为什么内部ORDER BY不起作用？

Question

以下是我的查询结构：

(select @rnk := @rnk + 2 `rank` ,* from table1, (select @rnk := -1) x order by col1)
union all
(select @rnk2 := @rnk2 + 2 `rank`,* from table2, (select @rnk2 := 0) x)
order by rank limit 10

根据结果，我认为order by col1不起作用。为什么呢？

我想要的是：首先，select先按order by col1排序，然后按order by rant替换最终结果。怎么办？

修改：想象一下这个结果：

select @rnk := @rnk + 2 `rank` ,* from table1, (select @rnk := -1) x order by col1

// output: blut, green, red


select @rnk2 := @rnk2 + 2 `rank`,* from table2, (select @rnk2 := 0) x

// output: five, three, one, six, seven, two

现在我想要输出

的订单

blue, five, green, three, red, one, six, seven, two

但是上面的查询并不像那样。为什么呢？

Answer 1

将order by col1放在另一级子查询中：

(SELECT @rnk := @rnk + 2 as rank, t.*
 FROM (SELECT * FROM table1 ORDER BY col1) AS t
 CROSS JOIN (SELECT @rnk := -1) AS x)
UNION ALL
(SELECT @rnk2 := @rnk2 + 2 AS rank, *
 FROM table2
 CROSS JOIN (SELECT @rnk2 := 0) AS x)
ORDER BY rank

有时在您分配rank列的同一查询中进行排序有效，但不能保证，所以最好在子查询中进行。