MySql查询：从表中为每个类别选择前3行

I have a table with records and it has a row called category. I have inserted too many articles and I want to select only two articles from each category.

我有一个包含记录的表，它有一行称为类别。我插入了太多文章，我想从每个类别中只选择两篇文章。

I tried to do something like this:

我试着这样做：

I created a view:

我创建了一个视图：

CREATE VIEW limitrows AS 
   SELECT * FROM tbl_artikujt ORDER BY articleid DESC LIMIT 2

Then I created this query:

然后我创建了这个查询：

SELECT * 
FROM tbl_artikujt 
WHERE 
   artikullid IN
   (
      SELECT artikullid
      FROM limitrows
      ORDER BY category DESC
   )
ORDER BY category DESC;

But this is not working and is giving me only two records?

但这不起作用，只给我两个记录？

4 个解决方案

#1

LIMIT only stops the number of results the statement returns. What you're looking for is generally called analytic/windowing/ranking functions - which MySQL doesn't support but you can emulate using variables:

LIMIT仅停止语句返回的结果数。您正在寻找的通常称为分析/窗口/排名函数 - MySQL不支持但您可以使用变量进行模拟：

SELECT x.*
  FROM (SELECT t.*,
               CASE 
                 WHEN @category != t.category THEN @rownum := 1 
                 ELSE @rownum := @rownum + 1 
               END AS rank,
               @category := t.category AS var_category
          FROM TBL_ARTIKUJT t
          JOIN (SELECT @rownum := NULL, @category := '') r
      ORDER BY t.category) x
 WHERE x.rank <= 3

If you don't change SELECT x.*, the result set will include the rank and var_category values - you'll have to specify the columns you really want if this isn't the case.

如果不更改SELECT x。*，则结果集将包含rank和var_category值 - 如果不是这种情况，则必须指定您真正想要的列。

#2

Read this article: Top N of a group

阅读本文：小组的前N名

#3

SELECT * FROM (   
    SELECT  VD.`cat_id` ,  
       @cat_count := IF( (@cat_id = VD.`cat_id`), @cat_count + 1, 1 ) AS 'DUMMY1', 
       @cat_id := VD.`cat_id` AS 'DUMMY2',
       @cat_count AS 'CAT_COUNT'   
     FROM videos VD   
     INNER JOIN categories CT ON CT.`cat_id` = VD.`cat_id`  
       ,(SELECT @cat_count :=1, @cat_id :=-1) AS CID  
     ORDER BY VD.`cat_id` ASC ) AS `CAT_DETAILS`
     WHERE `CAT_COUNT` < 4

------- STEP FOLLOW ----------  
1 . select * from ( 'FILTER_DATA_HERE' ) WHERE 'COLUMN_COUNT_CONDITION_HERE' 
2.  'FILTER_DATA_HERE'   
    1. pass 2 variable @cat_count=1 and  @cat_id = -1  
    2.  If (@cat_id "match" column_cat_id value)  
        Then  @cat_count = @cat_count + 1    
        ELSE @cat_count = 1      
    3. SET @cat_id = column_cat_id    

 3. 'COLUMN_COUNT_CONDITION_HERE'   
    1. count_column < count_number    

4. ' EXTRA THING '
   1. If you want to execute more than one statement inside " if stmt "
   2. IF(condition, stmt1 , stmt2 )
      1. stmt1 :- CONCAT(exp1, exp2, exp3) 
      2. stmt2 :- CONCAT(exp1, exp2, exp3) 
   3. Final "If" Stmt LIKE 
      1. IF ( condition , CONCAT(exp1, exp2, exp3) , CONCAT(exp1, exp2, exp3) )    
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#4

-4

Use group by instead of order by.

使用group by而不是order by。

#1