Question

我在MySql数据库中有这样的表。

                    "table1"
____________________________________________________
id      Category      Type     rating     Value
----------------------------------------------------
1          1           1         5          23
2          1           1         6          27
3          2           1         4          26
4          2           2         2          25
5          3           1         4          21
6          3           1         5          28

我想在此表中选择具有唯一类别和类型的特定数量的不同文档。像这样：

select distinct category, type from table1 order by rating desc limit 0,3

        "table2"
__________________________
id      Category      Type     
--------------------------
1          1           1       
5          3           1    
3          2           1

然后从表中选择具有此类别和类型的所有值。

select id,value from table1 where type and category is in table2

                    "table3"
____________________________________________________
id      Category      Type     rating     Value
----------------------------------------------------
1          1           1         5          23
2          1           1         6          27
5          3           1         4          21
6          3           1         5          28
3          2           1         4          26

我如何使用一个Sql语句来实现这个目标？

THX。

Answer 1

您可以使用查询生成派生表来执行此操作。然后将此派生表与原始表连接：

SELECT id, t1.Category, t1.Type, rating, Value
FROM table1 AS t1
INNER JOIN (SELECT DISTINCT category, type 
            FROM table1 
            ORDER BY rating DESC limit 0,2) AS t2
ON t1.category = t2.category AND t1.type = t2.type

但是上面并没有实际返回OP的预期结果集。如果您希望所有行的(Category, Type)值与前3个评级值相关，那么您可以使用：

SELECT id, t1.Category, t1.Type, rating, Value
FROM table1 AS t1
INNER JOIN (
  SELECT category, type
  FROM table1
  GROUP BY category, type
  ORDER BY MAX(rating) DESC LIMIT 0,3
) AS t2 ON t1.category = t2.category AND t1.type = t2.type

Demo here

如何在多列中选择特定和差异的记录数？

1 个答案: