简单了解标准SQL的update语句三种用法

DROP TABLE IF EXISTS t_test; 
CREATE TABLE t_test ( 
bs bigint(20) NOT NULL auto_increment, 
username varchar(20) NOT NULL, 
password varchar(20) default NULL, 
remark varchar(200) default NULL, 
PRIMARY KEY (bs) 
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=gbk; 
INSERT INTO t_test VALUES (1,'lavasoft','123456',NULL); 
INSERT INTO t_test VALUES (2,'hello',NULL,NULL); 
INSERT INTO t_test VALUES (3,'haha',zz,tt);