Sqlzoo习题练习：SUM and COUNT
下面为SUM and COUNT习题内容：
知识点：
SUM() 函数
SUM 函数返回数值列的总数(总额)。
SUM() 语法
SELECT SUM(column_name) FROM table_name
--#1
/*
Show the total population of the world.
表world包含的字段：
world(name, continent, area, population, gdp)
*/
SELECT SUM(population)
FROM world
知识点：
SELECT DISTINCT 语句
关键词 DISTINCT 用于返回唯一不同的值。
语法：SELECT DISTINCT 列名称 FROM 表名称
--#2
/*
List all the continents - just once each.
*/
SELECT DISTINCT(continent)
FROM world
--#3
/*
Give the total GDP of Africa
*/
SELECT SUM(GDP) FROM world
WHERE continent = 'Africa'
--#4
/*
How many countries have an area of at least 1000000
*/
SELECT COUNT(name) FROM world
WHERE area >= 1000000
--#5
/*
What is the total population of ('Estonia', 'Latvia', 'Lithuania')
*/
SELECT SUM(population) FROM world
WHERE name IN ('Estonia', 'Latvia', 'Lithuania')
知识点：
GROUP BY 语句
GROUP BY 语句用于结合合计函数，根据一个或多个列对结果集进行分组。
GROUP BY 语法
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
--#6
/*
For each continent show the continent and number of countries.
*/
SELECT continent,COUNT(name)
FROM world
GROUP BY continent
--#7
/*
For each continent show the continent and number of countries with populations of at least 10 million.
*/
SELECT continent,COUNT(name)
FROM world
WHERE population > 10000000
GROUP BY continent
知识点：
HAVING 子句
在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与合计函数一起使用。
HAVING 语法
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value
--#8
/*
List the continents that have a total population of at least 100 million.
*/
SELECT continent
FROM world
GROUP BY continent
HAVING SUM(population) > 100000000
