在SQL查询中获取唯一数据

Question

I've a database who contain some datas in that form:

icon(name, size, tag)
(myicon.png, 16, 'twitter')
(myicon.png, 32, 'twitter')
(myicon.png, 128, 'twitter')
(myicon.png, 256, 'twitter')
(anothericon.png, 32, 'facebook')
(anothericon.png, 128, 'facebook')
(anothericon.png, 256, 'facebook')

So as you see it, the name field is not uniq I can have multiple icons with the same name and they are separated with the size field. Now in PHP I have a query that get ONE icon set, for example :

$dbQueryIcons = mysql_query("SELECT * FROM pl_icon WHERE tag LIKE '%".$SEARCH_QUERY."%' GROUP BY name ORDER BY id DESC LIMIT ".$firstEntry.", ".$CONFIG['icon_per_page']."") or die(mysql_error());

With this example if $tag contain 'twitter' it will show ONLY the first SQL data entry with the tag 'twitter', so it will be :

(myicon.png, 16, 'twitter')

This is what I want, but I would prefer the '128' size by default. Is this possible to tell SQL to send me only the 128 size when existing and if not another size ?

In an another question someone give me a solution with the GROUP BY but in this case that don't run because we have a GROUP BY name. And if I delete the GROUP BY, it show me all size of the same icons.

Thanks !

Answer 1

Try using subqueries.

SELECT * FROM (

    SELECT * FROM pl_icon
    WHERE tag LIKE '%$SEARCH_QUERY%'
    ORDER BY IF(size = 128, 0, 1)

) pl_icon
GROUP BY name
ORDER BY id DESC
LIMIT ...

We find all matching rows in the inner query and place rows with size = 128 above all others. Then, for each unique name we choose only the first record using GROUP BY .

Answer 2

尝试以下ORDER BY子句（id或size？）：

ORDER BY case when size=128 then 999999 else size end DESC

Answer 3

If what you really want is get the record with the largest size then:

SELECT name, tag, MAX(size)
FROM pl_icon 
WHERE tag LIKE '%".$SEARCH_QUERY."%' 
GROUP BY name, tag

But what's confusing here is that the 'name' is not unique. Usually in such scenarios people want to identify the record where a maximum (or minimum) occurs - like the last meter reading, or biggest invoice. So if your table had an id field and you wanted to retrieve that:

SELECT pl2.*
FROM pl_icon pl2,
(SELECT name, tag, MAX(size) AS biggest
FROM pl_icon 
WHERE tag LIKE '%".$SEARCH_QUERY."%' 
GROUP BY name, tag) ilv
WHERE ilv.name=pl2.name
AND ilv.tag=pl2.tag
AND ilv.biggest=pl2.size;

However this can be rather inefficient for very large tables / selects - when you might want to try the MAX(CONCAT trick

C.