How to limit an SQL result set to not too common items

问题

Problem: I have a list of names and addresses. Some names (persons) have the same address (street, zip code, town) like others. I want to select all those names with addresses with no more than three occurrences and from the rest the first three names each of a bunch pointing to the same address. Example:

Albert | Adr1
Berta  | Adr1
Cesar  | Adr1
Donald | Adr1
Eric   | Adr2
Fritz  | Adr2
Gerd   | Adr2
Henry  | Adr3

The result set should be

Albert | Adr1 
Berta  | Adr1
Cesar  | Adr1
Eric   | Adr2
Fritz  | Adr2
Gerd   | Adr2
Henry  | Adr3

Donald is missing because he is the 4th of a group with the same address. Can this result be achieved with UNIONs and subqueries? Something like

select * from addresses where address in 
(select address from addresses group by address having count(address) <= 3)
UNION
select * from addresses where address in 
(select address from addresses group by address having count(address) > 3 limit 3)

I know that this query is wrong because it limits the complete result set of addresses with more than 3 occurences. I wonder if this can be done in a single SELECT with UNIONs and subqueries. I will do it now procedurally with PHP/MySQL, but just for fun would be interested in an SQL only solution.

I had a look at SQL query with limit on rows from one table, not the result set, but this does not reflect my situation - or does it?

回答1:

You can try something like

SELECT  PersonName,
        Address
FROM    (
            SELECT  *,
                    (SELECT COUNT(1) FROM addresses WHERE Address = a.Address AND PersonName < a.PersonName) CountLess
            FROM    addresses a
        ) sub
WHERE   sub.CountLess <= 2

回答2:

select name, address
from
 ( select *, row_number() over (partition by address order by name) as namenum
   from yourTable
 ) t
where namenum <= 3;

来源：https://stackoverflow.com/questions/2153410/how-to-limit-an-sql-result-set-to-not-too-common-items

标签

sql

subquery

union

limit