Question

So I have this MySQL 5.0.24 table

select state, county, population from state_pops order by state, county;

yields

Arizona   Yapavai    4
Arizona   Pueblo     5
Arizona   Pinal      8
Arizona   Maricopa  13
Michigan  Lawson     3
Michigan  Maple      4
Michigan  Appleton   8
Texas     Richmond   5
Texas     Dupree     7
Texas     Brighton  10

I need to determine which is the most heavily-populated county in each state.

select state, county, max( population) from state_pops group by state order by state;

yields

Arizona   Maricopa  13
Michigan  Appleton   8
Texas     Brighton  10

Easy. But now I need to somehow flag the most-populous county of each state while
listing all counties of all states, like so

Arizona   Yapavai    4  NO
Arizona   Pueblo     5  NO
Arizona   Pinal      8  NO
Arizona   Maricopa  13  YES
Michigan  Lawson     3  NO
Michigan  Maple      4  NO
Michigan  Appleton   8  YES
Texas     Richmond   5  NO
Texas     Dupree     7  NO
Texas     Brighton  10  YES

So I need to derive a column somehow, perhaps some form of CASE..WHEN Any ideas?

TIA,

Still-learning Stev

Answer 1

You can use this solution:

SELECT a.state, 
       a.county, 
       a.population, 
       COALESCE(b.isMostPop, 'NO') AS flag
FROM state_pops a
LEFT JOIN
(
    SELECT state, MAX(population) AS maxpop, 'YES' AS isMostPop
    FROM state_pops
    GROUP BY state
) b ON a.state = b.state AND a.population = b.maxpop

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See a SQL-Fiddle Demo