SQL SELECT for UPDATE

Example 1

MySQL
CREATE TABLE `tablea` (
 `userid` varchar(6) NOT NULL,
 `passwd` varchar(16) NOT NULL,
 PRIMARY KEY  (`userid`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE `tableb` (
 `userid` varchar(6) NOT NULL,
 `passwd` varchar(16) NOT NULL,
 PRIMARY KEY  (`userid`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

sql update password @tableb โดยอ้างถึง password จาก tablea

UPDATE tableb
SET tableb.passwd = (SELECT tablea.passwd FROM tablea WHERE tableaa.userid = tb.userid )

Example 2

UPDATE tableb
JOIN tablea ON tableb.userid=tablea.userid
SET tableb.passwd = tablea.passwd 

Example 3

UPDATE tableb
JOIN tablea ON tableb.userid=tablea.userid AND tablea.userid=5555555
SET tableb.passwd = tablea.passwd

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