Question

I have this working code. Every time I ran INSERT INTO "HR"."CITY" (CITY_ID) VALUES (0); the data in CITY_ID will increase by one like this CT0001, CT0002 ... CT0015.

 CREATE TABLE "HR"."CITY" 
   (    
    "CITY_ID" VARCHAR2(40 BYTE)
   ) ;


  CREATE OR REPLACE TRIGGER "HR"."PK_MAX_TRIGGER_CITY" 
BEFORE INSERT ON CITY
FOR EACH ROW
DECLARE
    CNT NUMBER;
    PKV CITY.CITY_ID%TYPE;
    NO NUMBER;
BEGIN
    SELECT COUNT(*)INTO CNT FROM CITY;

    IF CNT=0 THEN
        PKV:='CT0001';
    ELSE
        SELECT 'CT'||LPAD(MAX(TO_NUMBER(SUBSTR(CITY_ID,3,LENGTH(CITY_ID)))+1),4,'0') INTO PKV
        FROM CITY;
    END IF;
    :NEW.CITY_ID:=PKV;
END;
/
ALTER TRIGGER "HR"."PK_MAX_TRIGGER_CITY" ENABLE;

What I want to do is this E1, E2 ... E15.

I change the code to:

    IF CNT=0 THEN
        PKV:='E1';
    ELSE
        SELECT 'E'||LPAD(MAX(TO_NUMBER(SUBSTR(CITY_ID,3,LENGTH(CITY_ID)))+1),4,'0') INTO PKV

It work at first adding E1 to database but not the second time. What inside the bracket is just to hard for me to digest. I was hoping anyone here could explain to me what actually happen in the bracket and helping me to solve this thing.

Thanks in advance.

Answer 1

What you need to do is to modify your Else part query .

SELECT 'E'||(MAX(TO_NUMBER(SUBSTR(CITY_ID,2))+1)) INTO PKV
    FROM CITY;

Explaination
Whenever you want to find what is happening in the brackets start with the inner bracket .

See the result of this query:  SELECT SUBSTR(CITY_ID,2) FROM CITY;

Here,you will extract the string after E,where E always there in 1st position ,so we will find the string from the 2nd position .
Then

 SELECT TO_NUMBER(SUBSTR(CITY_ID,2)) FROM CITY ;

–this is converting the string into number.Then it will find the max number from the table CITY,and add 1 after that

SELECT MAX(TO_NUMBER(SUBSTR(CITY_ID,2)))+1 FROM CITY ;

And finally appending the constant E with the result .

SELECT 'E'||(MAX(TO_NUMBER(SUBSTR(CITY_ID,2))+1)) INTO PKV
    FROM CITY;

But take my suggestion ,create a sequence city_id_seq,this will be better solution than what you are doing now.

CREATE SEQUENCE city_id_seq
MINVALUE 1
MAXVALUE 99999999999
START WITH 1
INCREMENT BY 1
CACHE 20;


CREATE OR REPLACE TRIGGER "HR"."PK_MAX_TRIGGER_CITY" 
BEFORE INSERT ON CITY
FOR EACH ROW
DECLARE
 v_city_id PLS_INTEGER;  
BEGIN
  SELECT city_id_seq.nextval INTO v_city_id  FROM DUAL;

 :NEW.CITY_ID :='E'||v_city_id  ;

/*  if you are using oracle 11g 
:NEW.CITY_ID:='E'||city_id_seq.nextval; 
*/

END;
/