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SELECT password, username FROM A, B WHERE A.userid = B.userid AND A.username = 'Tom'
SELECT city.id, city.cityname, province.provincename
FROM city, province
WHERE city.provinceid = province.proid
SELECT COUNT(city.id), province.provincename
FROM city, province
WHERE city.provinceid = province.proid
GROUP BY province.provincename
1
select b.password,a.username from A,B where a.userid = b.userid and a.username = 'Tom'
2.1
select m.id ,m.cityname,isnull(n.provincename,'') from city m left join province n on m.provinceid = n.proid
select m.id ,m.cityname,n.provincename from city m , province where on m.provinceid = n.proid
2.2
select m.provincename , isnull(count(1),0) from province m left join city n on m.proid = n.provinceid group by m.provincename
select m.provincename , count(1) from province m , city n where m.proid = n.provinceid group by m.provincename