我试着做一个简单的触发器，如果第三列有一个特定的值，检查两列是否为null。

DELIMITER |
CREATE TRIGGER after_insert_ping AFTER INSERT
ON ping FOR EACH ROW
    BEGIN
        IF NEW.protocol_id = 'sip' THEN
            BEGIN
                IF NEW.sip_account_name IS NULL THEN
                    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'some error';
                ELSE IF NEW.is_sip_register_autotest_enabled IS NULL THEN
                    SIGNAL SQLSTATE '45001' SET MESSAGE_TEXT = 'some error';
                END IF;
            END;
        ELSE
            BEGIN
                IF NEW.sip_account_name IS NOT NULL THEN
                    SIGNAL SQLSTATE '45002' SET MESSAGE_TEXT = 'some error';
                ELSE IF NEW.is_sip_register_autotest_enabled IS NOT NULL THEN
                    SIGNAL SQLSTATE '45003' SET MESSAGE_TEXT = 'some error';
                END IF;
            END;
        END IF;
    END |

我知道了：

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ';
ELSE
BEGIN
    IF NEW.sip_account_name IS NOT NULL
    THEN
      ' at line 13

我在这里检查了mysql语法：https://dev.mysql.com/doc/refman/5.7/en/if.html
我读到我们需要在begin和end之间附上我们的多个语句，我试着这样做。
你知道为什么我在编译触发器时会有这个错误吗？