SQL: Comparing two counts from different Tables

问题

I have 3 Tables one with every Country in the World and its token

NAME              CODE
Afghanistan       AFG
Albania           AL
Algeria           DZ
American Samoa    AMSA
Andorra           AND
Angola            ANG
Anguilla          AXA
(...)

And and a Table of all lakes and another one of all mountains in these countries.

LAKE                 CODE
Bodensee             A
Neusiedlersee        A
Lake Prespa          AL
Lake Ohrid           AL
Lake Skutari         AL
Lake Eyre            AUS
Lake Jindabyne       AUS
Lake Hume            AUS
Lake Eucumbene       AUS
Lake Hume            AUS
Lake Burley Griffin  AUS
(...)

MOUNTAIN             CODE
Hochgolling          A
Hochgolling          A
Zugspitze            A
Grossglockner        A
Jezerce              AL
Korab                AL
Uluru                AUS
Mt. Kosciuszko       AUS
Mt. Bogong           AUS
Musala               BG
Illampu              BOL
Sajama               BOL
Licancabur           BOL
(...)

I now have to show the countries which have a smaller amount of mountains than lakes. And I tried now for hours but can´t find an approach to this problem. I tried to Join the 3 tables together - but I can't figure out what to do next. I'm sure I have to use nested SQL Commands.

Here is my attempt on counting the lakes and mountains of each country

SELECT Country.name, count(Geo_lake.code), count(Geo_mountain.code) 
From Country 
INNER JOIN Geo_lake On (Country.code = geo_lake.code)
INNER JOIn Geo_mountain On (Country.code = geo_mountain.code) 
Group by Country.name;

But somehow both columns count(Geo_lake.country) and count(Geo_mountain.country) appear to have the same values I don´t know exactly why.

回答1:

Start off by getting the sums for each individually:

# Sum of lakes
SELECT code, count(*) AS sum FROM lakes GROUP BY code

# Sum of mountains
SELECT code, count(*) AS sum FROM mountains GROUP BY code

Then join the results together and select all rows where the sum of the mountains for a country are less than the number of lakes:

SELECT l.code AS code, l.sum AS lake_count, m.sum AS mountain_count
FROM
(SELECT code, count(*) AS sum FROM lakes GROUP BY code) AS l JOIN
(SELECT code, count(*) AS sum FROM mountains GROUP BY code) AS m
ON l.code = m.code
WHERE m.sum < l.sum

来源：https://stackoverflow.com/questions/35215541/sql-comparing-two-counts-from-different-tables

标签

mysql

sql