How to Return the Top 5 Items in Each Category in MySQL: Handling Subquery Errors?

How to Return the Top 5 Items in Each Category in MySQL

Returning a limited number of records from each category is a common requirement in database applications. In the specific case of returning the top 5 menu items per menu, the challenge arises due to the subquery returning multiple rows for each menu, leading to the "Subquery returns more than 1 row" error.

To overcome this issue, a common approach is to use side-effecting variables. Here's a corrected query that addresses the error:

SELECT profilename, name
FROM
(
    SELECT m.profilename, s.name,
        @r:=case when @g=m.profilename then @r+1 else 1 end r,
        @g:=m.profilename
    FROM (select @g:=null,@r:=0) n
    cross join menus m 
    left join menuitems s on m.menuid = s.menuid
) X
WHERE r <= 5

Breaking down the query:

1. Side-Effecting Variables: @r and @g are side-effecting variables used to keep track of the current category and the number of records returned for that category.
2. Initialization: We initialize @g and @r to null and 0, respectively, using a subquery.
3. Loop over Categories: We use a cross join between the menus table and n to loop over each menu profile.
4. Category Group and Record Count: For each category, we calculate the row number (r) using a case statement. If the current category is different from the previous one, we reset r to 1; otherwise, we increment it by 1.
5. Update Category Group: We update @g to the current category profile to keep track of the current category.
6. Selecting Top 5: Finally, we select the profilename and name only if the record count r is less than or equal to 5.

By utilizing side-effecting variables, this query ensures that it returns at most 5 records for each category, resolving the original error.

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