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mysqlsql
vise
vise

Reputation: 23

MySQL - Selecting single entry per category

My apologies. I have edited the below into 2 table, Im just having a bit of confusion.

I have a tables very similar as the ones below and I wanted to show all the table2-class 1 but only 1 random item per each table1-category

Sample Item Table1

+---------+---------------+---------------+
|   ID    |   Item Name   |    Category   |
+---------+---------------+---------------+
|   01    |    Item A     |     Cat 1     |
|   02    |    Item B     |     Cat 1     |
|   03    |    Item C     |     Cat 2     |
|   04    |    Item D     |     Cat 2     |
|   05    |    Item E     |     Cat 3     |
|   06    |    Item F     |     Cat 3     |
+---------+---------------+---------------+

Sample Item Table2

+---------------+---------------+
|    Category   |     Class     |
+---------------+---------------+
|     Cat 1     |       1       |
|     Cat 2     |       1       |
|     Cat 3     |       2       |
+---------------+---------------+

I wanted to show all the table2-class 1 but only 1 random item per each table1-category

Desired Result

+---------+---------------+---------------+
|   02    |    Item B     |     Cat 1     |
|   03    |    Item C     |     Cat 2     |
+---------+---------------+---------------+

(This is within my PHP script)

Thanks in advance

Upvotes: 1

Views: 223

Answers (4)

vise
vise

Reputation: 23

Prior to the above edited scenario, I used the below query and it works fine except that it doesn't randomize the entry of each category:

SELECT * FROM Table1,Table2
WHERE Table2.Class = '1'
 AND Table1.Category = Table2.Category
GROUP BY Table1.Category ORDER BY RAND()

Upvotes: 0

Gordon Linoff
Gordon Linoff

Reputation: 1271231

The safest way to do this is with a correlated subquery. To get the item_id:

select category,
       (select item_id from sample s where s2.category = s.category order by rand() limit 1) as item_id
from sample s
group by category;

To get the rest of the item information, join that back in:

select s.*
from (select category,
             (select item_id from sample s where s2.category = s.category order by rand() limit 1) as item_id
      from sample s
      group by category
     ) c join
     sample s
     on s.item_id = c.item_id;

Upvotes: 0

Rafa Paez
Rafa Paez

Reputation: 4870

Try something like this:

SELECT id, itemname, category FROM (
  SELECT id, itemname, category FROM sample_table 
  ORDER BY RAND()
) AS tmp
GROUP BY category

Note that this query is totally valid in MySQL http://dev.mysql.com/doc/refman/5.0/en/group-by-extensions.html

Upvotes: 0

peterm
peterm

Reputation: 92845

You can do something like this

SELECT t.id, itemname, category
  FROM
(
  SELECT 
  (
    SELECT id
      FROM table1
     WHERE category = t.category
     ORDER BY RAND()
     LIMIT 1
  ) id
    FROM table1 t
   GROUP BY category
) q JOIN table1 t
    ON q.id = t.id

Note: using RAND() is very costly

Output:

| ID | ITEMNAME | CATEGORY |
|----|----------|----------|
|  1 |   Item A |    Cat 1 |
|  3 |   Item C |    Cat 2 |
|  6 |   Item F |    Cat 3 |

Here is SQLFiddle demo

Upvotes: 3

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