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SUM and COUNT

刷题网站SQLZoo，刷题语言MySQL

文章目录

• SUM and COUNT
• • 知识点
  • • 1. SUM
    • 2. Count
    • 3. MAX
    • 4. DISTINCT
    • 5. ORDER BY
    • 6. HAVING
  • 题目
  • • 1 Total world population
    • 2 List of continents
    • 3 GDP of Africa
    • 4 Count the big countries
    • 5 Baltic states population
    • 6 Counting the countries of each continent
    • 7 Counting big countries in each continent
    • 8 Counting big continents

知识点

这套题目需要使用：SUM, Count, MAX, DISTINCT 和 ORDER BY.

1. SUM

 SUM 函数返回数值列的总数（总额），语法是SELECT SUM(column_name) FROM table_name

2. Count

 COUNT() 函数返回匹配指定条件的行数（空值不计），语法为：SELECT COUNT(column_name) FROM table_name

3. MAX

 MAX 函数返回一列中的最大值（NULL 值不包括在计算中），语法为：SELECT MAX(column_name) FROM table_name

4. DISTINCT

 关键词 DISTINCT 用于返回唯一不同的值，语法为SELECT DISTINCT 列名称 FROM 表名称

5. ORDER BY

 ORDER BY 语句用于根据指定的列对结果集进行排序，默认按照升序对记录进行排序，如果希望按照降序对记录进行排序，可以使用 DESC 关键字。语法是：SELECT column_name FROM table_name ORDER BY column_name

6. HAVING

 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。具体用法为：

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

题目

World Country Profile: Aggregate functions

This tutorial is about aggregate functions such as COUNT, SUM and AVG. An aggregate function takes many values and delivers just one value. For example the function SUM would aggregate the values 2, 4 and 5 to deliver the single value 11.

1 Total world population

Show the total population of the world.

world(name, continent, area, population, gdp)

SELECT SUM(population) FROM world

2 List of continents

List all the continents - just once each.

select distinct continent from world

3 GDP of Africa

Give the total GDP of Africa

select sum(gdp) from world 
  where continent = 'Africa'

4 Count the big countries

How many countries have an area of at least 1000000

select count(name) from world
 where area > 1000000

5 Baltic states population

What is the total population of (‘Estonia’, ‘Latvia’, ‘Lithuania’)

select sum(population) from world
 where name in  ('Estonia', 'Latvia', 'Lithuania')

6 Counting the countries of each continent

For each continent show the continent and number of countries.

select continent, count(name) from world
 group by continent

7 Counting big countries in each continent

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

8 Counting big continents

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

select continent from world 
  group by continent
  having sum(population)>100000000

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