以下是患者表的定義：

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

我在 MySQL 中的該表上有以下觸發器：

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 ;

語句 SET @yearly_id = 1; 正常運作。但是如果我將其更改為 SET @yearly_id = @last_id; ，則觸發失敗。我在做這個作業時做錯了什麼？ last_id有預設值，所以我需要初始化它嗎？