//show every user details and how many cars that each user can drive
SELECT a.userid,a.username,b.carid from user a,usercards b where a.userid = b.userid//design a sql to list all users and how many cars that user can drive
SELECT a.userid,a.username,count(*) from user a,usercards b where a.userid = b.userid group by a.userid,a.username
SELECT a.userid,a.username,b.carid from user a,usercards b where a.userid = b.userid//design a sql to list all users and how many cars that user can drive
SELECT a.userid,a.username,count(*) from user a,usercards b where a.userid = b.userid group by a.userid,a.username
usercars b on a.userid=b.userid
A.UserID,
A.UserName ,
B.CarNo
FROM User A,UserCars B
WHERE A.USERID=B.USERID
Can you tell me why we need group by in that SQL?
group by是按组求count(*)数量