this statement to check ,

 SELECT idpledge 
    INTO lv_id
    FROM dd_pledge;

This will evaluate the whole tale and hence it is returning multiple rows, and pledgeId will always have either 'Y' or 'N' apart from the newly entered value.

To escape this problem you will have to introduce a where clause in your select clause using

where pledgeid=:new.pledgeId  -- (using for each row ,  in your trigger),

but even this will create a problem of mutating table, as you would be updating a row which is newly inserted and the trigger can see it as the new row isnt commited or rollbacked.

you will have to rework your logic or find a workaround.