计算2列之间的平均值SQL

Question

I have a table called validation_errors that looks like this:

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| link        | varchar(200) | NO   | MUL | NULL    |                |
| message     | varchar(500) | NO   |     |         |                |
| explanation | mediumtext   | NO   |     | NULL    |                |
| type        | varchar(50)  | NO   |     |         |                |
| subtype     | varchar(50)  | NO   |     |         |                |
| message_id  | varchar(50)  | NO   |     |         |                |
+-------------+--------------+------+-----+---------+----------------+

Link table looks like this:

+-----------+--------------+------+-----+---------+-------+
| Field     | Type         | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| link      | varchar(200) | NO   | PRI | NULL    |       |
| visited   | tinyint(1)   | NO   |     | 0       |       |
| validated | tinyint(1)   | NO   |     | 0       |       |
+-----------+--------------+------+-----+---------+-------+

I wish to calculate the average number of validation errors per page per topdomain. I have a query that can fetch the amount of pages per topdomain:

    SELECT substr(link, - instr(reverse(link), '.')) as domain , count(*) as count
    FROM links
    GROUP BY domain
    ORDER BY count desc
    limit 30;

And have a sql query that can fetch the amount of validation errors per top domain:

    SELECT substr(link, - instr(reverse(link), '.')) as domain ,count(*) as count
    FROM validation_errors
    GROUP BY domain
    ORDER BY count desc
    limit 30;

What i now need to do is combine them into a query and divise the results of one column with the other and i can't figure out how to do it.

Any help would be greatly apriciated.

Answer 1

First, use substring_index() , rather than your construct. Here is the query to join them together:

select domain, sum(numviews) as numviews, sum(numerrors) as numerrors,
       sum(numerrors) / nullif(sum(numviews), 0) as error_rate
from ((SELECT substring_index(link, '.', -1) as domain , count(*) as numviews, 0 as numerrors
       FROM links
       GROUP BY domain
      ) UNION ALL
      (SELECT substring_index(link, '.', -1) as domain , 0, count(*)
       FROM validation_errors
       GROUP BY domain
      )
     ) d
GROUP BY domain;

With both variables, I don't know which 30 you want to choose, so I haven't included an order by .

Note that this doesn't use a join , it uses union all with aggregation. This ensures that you will get all domains, even those with no views and those with no errors.