如何在mysql中获取不重复的结果

Question

I am trying to get results from mysql by not letting a specific field gets repeated more than 5 times.

For example, lets say that I have a table like this :

-------------
Name        City    
person1     Nashville
person2     Nashville
person3     Nashville
person4     Nashville
person5     Nashville
person6     Nashville
person7     New York
-------------

What I want it to return is this :

-------------
Name        City    
person1     Nashville
person2     Nashville
person3     Nashville
person4     Nashville
person5     Nashville
person7     New York
-------------

where the city field cannot be repeated more than five times! if it was repeated more than fix times, only return the first five results.

This question could be impossible (that is what I am guessing) but if there is a solution or anyway to work around what I am trying to achieve, please tell me! I can use PHP.

Answer 1

How about something like this which uses variables to check the current row to the previous row, if they are the same then the row number will increase if they are not the row number will reset:

select name, city
from
(
  select name, 
    city,
    @row:=(case when @prev=city then @row else 0 end) + 1 as rownum,
    @prev:=city pcity
  from yourtable
  order by city, name
) src
where rownum <= 5

See SQL Fiddle with Demo

The result is:

|    NAME |      CITY |
-----------------------
| person1 | Nashville |
| person2 | Nashville |
| person3 | Nashville |
| person4 | Nashville |
| person5 | Nashville |
| person7 |  New York |

Answer 2

    SELECT x.*
      FROM my_table x 
      JOIN my_table y
        ON y.name <= x.name 
       AND y.city = x.city 
     GROUP 
        BY x.city
         , x.name 
    HAVING COUNT(*) <= 5;

Answer 3

Are you allowed to perform more than one MySql command? If so, you can try doing two separate calls --

SELECT name, city FROM table WHERE city = 'Nashville' ORDER BY name LIMIT 5;
SELECT name, city FROM table WHERE city <> 'Nashville' ORDER BY name;

Someone may need to check on this but the general idea stands. The only problem with this is your end result would not have the table ordered by name. Rather, it'd have the people with the city has 'Nashville' first, and then everyone else.