如何使“知道”哪个实体具有更多元素的SELECT？

Question

I have a simple database that has three tables ( company , phone , email ) which are related through the rules:

• A company may have zero or more phone numbers;
• A company may have zero or more e-mail addresses.

Here is the MySQL code: Pastebin

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Let's suppose the following 2 cases:

• A company has 3 phones and 2 e-mails ( case 1 )
• A company has 2 phones and 3 e-mails ( case 2 )

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I would like to make a SELECT that returns the following:

Case 1

Company         Phone       E-mail
--------------------------------------------
Test Company    1111222     export@testco.co
Test Company    2222222     import@testco.co
Test Company    3333222 
--------------------------------------------

Case 2

Company         Phone       E-mail
--------------------------------------------
Test Company    1111222     export@testco.co
Test Company    2222222     import@testco.co
Test Company                sales@testco.co
--------------------------------------------

---

That SELECT should "know", for a given company, which entity has more elements ( phone or email ) and then return a result where the number of rows is equal to number of elements from the entity with more elements ( phone in case 1; email in case 2).

I could not figure out how to do that. Here's the best I could do: Pastebin

Answer 1

This is a pain to do in MySQL, but it is possible. Here is the logic. Enumerate the values for the emails and phones for each company. Then aggregate these based on the company and sequence number:

select ep.company, min(ep.email) as email, min(ep.phone) as phone
from ((select e.company, e.email, NULL as phone,
              (@rne := if(@company = company, @rne + 1, if(@company := company, 1, 1)) ) as seqnum
       from email e cross join
            (select @rne := 0, @company := '') vars
       order by e.company
      ) union all
      (select p.company, NULL, p.phone,
              (@rnp := if(@companyp = company, @rnp + 1, if(@companyp := company, 1, 1)) ) as seqnum
       from phone p cross join
            (select @rnp := 0, @companyp := '') vars
       order by p.company
      )
     ) ep
group by ep.company, ep.seqnum
order b ep.company, ep.seqnum;

The expression:

 (@rne := if(@company = company, @rne + 1, if(@company := company, 1, 1)) ) as seqnum

for setting seqnum is a way of safely using the variables in MySQL. Note that MySQL does not guarantee the order of evaluation of expressions in a select, so the more natural:

 @rne := if(@company = company, @rne + 1, 1), @company := company

may not work, because the second would be evaluated first. This method with the nested assignment doesn't have this problem.