如何从MySQL中的FROM字段中删除子查询

Question

I'm new to MySQL and databases and I've seen in many places that it is not considered a good programming practice to use subqueries in the FROM field of SELECT in MySQL, like this:

select userid, avg(num_pages) 
from (select userid , pageid , regid , count(regid) as num_pages 
      from reg_pag 
      where ativa = 1 
      group by userid, pageid) as from_query 
group by userid;

which calculates the average number of registers per page that the users have.

The reg_page table looks like this:

Questions:

1. How to change the query so that it doesn't use the subquery in the FROM field?

2. And is there a general way to "flatten" queries like this?

Answer 1

One way is to use count(distinct) :

select userid, count(*) / count(distinct pageid)
from reg_pag 
where ativa = 1
group by userid;

Answer 2

The average number of registers per page per user can also be calculated as number of registers per user divided by number of pages per user. Use count distinct to count only distinct psgeids per user:

select userid, count(regid) / count(distinct pageid) as avg_regs
from reg_pag 
where ativa=1
group by userid;

There is no general way of flattening such queries. It may not even be possible to flatten some of them, otherwise there would be little point in having this feature in the first place. Do not get scared of using subqueries in the from clause, in some occasions they may be even more effective, than a flattened query. But this is vendor and even version specific.