https://blog.csdn.net/nxjhi/article/details/51340018

最近学习数据库，在codecademy中， 遇到如下语句

1. SELECT a.dep_month,

2. a.dep_day_of_week,

3. AVG(a.flight_count) AS average_flights

4. FROM (

5. SELECT dep_month,

6. dep_day_of_week,

7. dep_date,

8. COUNT(*) AS flight_count

9. FROM flights

10. GROUP BY 1,2,3

11. ) a

12. GROUP BY 1,2

13. ORDER BY 1,2;

看到group by 1，2 和 order by 1， 2。看不懂，google，搜到了Stack Overflow 上有回答 What does SQL clause “GROUP BY 1” mean? 

大概意思就是，group by， order by 后面跟数字，指的是 select 后面选择的列（属性），1 代表第一个列（属性），依次类推。

因而上面最外层的

1. GROUP BY 1,2

2. ORDER BY 1,2

等价于

1. GROUP BY <span style="font-family: Arial, Helvetica, sans-serif;">a.dep_month, </span><span style="font-family: Arial, Helvetica, sans-serif;">a.dep_day_of_week</span><span style="font-family: Arial, Helvetica, sans-serif;">

2. </span> ORDER BY <span style="font-family: Arial, Helvetica, sans-serif;">a.dep_month, </span><span style="font-family: Arial, Helvetica, sans-serif;">a.dep_day_of_week</span>

注意，这边从1开始数，而不是0

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