One to many association in MySQL to obtain the latest data in multiple tables

background

stay msyql One to many queries often appear in queries , Only query the latest data in one of the multiple tables or the data that meets a certain condition . If only GROUP BY The query results cannot be satisfied , At this point, the query becomes complex .

for instance , School watch 、 The curriculum 、 Course sequencing table , I need the top course in each school .

In some cases, we can see that the method used by people is ：

``SELECT c.Id FROMt_course_rank AS ocrLEFT JOIN ( SELECT * FROM t_course WHERE coursetype = 2 ORDER BY Visit DESC ) AS c ON c.Id = ocr.courseidLEFT JOIN t_school AS s ON c.LoginId = s.LoginIdWHEREc.LoginId IN (19) GROUP BY c.LoginId

Complete the latest data in a condition through temporary table Association , But this kind of query is not accurate , Can't meet the conditions .

Solution

1. Find out all the data first , Then traverse to query (php in foreach Query in ), Simple , But it's inefficient , Process can be complex .

2. Sort first and then call