Every derived table must have its own alias(每一个衍生表必须有它自己的别名)

Every derived table must have its own alias(每一个派生表必须有它自己的别名)
每一个派生表必须有它自己的别名
select count(*) from (select t.a from test t group by t.a)
解决办法：
select count(*) from (select t.a from test t group by t.a)c