使用GROUP_CONCAT在查询中忽略LIMIT

Question

I need to select some rows from second table and concatenate them in comma-separated string. Query works well except one problem - It always selects all rows and ignores LIMIT.

This is part of my query which gets that string and ignores LIMIT:

select 
    group_concat(value order by `order` asc SEPARATOR ', ') 
from slud_data 
    left join slud_types on slud_types.type_id=slud_data.type_id 
where slud_data.product_id=18 and value!='' and display=0 limit 3;


// Result:
+---------------------------------------------------------+
| group_concat(value order by `order` asc SEPARATOR ', ') |
+---------------------------------------------------------+
| GA-XXXX, Bentley, CONTINENTAL FLYING SPUR, 2006         |
+---------------------------------------------------------+

// Expected result: (only 3 comma-separated records, not 4)

Full query:

SELECT *,product_id id,
    (select group_concat(value order by `order` asc SEPARATOR ', ') from slud_data left join slud_types on slud_types.type_id=slud_data.type_id where slud_data.product_id=t1.product_id and value!='' and display=0 limit 3) text
FROM slud_products t1 
WHERE 
    now() < DATE_ADD(date,INTERVAL +ttl DAY) and activated=1
ORDER BY t1.date desc

Answer 1

The LIMIT clause limits the number of rows in the final result set, not the number of rows used to construct the string in the GROUP_CONCAT. Since your query returns only one row in the final result the LIMIT has no effect.

You can solve your issue by constructing a subquery with LIMIT 3, then in an outer query apply GROUP_CONCAT to the result of that subquery.

Answer 2

Your query is not working as you intended for the reasons @Mark Byers outlined in the other answer . You may want to try the following instead:

SELECT  GROUP_CONCAT(`value` ORDER BY `order` ASC SEPARATOR ', ') 
FROM    (
           SELECT    `value`, `order`
           FROM      slud_data 
           LEFT JOIN slud_types ON slud_types.type_id = slud_data.type_id 
           WHERE     slud_data.product_id = 18 AND value != '' AND display = 0 
           LIMIT     3
        ) a;

Answer 3

An example of Mark Byers idea:

SELECT GROUP_CONCAT(id, '|', name) 
FROM (
SELECT id, name
FROM users
LIMIT 3) inner