Mysqli-使用平均，总和，两个表分组

Question

I know the basics when it comes to mysqli queries, but I'm having trouble with the following:

I have a large table called caseStatistics with the following columns:

idSchool (id for school), timeReported (unix timestamp), timeRead (unix timestamp), timeSolved (unix timestamp)

I have another large table called school with the following columns:

idSchool (id for school), name (name of school)

From these two tables, I want to output the name of the school, total cases, the avarage time it has taken for operators to respond to a case (after it was reported), and the avarage time on how long it has taken to solve the case (after it was reported). I also need the results to be grouped by the school name.

So it should output something like this:

Name : BlaBlaSchool

Total cases : 49

Avarage respond time : 2 hours

Avarage solve time : 16 hours

(For each school)

I don't need assistance with converting from unix to days, hours, minutes etc, only the mysqli query. Any help is greatly appreciated, thank you!

Answer 1

All you need to do is add the functions into the column list in the select statement:

SELECT
  s.`name`,
  COUNT(*) AS totalCases,
  AVG(cs.`timeRead` - cs.`timeReported`) AS avgRespond,
  AVG(cs.`timeSolved` - cs.`timeReported`) AS avgSolved
FROM `caseStatistics` AS cs
LEFT JOIN `school` AS s ON s.`idSchool` = cs.`idSchool`
GROUP BY s.`name`

You can use the HAVING keyword to narrow results using the aggregate values (similar to WHERE ). Also, make sure to check out: https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html