I have a database of posts, each of which have tags. These tables are named Posts and Tags respectively. I also have a third table, called Posts_Tags which maintains a many-to-many relationship between these two tables.
In order to do this, both my posts and my tags tables have an id column. My Posts_Tags table, therefore, has both a postid and tagid column to store the mappings.
I am querying, for example, all posts with the word "class" in the title. I can do this easily with this query:
SELECT * FROM Posts WHERE title LIKE '%{class}%'
However, now I want to query all posts which not only have "class" in the title, but are also tagged with the "Java" tag. I could do this in two separate queries, where I first get the id of the Java tag:
SELECT id FROM Tags WHERE name='Java'
Then I could plug that into my first query, like this:
SELECT Posts.*
FROM Posts
INNER JOIN Posts_Tags
ON Posts.id=Posts_Tags.postid
WHERE Posts_Tags.tagid='$java_tag_id'
AND title LIKE '%{class}%'
However, I know I can do this in a single query, I just don't know how. I still have to think a lot about joins when doing just one, and doing multiple joins in the same query makes my head spin. How should I structure my query to perform this operation?
SELECT p.*
FROM Posts p
JOIN Posts_Tags pt
ON pt.postid = p.id
JOIN tags t
ON t.id = pt.tagid
WHERE t.tag='java'
AND p.title LIKE '%{class}%';
SELECT
p.*
FROM posts as p
INNER JOIN Posts_Tags pt ON pt.post_id = p.id
INNER JOIN Tags as t ON pt.tags_id = t.id
WHERE t.tag='java'
AND p.title LIKE '%keyword%';
Related
Here is my actual query:
$result = mysqli_query($link,"select ids_billets from pref_tags where id='$_GET[id]'");
$tags_ids = $result->fetch_object();
$result = mysqli_query($link,"select id, html from pref_posts where id in ($tags_ids->ids_billets)");
while($posts = $result->fetch_object()) {
.....
}
I have ids in one varchar field of the pref_tags table (ids_billets) - example : "12,16,158"
Is there a better way to query this?
Thanks
Instead of one row with a comma-separated list, I would create a new table linking posts to tags, with one row per post/tag combo, i.e.:
posts
---------------------
post_id | html | etc.
posts_tags
----------------
post_id | tag_id
tags
------------------------
tag_id | tag_name | etc.
Then do something like this:
SELECT p.post_id, p.html
FROM posts p
INNER JOIN posts_tags pt
ON p.post_id = pt.post_id
INNER JOIN tags t
ON pt.tag_id = t.tag_id
WHERE t.tag_name = ?
Or if you already have the tag_id, like you seem to:
SELECT p.post_id, p.html
FROM posts p
INNER JOIN posts_tags pt
ON p.post_id = pt.post_id
WHERE pt.tag_id = ?
You can also do the same query in a different form, using a subquery:
SELECT post_id, html
FROM posts
WHERE post_id IN (SELECT post_id FROM posts_tags WHERE tag_id = ?)
Also, look at prepared statements, which will make it easy to avoid the serious SQL injection problems your current code has.
You should not put multiple values in a single column, because doing so breaks first normal form. Have a look at that link for examples of the problems you're likely to come across, and how to fix them.
Rather, create a separate table where you can have the ID and the Tag ID in separate columns. Then you can pull back the IDs in a subquery in your second example query, and get the benefits of being able to search for and manipulate individual IDs in other queries.
So I have three tables, one is posts , having columns id,title,content,timestamp . Other is tags, having columns id,tag and third posttags describes one to many relation between posts and tags , having columns postid,tagid .
Now instead of having columns like hidden,featured etc in the table posts to describe whether a post should be visible to all or should be displayed on a special featured page, I thought why not use tags to save time. So what I decided is that all posts that have a tag #featured will be featured and all posts with tag #hidden will be hidden.
Implementing first one was easy as I could use a join query and in my where clause I could mention WHERE tag='featured' and this would get all the featured posts for me.
