第一部分 GROUP BY & HAVING
For each continent show the number of countries:
world(name, continent, area, population, gdp)
SELECT continent, COUNT(name) FROM world GROUP BY continent
For each continent show the total population:
SELECT continent, SUM(population) FROM world GROUP BY continent
WHERE and GROUP BY. The WHERE filter takes place before the aggregating function. For each relevant continent show the number of countries that has a population of at least 200000000.
WHERE and GROUP BY. The WHERE filter takes place before the aggregating function. For each relevant continent show the number of countries that has a population of at least 200000000.
GROUP BY and HAVING. The HAVING clause is tested after the GROUP BY. You can test the aggregated values with a HAVING clause. Show the total population of those continents with a total population of at least half a billion.
SELECT continent, SUM(population)
FROM world
GROUP BY continent
HAVING SUM(population)>500000000
第二部分 SUM COUNT
Show the total population of the world.
SELECT SUM(population)
FROM world
List all the continents - just once each.
SELECT distinct continent FROM world
Give the total GDP of Africa
SELECT SUM(gdp) FROM world WHERE continent = 'Africa'
How many countries have an area of at least 1000000
SELECT COUNT(name) FROM world WHERE area >= 1000000
What is the total population of ('Estonia', 'Latvia', 'Lithuania')
SELECT SUM(population) FROM world WHERE name IN ('Estonia', 'Latvia', 'Lithuania')
For each continent show the continent and number of countries.
SELECT continent, COUNT(name) FROM world GROUP BY 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
List the continents that have a total population of at least 100 million.
SELECT continent FROM world GROUP BY continent HAVING SUM(population) > 100000000
