SQL查询优化帮助

Question

my foundation on SQL is pretty weak so I hope you could bear with me. I have three tables: contents, categories, and categorization. The setup was chosen since some content will belong to one or more categories.

I want to fetch contents and its corresponding categories.

This is an overly-simplified version of the current script, without error-checking routines:

$q = "SELECT * FROM contents WHERE contents.foo = 'bar'"
$resource = mysql_query($q);
$categoryFilter = array();

$q2 = "SELECT * FROM categorization WHERE ";
while($content = mysql_fetch_assoc($resource))
{
    $categoryFilter[] = "content_id='" . $content["id"] . "'";
}

if(count($categoryFilter))
{
    $q2 .= implode(" OR ", $categoryFilter);
    mysql_query($q2);
}

That's the gist of it. I hope you get what I am trying to do. I don't know if I can actually use JOINS the content_id may be present in multiple rows in categorization . So what I did was to simply append multiple OR 's, trying to fetch items one by one. I really would not like to use multiple queries in this scenario. I hope anyone could suggest an approach

Thanks for your time

Answer 1

One query should be enough to fetch data from all three tables:

SELECT categories.category_id #, other fields
FROM contents
INNER JOIN categorization ON contents.content_id = categorization.content_id
INNER JOIN categories ON categorization.category_id = categories.category_id
WHERE contents.content_id = 1 # AND other filters

Tweak the columns in the SELECT clause and/or conditions in WHERE clause according to your needs.

Answer 2

If I understand it correctly, you can do this in one sql statement

SELECT *
FROM contents t1 
JOIN categorization t2
WHERE t1.content_id = t2.content_id AND t1.foo = 'bar'

Also ensure that content_id is indexed both in 'content' and 'categorization'. You may find it worthwhile indexing 'foo' aswell, but it depends on how you are actually searching.

Answer 3

This should do the same thing as in your example:

$q = "
    SELECT *
    FROM
        contents c
        categorization ctg ON ctg.content_id = c.id
    WHERE c.foo = 'bar'
";

$result = mysql_query($q);