数据分组

SELECT
    Continent,
    Region,
    COUNT(*) AS CountriesCount,
    AVG(Population) AS AvgPopulation
FROM country
GROUP BY Continent, Region


Continent Region CountriesCount AvgPopulation
Asia Eastern Asia 8 188,416,000.00
Asia Middle East 18 10,465,594.44
Asia Southeast Asia 11 47,140,090.90
Asia Southern and Central Asia 14 106,484,000.00
Europe Baltic Countries 3 2,520,633.33
Europe British Islands 2 31,699,250.00
Europe Eastern Europe 10 30,702,600.00
Europe Nordic Countries 7 3,452,342.85
Europe Southern Europe 15 9,644,946.66
...

GROUP BY 中可以写任意多个列，这样分组可以嵌套，对数据的分组方式控制得更细。
除聚合计算之外，SELECT 中出现的每一列都必须写进 GROUP BY。