mysql - MySQL：将每个组的前 N ​​行插入到表中。假设数百万组

Question

为一组执行此操作很简单：

INSERT INTO top_dancers_by_group
    SELECT group_id, dancer_id
    FROM dancers
    WHERE group_id = 1
    ORDER BY how_good_they_do_a_half_turn DESC
    LIMIT 1000

现在让我们假设有一个groups包含数千个组 ID 的表。如何为纯 MySQL 中的每个组 ID 执行此插入？

Answer 1

我会使用存储过程：

delimiter $$
create procedure insert_top_dancers_by_group()
begin
    declare gId int;
    declare done tinyint default 0;
    declare curGroup cursor for
        select distinct group_id from dancers;
    declare continue handler for not found
        set done = 1;

    open curGroup;
    group_insert: while done=0 do
        fetch curGroup into gId;
        if done = 0 then
            -- If you want to remove the previous stored dancers for this group:
            delete from top_dancers_by_group where group_id = gId;
            -- Insert the top dancers for this group:
            insert into top_dancers_by_group
                select group_id, dancer_id
                from dancers
                where group_id = gId
                order by how_good_they_do_a_half_turn DESC
                limit 1000;
        end if;
    end while;
    close curGroup;
end $$
delimiter ;

希望这可以帮助。

---

您还可以在此过程中使用参数来定义插入的行数：

delimiter $$
create procedure insert_top_n_dancers_by_group(n int)
begin
    declare gId int;
    declare done tinyint default 0;
    declare curGroup cursor for
        select distinct group_id from dancers;
    declare continue handler for not found
        set done = 1;

    open curGroup;
    group_insert: while done=0 do
        fetch curGroup into gId;
        if done = 0 then
            -- If you want to remove the previous stored dancers for this group:
            delete from top_dancers_by_group where group_id = gId;
            -- Insert the top dancers for this group:
            insert into top_dancers_by_group
                select group_id, dancer_id
                from dancers
                where group_id = gId
                order by how_good_they_do_a_half_turn DESC
                limit n;
        end if;
    end while;
    close curGroup;
end $$
delimiter ;

创建过程后，您可以像这样调用它们：

call insert_top_dancers_by_group();