链接查询
查询user id，username
provinces，proName
SELECT user.id，username，proName FROM user，provinces;
查询user表中id，username，email，sex
查询provinces表中proname
SELECT u,id,u,username,u,email,u.sex,p.proName
FROM user AS u
INNER JOIN provinces AS p 等同CROSS JOIN provinces AS p
ON u.priID=p.id;
