GROUP BY分组

练习内容

-- 按城市分组统计用户数\nSELECT city, COUNT(*) AS user_count\nFROM users\nGROUP BY city;\n\n-- 按日期分组统计订单数\nSELECT DATE(created_at) AS order_date, COUNT(*) AS order_count\nFROM orders\nGROUP BY DATE(created_at);