这个mysql查询是什么意思？

Question

我正在试图找出这个查询的确切执行情况。特别是在其中使用变量@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，但对我来说不太清楚的是列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

Answer 1

当user_id发生变化时，它会创建一个重置为1的行号。

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

Answer 2

@rn := if(@uid = t1.user_id, @rn + 1, if(@uid := t1.user_id, 1, 1)) 是

assignement operator就像这样

  

IF（表达式，expression_if_true，expression_if_false）;

此：

if (@uid == t1.user_id) {  // <------ t1.user_id is COMPARED with @uid

    @rn = @rn + 1 ; // or @rn++, ie we increment it by 1

}else{

        if (@uid = t1.user_id) { // <------ t1.user_id is ASSIGNED to @uid

            @rn = 1;

        }else{

            @rn = 1;

        }

}

可以像PHP / C风格那样分解：

@rn

第二个 if 始终将相同的值1分配给t1.user_id，但它还将@uid的值分配给{{1}}