Query help

accountid.   name

1                  bill

2.                 james

3                  sarah

4                  carl

transaction

id.         amount.      accountid.     name

1.          50.              1                   bill

2.          25.              2                   james

3           35               4                   carl 

4.          75.              1                   bill

5           25.              1                   bill

6           50               3                   sarah

results wanted-all transactions where account total >= 50

I've tried to understand how to use GROUP BY and HAVING, but the penny won't drop. I keep getting errors saying that all columns in the SELECT have to also be in the GROUP BY, but nothing I've done seems to produce the correct results. I think because the GROUP BY contains multiple columns, so each row is treated as a group. It also is difficult to parse out since in the real world, many more tables and columns are involved.