SQL 遇到的问题

每个派生出来的表都要有别名

select max(count) from (select count(*) as count from instructor group by dept_name) as T;

如果没加as T 会爆错

P59 3.9 (g)

select money ,dept_name from (select avg(salary) as money ,dept_name from instructor group by dept_name) as T where T.money >(select avg(salary) from instructor  where dept_name="Comp. Sci." ) ; 才会显示正确结果

select money ,dept_name from (select avg(salary) as money ,dept_name from instructor group by dept_name) as T where T.money >(select avg(salary) from instructor group by dept_name and  dept_name="Comp. Sci." ) ; 会缺少一些元组，原因暂时未知