集約関数とGROUP BY

集約関数

件数
SELECT count(id) as count_id FROM members;

最大値
SELECT max(age) as max_age FROM members;

最小値
SELECT min(age) as min_age FROM members;

平均値
SELECT avg(age) as avg_age FROM members;

合計値
SELECT sum(age) as sum_age FROM members;

GROUP BY

SELECT gender FROM members GROUP BY gender;

SELECT gender, avg(age) as avg_age FROM members GROUP BY gender;

SELECT threads.id, threads.name, count(responses.id) as count FROM threads LEFT JOIN responses ON threads.id = responses.thread_id GROUP BY threads.id, threads.name;