Question

+----+--------+-------------+
| ID |  Name  |  Category   |
+----+--------+-------------+
|  1 | Cat    | Animal      |
|  2 | Loin   | Animal      |
|  3 | Dog    | Animal      |
|  4 | Red    | Color       |
|  5 | Hen    | Bird        |
|  6 | Parrot | Bird        |
+----+--------+-------------+

我想要类别明智的序列号。这意味着输出

+----+--------+----------+--------+
| ID |  Name  | Category | Count  |
+----+--------+----------+--------+
|  1 | Cat    | Animal   |      1 |
|  2 | Loin   | Animal   |      2 |
|  3 | Dog    | Animal   |      3 |
|  4 | Red    | Color    |      1 |
|  5 | Hen    | Bird     |      1 |
|  6 | Parrot | Bird     |      2 |
+----+--------+----------+--------+

有人可以帮忙吗？

Answer 1

一个非常好的解决方案是使用用户定义的变量，例如

select
id,
name,
category,
rn as `count`
from (
  select
  id,name,category,
  @rn:= if(category = @prev_category,@rn+1,1) as rn,
  @prev_category:= category
  from my_table,(select @rn:=1,@prev_category:= null)x
  order by category,id
)x

Answer 2

基本上你需要等同于ROW_NUMBER OVER(PARTITION)，你可以使用自联接和聚合：

SELECT a.id, a.name, a.category, count(*) AS `Count`
FROM tab a
JOIN tab b 
  ON a.Category= b.Category AND a.ID >= b.ID
GROUP BY a.id, a.name, a.category;

的 LiveDemo

输出：

╔════╦════════╦══════════╦═══════╗ ║ ID ║ Name ║ Category ║ Count ║ ╠════╬════════╬══════════╬═══════╣ ║ 1 ║ Cat ║ Animal ║ 1 ║ ║ 2 ║ Loin ║ Animal ║ 2 ║ ║ 3 ║ Dog ║ Animal ║ 3 ║ ║ 4 ║ Red ║ Color ║ 1 ║ ║ 5 ║ Hen ║ Bird ║ 1 ║ ║ 6 ║ Parrot ║ Bird ║ 2 ║ ╚════╩════════╩══════════╩═══════╝

如何检查MySQL中的前一条记录？

2 个答案: