sqlzoo练习17-group by and having

GROUP BYandHAVING

By including a GROUP BY clause functions such as SUM and COUNT are applied to groups of items sharing values. When you specify GROUP BY continent the result is that you get only one row for each different value of continent. All the other columns must be “aggregated” by one of SUM, COUNT …

The HAVING clause allows use to filter the groups which are displayed. The WHERE clause filters rows before the aggregation, the HAVING clause filters after the aggregation.

• where 过滤在前
• group by 中间
• having 过滤在后

-- 1
SELECT continent, COUNT(name) 
FROM world
GROUP BY continent

-- 2
SELECT continent, SUM(population)
FROM world
GROUP BY continent

-- 3
/*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.*/
select continent, count(name)
from world 
where population > 200000000
group by continent;

-- 4
/*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   -- 分组之后再把大于50000000的筛选出来

-- 5
/*For each continent show the continent and number of countries.*/
select continent, count(name)   -- 分清sum 和 count的关系
from world
group by continent;

-- 6
/*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

-- 7
/*List the continents that have a total population of at least 100 million.*/
select continent
from world 
group by continent
having sum(population) >= 100000000;