聚合函数 + CASE

SELECT 
	c.name AS country,
    -- Count games from the 2012/2013 season
	COUNT (CASE WHEN m.season = '2012/2013' 
        	THEN m.id ELSE NULL END) AS matches_2012_2013
FROM country AS c
LEFT JOIN match AS m
ON c.id = m.country_id
-- Group by country name alias
GROUP BY country;

Count在 值是NULL是 不统计数， （count(‘任意内容’)都会统计出所有记录数，因为count只有在遇见null时不计数，即count(null)==0，因此前者单引号内不管输入什么值都会统计出所有记录数

用CASE SHEN来统计logical values

In R or Python, you have the ability to calculate a SUM of logical values (i.e., TRUE/FALSE) directly. In SQL, you have to convert these values into 1 and 0 before calculating a sum.

SELECT 
	c.name AS country,
    -- Sum the total records in each season where the home team won
	SUM(CASE WHEN m.season = '2012/2013' AND m.home_goal > m.away_goal 
        THEN 1 ELSE 0 END) AS matches_2012_2013,
 	SUM(CASE WHEN m.season = '2013/2014' AND m.home_goal > m.away_goal 
        THEN 1 ELSE 0 END) AS matches_2013_2014,
	SUM(CASE WHEN m.season = '2014/2015' AND m.home_goal > m.away_goal 
        THEN 1 ELSE 0 END) AS matches_2014_2015
FROM country AS c
LEFT JOIN match AS m
ON c.id = m.country_id
-- Group by country name alias

这里用sum()，而不是count()了

SELECT 
	c.name AS country,
    -- Round the percentage of tied games to 2 decimal points
	ROUND(AVG(CASE WHEN m.season='2013/2014' AND m.home_goal = m.away_goal THEN 1
			 WHEN m.season='2013/2014' AND m.home_goal != m.away_goal THEN 0
			 END)*100,2) AS pct_ties_2013_2014,
	ROUND(AVG(CASE WHEN m.season='2014/2015' AND m.home_goal = m.away_goal THEN 1
			 WHEN m.season='2014/2015' AND m.home_goal != m.away_goal THEN 0
			 END)*100,2) AS pct_ties_2014_2015
FROM country AS c
LEFT JOIN matches AS m
ON c.id = m.country_id
GROUP BY country;

另一种用法 =》 avg()，符合条件的1，不符合的0