这个MySQL查询总是返回预期的结果吗？

Question

I wrote a query as follows:

  SELECT COUNT(*) AS count, email
    FROM sometable
GROUP BY email
ORDER BY count DESC 
   LIMIT 4

I am interested in seeing the four most duplicated email entries in the table. So far, it seems to return exactly what I want:

count   email
12      very-duplicated@email.com
2       duped-twice@email.com
2       also-twice@email.com
1       single@email.com

When I don't use LIMIT , I get the same result (albeit with many more rows having a count = 1). What I'm wondering about is the LIMIT . In the future, when the numbers change, will my query above still return the four most used emails? or does the query need to scan the entire database to remain accurate?

(note: I am not trying to prevent duplicates, I'm trying to see the most frequently used email.)

Answer 1

I'm not sure. But if you're concerned, you could apply a limit to a subquery:

select *
from 
(
  SELECT COUNT(*) AS count, email
  FROM sometable
  GROUP BY email
  ORDER BY count DESC 
)
limit 4

Alternateively, you could do something like this to see all duplicated email address (may return more or less than 4):

  SELECT COUNT(*) AS count, email
    FROM sometable
GROUP BY email
having COUNT(email) > 1
ORDER BY count DESC 

Answer 2

Well first thing is, the query does not only return you the duplicate entries. Look at 4th row which says count = 1 which means it occurs only once in the table. To list duplicate records you need to modify your query as -

SELECT COUNT(*) AS count, email
FROM sometable
GROUP BY email
HAVING COUNT(*) > 1
ORDER BY count DESC     
LIMIT 4

Then, this will always return you 4 topmost duplicate entries in your table as the order mentioned.