World Country Profile: Aggregate functions（世界國家概況:綜合功能）

題目7：Counting big countries in each continent

?For each continent show the continent and number of countries with populations of at least 10 million.

（對于每個大洲，顯示大洲和人口數量超過10000萬的國家數量）

解題7:

SELECT continent, COUNT(name) FROM world WHERE population >= 10000000 GROUP BY continent

題目8：Counting big continents

List the continents that have a total population of at least 100 million.

（列出大洲，大洲的總人口數至少是100000萬）

解題8：

SELECT continent FROM world GROUP BY continent HAVING SUM(population) >= 100000000

注意：對比題目7和題目8，WHERE,GROUP BY 與 GROUP BY , HAVING 的關系。前者在分組之前進行篩選，后者在分組后進行篩選。

測試 SUM and COUNT Quiz

3. Select the list of core SQL aggregate functions（sql中的聚集函數）

AVG(), COUNT(), MAX(), MIN(), SUM()

?

4. Select the result that would be obtained from the following code:

SELECT region, SUM(area)FROM bbc WHERE SUM(area) > 15000000 GROUP BY region

5. Select the statement that shows the average population of 'Poland', 'Germany' and 'Denmark'

?解題5：SELECT AVG(population) FROM bbc WHERE name IN ('Poland', 'Germany', 'Denmark')

注意：SQL 中等于 后面是某一個特定值，而IN 后面可以是一個結果集。如果結果集只有一個值時“=”等價于“IN” ，如果結果集有多個值時，IN 等價于 多個 “=”或。

col = 'A' 等價于 col IN ('A')
col = 'A' or col ='B' or col ='C' 等價于 IN('A','B','C')

6. Select the statement that shows the medium population density of each region

查找每個區域的人口密度，注意此處的人口密度是針對區域來說，也就是區域內的指標總和。

SELECT region, SUM(population)/SUM(area) AS density FROM bbc GROUP BY region

?

7.Select the statement that shows the name and population density of the country with the largest population

查找人口數量最多的國家和人口密度

SELECT name, population/area AS density FROM bbc WHERE population = (SELECT MAX(population) FROM bbc)

The nobel table can be used to practice more SUM and COUNT functions.

題目9：Show the years in which three prizes were given for Physics.

nobel(yr, subject, winner)

解題9：

SELECT yr FROM nobel WHERE subject = 'Physics' GROUP BY yr HAVING COUNT(winner) = 3

注意：根據題意的理解，確定WHERE ,GROUP BY 和 HAVING 中各自對應什么條件。

題目12：GROUP BY yr, subject

Show the year and subject where 3 prizes were given. Show only years 2000 onwards.

顯示頒發3個獎項的年份和主題。只顯示2000年及以后。

nobel(yr, subject, winner)

解題12：

SELECT yr, subject FROM nobel WHERE yr >= 2000 GROUP BY yr, subject HAVING COUNT(subject) = 3

總結

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