MySQL 5.6 - DENSE_RANK之类没有Order By的功能

Question

我有这样一张桌子：

+------+-----------+
|caseID|groupVarian|
+------+-----------+
|1     |A,B,C,D,E  |
+------+-----------+
|2     |A,B,N,O,P  |
+------+-----------+
|3     |A,B,N,O,P  |
+------+-----------+
|4     |A,B,C,D,F  |
+------+-----------+
|5     |A,B,C,D,E  |
+------+-----------+

我想得到一个新的列nameVarian，使得相同的groupVarian值具有由nameVarian表示的相同排名（例如：v1，v2等）。但是，分配给特定nameVarian的groupVarian值应该按照caseID的顺序排列（按照它们出现在表格中的顺序）。

输出应该是这样的：

+------+-----------+----------+
|caseID|groupVarian|namevarian
+------+-----------+----------+
|1     |A,B,C,D,E  |v1        |
+------+-----------+----------+
|2     |A,B,N,O,P  |v2        |
+------+-----------+----------+
|3     |A,B,N,O,P  |v2        |
+------+-----------+----------+
|4     |A,B,C,D,F  |v3        |
+------+-----------+----------+
|5     |A,B,C,D,E  |v1        |
+------+-----------+----------+

Answer 1

对于MySQL版本<8.0（OP's version is 5.6）：

问题陈述看起来需要DENSE_RANK上的groupVarian功能;但事实并非如此。 As explained by @Gordon Linoff：

您似乎希望按照它们在数据中出现的顺序枚举它们。

假设您的表名是t（请根据您的代码更改表和字段名称）。这是一个approach utilizing session variables（对于旧版本的MySQL），给出了期望的结果（DB Fiddle）：

SET @row_number = 0;
SELECT t3.caseID, 
       t3.groupVarian, 
       CONCAT('v', t2.num) AS nameVarian
FROM
  (
   SELECT 
     (@row_number:=@row_number + 1) AS num, 
     t1.groupVarian 
   FROM 
     (
      SELECT DISTINCT groupVarian 
      FROM t 
      ORDER BY caseID ASC 
     ) AS t1 
  ) AS t2 
INNER JOIN t AS t3 
  ON t3.groupVarian = t2.groupVarian 
ORDER BY t3.caseID ASC 

另外：我之前尝试模仿DENSE_RANK功能，效果很好。虽然以前的查询也可以稍微调整，以实现DENSE_RANK功能。但是，以下查询更有效，因为它创建较小的派生表，并避免在groupVarian上加入：

SET @row_number = 1;
SET @group_varian = '';

SELECT inner_nest.caseID, 
       inner_nest.groupVarian, 
       CONCAT('v', inner_nest.num) as nameVarian 
FROM (
        SELECT 
            caseID, 
            @row_number:=CASE
                           WHEN @group_varian = groupVarian THEN @row_number
                           ELSE @row_number + 1
                         END AS num, 
            @group_varian:=groupVarian as groupVarian 
        FROM
            t  
        ORDER BY groupVarian
     ) AS inner_nest 
ORDER BY inner_nest.caseID ASC 

Answer 2

你可以使用DENSE_RANK（MySQL 8.0）：

SELECT *, CONCAT('v', DENSE_RANK() OVER(ORDER BY groupVarian)) AS namevarian
FROM tab
ORDER BY CaseID;

db<>fiddle demo

Answer 3

基本上，您想要枚举变体。如果您只想要一个数字，那么您可以使用最小ID：

select t.*, min_codeId as groupVariantId
from t join
     (select groupVariant, min(codeId) as min_codeId
      from t
      group by groupVariant
     ) g
     on t.groupVariant = g.groupVariant;

但那不是你想要的。您似乎希望按照它们在数据中出现的顺序枚举它们。为此，您需要变量。这有点棘手，但是：

select t.*, rn as groupVariantId
from t join
     (select g.*,
             (@rn := if(@gv = groupvariant, @gv,
                        if(@gv := groupvariant, @gv+1, @gv+1)
                       )
             ) as rn
      from (select groupVariant, min(codeId) as min_codeId
            from t
            group by groupVariant
            order by min(codeId)
           ) g cross join
           (select @gv := '', @rn := 0) params
     ) g
     on t.groupVariant = g.groupVariant;

使用变量很棘手。一个重要的考虑因素：MySQL不保证SELECT中表达式的评估顺序。这意味着变量不应该在一个表达式中分配，然后在另一个表达式中使用 - 因为它们可能以错误的顺序进行评估（另一个答案有这个错误）。

此外，order by需要在子查询中进行。 MySQL不保证在排序之前发生变量赋值。