有两个表用户表user表和用户积分表score表，两个表是一对一关系，score表有user_id字段关联了user表的id，现需求是score表即将废弃，需要将score表的字段num的值作为user表的字段score_num的值，
有以下SQL写法

UPDATE user,score SET user.score_num = score.num WHERE score.user_id = user.id

或者

UPDATE user set user.score_num = (SELECT score.num FROM score WHERE score.user_id = user.id)

或者

UPDATE user SET user.score_num = score.num FROM user INNER JOIN score on score.user_id = user.id