But take an example of a post tagged #sports and #hidden if I were to use the query
SELECT * FROM posts
INNER JOIN posttags ON posttags.postid = posts.id
INNER JOIN tags ON posttags.tagid = tags.id
WHERE tag !='hidden'
but that'd still return the post tagged hidden since its also tagged sports
PS my question is different from this question : Select a post that does not have a particular tag since it uses tagid directly and I'm unable to achieve same result using double join to check against tag name instead of tagid. And also I wish to retrieve the other tags of the post in same query which is not possible using the method in that question's answers
Group the tags by post, then use the HAVING clause to filter the groups for those that do not contain a 'hidden' tag. Because of MySQL's implicit type conversion and lack of genuine boolean types, one can do:
SELECT posts.*
FROM posts
JOIN posttags ON posttags.postid = posts.id
JOIN tags ON posttags.tagid = tags.id
GROUP BY posts.id
HAVING NOT SUM(tag='hidden')
You can do this with a NOT EXISTS subquery:
SELECT p.*, t.* -- what columns you need
FROM posts AS p
INNER JOIN posttags AS pt
ON pt.postid = p.id
INNER JOIN tags AS t
ON pt.tagid = t.id
WHERE NOT EXISTS
( SELECT *
FROM posttags AS pt_no
INNER JOIN tags AS t_no
ON pt_no.tagid = t_no.id
WHERE t_no.tag = 'hidden'
AND pt_no.postid = p.id
) ;
or the equivalent LEFT JOIN / IS NULL:
SELECT p.*, t.*
FROM posts AS p
LEFT JOIN posttags AS pt_no
INNER JOIN tags AS t_no
ON t_no.tag = 'hidden'
AND pt_no.tagid = t_no.id
ON pt_no.postid = p.id
INNER JOIN posttags AS pt
ON pt.postid = p.id
INNER JOIN tags AS t
ON pt.tagid = t.id
WHERE pt_no.postid IS NULL ;
Thsi type of queries are called anti-semijoins or just anti-joins. It's slightly more complex in your case because the condition (tag='hidden') is in a 3rd table.
I'm trying to load rows form a posts table based on whether they have multiple rows in another table. Take the below table structures:
posts
post_id post_title
-------------------
1 My Post
2 Another Post
post_tags
post_tag_id post_tag_name
--------------------------
1 My Tag
2 Another Tag
postTags
postTag_id postTag_tag_id postTag_post_id
------------------------------------------
1 1 1
2 2 1
Unsurprisingly, post and post_tags stores the posts and tags, and postTags joins which posts have which tags.
What I'd normally do to join the tables is this:
SELECT * FROM (`posts`)
JOIN `postTags` ON (`postTag_post_id` = `post_id`)
JOIN `post_tags` ON (`post_tag_id` = `postTag_tag_id`)
Then I'd have information on the tags, and can have additional stuff later in the query to search tag names for search terms etc, and then GROUP once I have posts that match the search terms.
What I'm trying to do is only select from posts where a post has both tag 1 AND tag 2, and I can't work out the SQL for it. I think it needs to be done in the actual JOIN rather than having a WHERE clause for it as when I run the join above I'd obviously get two rows back, so I can't have something like
WHERE post_tag_id = 1 AND post_tag_id = 2
as each row will only have one post_tag_id, and I can't check different values for the same column in one row.
What I've tried to do is something like this:
SELECT * FROM (`posts`)
JOIN `postTags` ON (postTag_tag_id = 1 AND postTag_tag_id = 2)
JOIN `post_tags` ON (`post_tag_id` = `postTag_tag_id`)
but this is returning 0 results when I run it; I've put conditions like this in JOINS before for similar things and I'm sure it's close but can't quite work out what to do if this doesn't work.
Am I at least on the right track? Hopefully I'm not missing something obvious.
Thanks.
You are trying to ask the postTags row to be at the same time one thing and another.
You either need to do two joins to post_tags and postTags so you get both. Or you can say that the post can have whatever tag between those two and the total amount of tags must equal two (assuming a post cannot related to the same tag more than once).
First approach:
SELECT *
FROM `posts` as p
WHERE p.`post_id` IN (SELECT pt.`postTag_post_id`
FROM `postTags` as pt
WHERE pt.`postTag_tag_id` = 1)
AND p.`post_id` IN (SELECT pt.`postTag_post_id`
FROM `postTags` as pt
WHERE pt.`postTag_tag_id` = 2);
Second approach:
SELECT *
FROM posts as p
WHERE p.post_id IN (SELECT pt.postTag_post_id
FROM (SELECT count(0) as c, pt.postTag_post_id
FROM postTags as pt
WHERE pt.postTag_tag_id IN (1, 2)
GROUP BY pt.postTag_post_id
HAVING c = 2) as pt);
I want also to add that if you use IN or EXISTS in the first approach then you won't have multiple lines for the same post row just because you have more than one tag. This way you save one DISTINCT later that would make your query slower.
I've used an IN in the second approach just as a rule of thumb I use: if you don't need to show the data you don't need to do a JOIN in the FROM section.
SELECT p.*, t1.*, t2.* FROM posts p
INNER JOIN postTags pt1 ON pt1.postTag_post_id = p.id AND pt1.postTag_tag_id = 1
INNER JOIN postTags pt2 ON pt2.postTag_post_id = p.id AND pt2.postTag_tag_id = 2
INNER JOIN post_tags t1 ON t1.post_tag_id = pt1.postTag_tag_id
INNER JOIN post_tags t2 ON t2.post_tag_id = pt2.postTag_tag_id
Without actually building a db the same as yours this is hard to verify but it should work.
Let me start by saying that this type of query is much easier and much more performant in a database that supports analytic queries (Oracle, MS SQL Server). So in MySQL you have to do it the old, crappy, aggregate way.
