MySQL版本：5.7

问题：
[Err] 1064 - 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 'DECLARE userName VARCHAR(20)

解决：
需将DECLARE 放在在存储过程、函数或者触发器中，再调用它（call 函数名或存储过程名）
例：
CREATE PROCEDURE procd1()
BEGIN
DECLARE userName VARCHAR(20);
DECLARE userEmail VARCHAR(20);
DECLARE i INT DEFAULT 4;
WHILE i<= 400 DO
set userName = CONCAT(‘test’,LPAD(i,3,‘0’));
set userEmail = CONCAT(‘userName’, ‘@test.com’);
INSERT INTO jforum_users(user_id,user_active,username,user_password,user_regdate,user_email) VALUES(i,
‘1’, userName,‘abcd1234’, ‘2021-05-10 06:19:34’, userEmail);
INSERT INTO jforum_user_groups VALUES(3,i);
SET i = i+1;
END WHILE;
END

最后执行procd1：call procd1