SQL：比較來自不同表的兩個計數

Question

我有3張桌子，每個國家和地區都有它的代幣

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

還有這些國家中所有湖泊和所有山巒中的一個的表。

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
(...)

現在，我必須說明山峰少於湖泊的國家。 我現在嘗試了幾個小時，但找不到解決此問題的方法。 我試圖將3個表連接在一起-但我不知道下一步該怎么做。 我確定我必須使用嵌套的SQL命令。

這是我對每個國家的湖泊和山脈計數的嘗試

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;

但是以某種方式，兩列的count（Geo_lake.country）和count（Geo_mountain.country）似乎都具有相同的值，我不知道為什么。

Answer 1

首先獲取每個人的總和：

# 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

然后將結果合並在一起，並選擇一個國家的山之和小於湖泊數量的所有行：

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