内连接时加上 inner 关键字

显示表达优于隐式，这样连接类型会非常清晰：

-- Good
select
    users.email,
    sum(charges.amount) as total_revenue
from users
inner join charges on users.id = charges.user_id

-- Bad
select
    users.email,
    sum(charges.amount) as total_revenue
from users
join charges on users.id = charges.user_id