Question

我如何将以下3个查询组合（垂直堆叠）到一个查询，返回100行，类别1中的50行，来自类别2的25个，来自类别3的25个，都是随机选择的。我试过UNION但似乎没有用。

select * from table where category_id = 1 order by rand() limit 50;

select * from table where category_id = 2 order by rand() limit 25;

select * from table where category_id = 3 order by rand() limit 25;

Answer 1

要将ORDER BY或LIMIT应用于个人SELECT，请将该子句放在括起SELECT的括号内：

(select * from table where category_id = 1 order by rand() limit 50)
UNION ALL
(select * from table where category_id = 2 order by rand() limit 25)
UNION ALL
(select * from table where category_id = 3 order by rand() limit 25);

Answer 2

您正在寻找的是UNION ALL语法（链接到MySQL文档）。

select * from table where category_id = 1 order by rand() limit 50
UNION ALL
select * from table where category_id = 2 order by rand() limit 25
UNION ALL
select * from table where category_id = 3 order by rand() limit 25;

编辑：删除分号，谢谢@Matt Fenwick

垂直堆栈MySQL导致单个查询

2 个答案: