这个mysql查询是什么意思？

debugcn 发表于 Dev

优格

我试图弄清楚这个查询究竟执行了什么。特别是使用变量@和赋值的部分:=。第一部分非常简单，因为我们有一个来自派生表 t1 的嵌套查询，但我不太清楚的是列 rn 的结果。这是查询：

SELECT 
t1.user_id, 
t1.percentage, 
t1.id, 
t1.name,
(@rn := if(@uid = t1.user_id, @rn + 1,
   if(@uid := t1.user_id, 1, 1))
) as rn

FROM

(SELECT 
 pbt.user_id, 
 pbt.percentage, 
 t.id, t.name
 FROM 
 user_purchased_brand_tags AS pbt 
 JOIN tags t on t.id = pbt.tag_id 
 ORDER BY pbt.user_id, pbt.percentage desc) t1

花哨的裤子

它创建一个行号，当 user_id 更改时该行号重置为 1。

if 函数的参数是 (condition, true part, false part)。

(@rn := /* assign the new value to the variable @rn */

if(@uid = t1.user_id, @rn + 1, /* when user_id is the same as the value in @uid, return @rn + 1. 
                                  The comparison is done before assigning the value of the current row below. Therefore the @uid variable still holds the value of the previous row */

   if(@uid := t1.user_id, 1, 1)) /* this applies when above condition is not true.
                                    It's a clever combination of assigning the value of the current row to @uid and returning 1 at the same time. */

) as rn

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