MySQL trigger statement fails on variable assignment

Question

The following is the definition of the patient table:

Patients table Columns:
PatientId int AI PK 
FirstName longtext 
LastName longtext 
Email longtext 
NIC longtext 
Phone longtext 
Address longtext 
City longtext 
Country longtext 
DOB datetime(6) 
Gender longtext 
Reference longtext 
SerialNumberYear smallint 
SerialNumber int 
DoctorId int 
CreatedOn datetime(6) 
UpdatedOn datetime(6) 
CreatedBy longtext 
SMS_Allowed tinyint(1)
Email_Allowed tinyint(1) 
SpecialConcern1 longtext 
SpecialConcern2 longtext 
SpecialConcern3 longtext 
SpecialConcern4 longtext 
CustomYearlyId longtext 
YearlyId int

I have the following trigger on this table in MySQL:

CREATE TRIGGER generate_yearly_id BEFORE INSERT ON clinic.patients
       FOR EACH ROW
       BEGIN
           DECLARE last_id INTEGER default 10;
           DECLARE current_year DATE;
           DECLARE yearly_id INTEGER default 0;
           IF NEW.CustomYearlyId IS NULL THEN
           BEGIN
                INSERT INTO DEBUG VALUES (null,"start trigger");
                 #SET @last_id := (SELECT max(yearlyid) FROM patients WHERE 
                 #      YEAR(createdon)=YEAR(CURDATE()));
                IF last_id IS NOT NULL THEN
                BEGIN
                    SET @yearly_id = 1;
                    INSERT INTO DEBUG VALUES (null, concat("in if lastid is not null ",@yearly_id));
                END;
                ELSE
                BEGIN
                    #SET @yearly_id := 1;
                END;
                END IF;
                SET NEW.yearlyid := @yearly_id;
           END;
           END IF;
       END;//
       delimiter ;

Statement SET @yearly_id = 1; works fine. But if I change it to SET @yearly_id = @last_id; the trigger fails. What did I do wrong while doing this assignment? last_id has default value so do I need to initialize it?

Answer 1

The code below works perfectly. The difference is 1. The variable has been initialized before use. Apparently the default value is not enough. 1. Used count in "if" condition instead of testing "is null".

CREATE TRIGGER generate_yearly_id BEFORE INSERT ON clinic.patients
       FOR EACH ROW
       BEGIN
           DECLARE last_id INTEGER default 0;
           DECLARE current_year DATE;
           DECLARE yearly_id INTEGER default 1;
           declare count INTEGER default 0;
           IF NEW.CustomYearlyId IS NULL THEN
           BEGIN
                #init last_id to some value before using
                SET @last_id = 0;
                SET @yearly_id = 0;
                #INSERT INTO DEBUG VALUES (null,"start trigger");
                set @count := (select count(*) from patients where
                        YEAR(CreatedOn)=YEAR(curdate()));
                IF @count > 0 THEN
                BEGIN
                    SET @last_id := (SELECT max(yearlyid) FROM patients WHERE 
                       YEAR(createdon)=YEAR(CURDATE()));
                    SET @yearly_id = @last_id + 1;
                    SET NEW.yearlyid := @yearly_id;
                    #INSERT INTO DEBUG VALUES (null, concat("in if lastid is not null ",@yearly_id));
                END;
                ELSE
                BEGIN
                    #INSERT INTO DEBUG VALUES (null, concat("in else ",@yearly_id));
                    #SET @yearly_id = 1;
                    SET NEW.yearlyid := 1;
                END;
                END IF;
                #Set NEW.yearlyid := 1;
           END;
           END IF;
       END;//
       delimiter ;