在sql中的触发器中如何进行嵌套？

Question

此声明有什么问题？它说我在“何时”出现错误，我需要检查以确保没有外键问题，之后，我需要检查外键问题是否源于错误表中输入数据的错误。

CREATE TRIGGER trigger3
            BEFORE INSERT ON sightings
            FOR EACH ROW
                WHEN ((SELECT flowers.comname FROM flowers WHERE comname = NEW.name) IS NULL) 
                BEGIN
                    WHEN ((SELECT genus, species FROM flowers WHERE CONCAT(species,' ',genus) = NEW.name) IS NOT NULL)
                    THEN
                        INSERT INTO sightings VALUES (comname, NEW.person, NEW.location, NEW.sighted)
                    END IF;
                END; 
            END;

Answer 1

使用IF代替WHEN。

CREATE TRIGGER trigger3
            BEFORE INSERT ON sightings
            FOR EACH ROW
                BEGIN
                    IF ((SELECT flowers.comname FROM flowers WHERE comname = NEW.name) IS NULL) 
                    BEGIN
                        IF ((SELECT genus, species FROM flowers WHERE CONCAT(species,' ',genus) = NEW.name) IS NOT NULL)
                        BEGIN
                            INSERT INTO sightings VALUES (comname, NEW.person, NEW.location, NEW.sighted)
                        END ;
                    END; 
                END;
            END;