Working with Data Sources 6

1. GROUP BY function

　　The group by function will categroy the table according to different requirement:

　　Select Employed, Major_category, SUM(Employed) FROM recent_grads GROUP BY Major_category; #Here, in the table, the program first group by Employed and Major_category. Then they use aggregate function to get the result.

2. If we would like to change name of each column in the query result, we can use AS function:

　　Select sum(Men) as total_men, sum(Women) as total_women from recent_grads;

3. When we use AS, we choose HAVING instead of WHERE:

　　Select Major_category, Avg(low_wage_jobs)/ Avg(Total) as share_low_wage from recent_grads group by Major_category having share_low_wage > 0.1; 　　

4. We use ROUND function to round values when query them:

　　SELECT Major_category, ROUND(ShareWomen, 2) AS rounded_share_women 

　　FROM recent_grads;  # Round the ShareWomen column to 2 decimal places

 

转载于:https://www.cnblogs.com/kingoscar/p/6079799.html