如何在MySQL中执行动态UNION查询？

Question

mytable has an auto-incrementing id column which is an integer, and for all intents and purposes in this case you can safely assume that the higher ID represents a more recent value. mytable also has an indexed column called group_id which is a foreign key to the groups table.

I want a quick and dirty query to select the 5 most recent rows for each group_id from mytable .

If there were only three groups, this would be easy, as I could do this:

SELECT * FROM `mytable` WHERE `group_id` = 1 ORDER BY `id` DESC LIMIT 5 
UNION ALL 
SELECT * FROM `mytable` WHERE `group_id` = 2 ORDER BY `id` DESC LIMIT 5 
UNION ALL 
SELECT * FROM `mytable` WHERE `group_id` = 3 ORDER BY `id` DESC LIMIT 5 

However , there is not a fixed number of groups. Groups are determined by the what's in the groups table, so there is an indeterminate number of them.

My thoughts so far :

• I could grab a CURSOR on the groups table and build a new SQL query string, then EXECUTE it. However, that seems really messy and I'm hoping there's a better way of doing it.
• I could grab a CURSOR on the groups table and insert things into a temporary table, then select from that. However, that also seems really messy.
• I don't know if I could just grab a CURSOR and then start returning rows directly from there. Is there perhaps something similar to SQL Server's @table type variables?
• What I'm hoping most of all is that I'm overthinking this and there is a way to do this in a SELECT statement.

Answer 1

To get n most recent rows per group can be best handled by window functions in other RDBMS (SQL Server,Postgre Sql,Oracle etc), But unfortunately MySql don't have any window functions so for alternative there is a solution to use user defined variables to assign a rank for rows that belong to same group in this case ORDER BY group_id,id desc is important to order the results properly per group

SELECT  c.*
FROM (
SELECT *,
@r:= CASE WHEN @g = group_id THEN @r + 1 ELSE 1 END rownum,
@g:=group_id
FROM mytable
CROSS JOIN(SELECT @g:=NULL ,@r:=0) t
  ORDER BY group_id,id desc
) c
WHERE c.rownum <=5

Above query will give you 5 recent rows for each group_id and if you want to get more than 5 rows just change where filter of outer query to your desired number WHERE c.rownum <= n