avg.sql

create table things(id int not null auto_increment, created_at date, user varchar(32), amount decimal(10,2), primary key(id));
insert into things(created_at, user, amount) values (date'2021-10-02', 'michael', 5);
insert into things(created_at, user, amount) values (date'2021-10-02', 'markus', 10);
insert into things(created_at, user, amount) values (date'2021-10-03', 'markus', 15);
insert into things(created_at, user, amount) values (date'2021-10-04', 'markus', 5);

// Correct
 select avg(amount) from things;

// Same value but really hard to understand
// https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html
select avg(amount), created_at from things;

// Irritating at least (grouping by one column and use another to return)
select avg(amount), user from things group by created_at;

// avg per day
select avg(amount), created_at from things group by created_at;

// avg per day
select avg(amount), user from things group by user;