I also want to say that having a table that stores the names of the tags in post_tags and then the mapping of post tags to posts in postTags is confusing. If it were me, I would change the name of the mapping table to post_tags_map or post_tags_to_post_map. So you would have posts with post_id, post_tags with post_tags_id, and post_tags_map with post_tags_map_id. And those id columns would be named the same in every table. Having the same column that is named differently in other tables is also confusing.
Anyways, let's solve your problem.
First you want a result set that is 1 post id per row, and only the posts that have tags 1 & 2.
select postTag_post_id, count(1) cnt from (
select postTag_post_id from postTags where postTag_tag_id in (1, 2)
) group by postTag_post_id;`
That should give you back data like this:
postTag_post_id | cnt
1 | 2
Then you can join that result set back to your posts table.
select * from posts p,
(
select postTag_post_id, count(1) cnt from (
select postTag_post_id from postTags where postTag_tag_id in (1, 2)
) group by postTag_post_id;
) t
where p.post_id = t.postTag_post_id
and t.cnt >= 2;
If you need to do another join to the post_tags table in order to get the postTag_tag_id from the post_tag_name, your inner most query would change like so:
select postTag_post_id
from postTags a,
post_tags b
where a.postTag_tag_id = b.post_tag_id
and b.post_tag_name in ('tag 1', 'tag 2');
That should do the trick.
Assuming you already know tag IDs (1 and 2), you could do something like this:
SELECT post_id, post_title
FROM posts JOIN postTags ON (postTag_post_id = post_id)
WHERE postTag_tag_id IN (1, 2)
GROUP BY post_id, post_title
HAVING COUNT(DISTINCT postTag_tag_id) = 2
NOTE: DISTINCT is not necessary if there is an alternate key on postTags {postTag_tag_id, postTag_post_id}, as it should be.
NOTE: If you don't have tag IDs (and just have tag names), you'll need another JOIN (towards the post_tags table).
BTW, you should seriously consider ditching the surrogate PK in the junction table (postTags.postTag_id) and just having the natural PK {postTag_tag_id, postTag_post_id}. InnoDB tables are clustered, and secondary indexes in clustered tables are fatter and slower than in heap-based tables. Also, some queries can benefit from storing posts tagged by the same tag physically close together (or storing tags of the same post close together, if you reverse the PK).
Consider an standard normalized many-to-many tag system (three tables of articles, tags, tag_map). I want to get a list of tags with associated articles; for example
Tag Article_IDs
tag1 1,5,7
tag2 3,4,5,7,8
.....
How should I JOIN the tables to generate this list?
The naive way to simply count the number of rows in tag_map WHERE tag='something'. Badly, for this method, we need a separate query for every tag. For example, to generate a list for 20 tags, we need 20 queries (which is not rational). I hope to do this is one query with JOIN.
You can acomplish this with group_concat aggregation function. They are a lot of samples in stackoverflow.
SELECT tag.id,
GROUP_CONCAT(post.post_id)
FROM
posts
inner join
post_tag on ...
inner join
tags on ...
GROUP BY tag.id;
Something like this should work (but this is Oracle only, I don't know the equivalent of wm_concat() on other databases):
select t.tagname, wm_concat(m.articleid) from tags t, tag_map m where t.id = m.tagid group by t.tagname;
Select t.Tag, Group_Concat(a.Article_ID)
From tag_map tm
Join tag t on tm.tag_id = t.tag_id
Join articles a on tm.article_id = a.article_id
Group By t.Tag
Here's what I want to accomplish:
Select all rows from table "posts" while also fetching linked entities, such as the author (from table "users") and the names of the categories the post belongs to (from table "categories").
These are my two queries so far:
This one fetches the posts:
SELECT
posts.*
, users.authorName AS author
FROM posts
INNER JOIN users
ON users.id = posts.author
And this one fetches a comma separated list of categories for a specific post:
SELECT
GROUP_CONCAT(categories.category) AS categories
FROM categories
INNER JOIN post_category
ON post_category.categoryID = categories.id
WHERE
post_category.postID = ?
The two queries on their own work fine.
Naturally when the two are combined, I wouldn't need the WHERE clause of the second query.
I tried using the second query as a sub-query within the first one's SELECT clause, but that fetched a comma separated list of ALL categories for ALL posts. I want only the categories for the post I'm currently iterating over.
Any help would be greatly appreciated, and I apologize if any of this is unclear - it's hard enough for me to think about, let alone describe to other people ;)
Something like this:
SELECT posts.*, users.authorName AS author,
GROUP_CONCAT(categories.category) AS categories
FROM posts, users, categories, post_category
WHERE users.id = posts.author
AND post_category.categoryID = categories.id
AND post_category.postID = posts.id
GROUP BY posts.* /* list actual columns here */, author
Or did I miss something?
I think your subquery approach should work fine, but you still need the where clause in your subquery. How else would MySQL know which rows to retrieve? Try:
SELECT posts.*, users.authorName AS author,
(SELECT GROUP_CONCAT(categories.category)
FROM categories
INNER JOIN post_category ON post_category.categoryID = categories.id
WHERE post_category.postID = posts.postID) AS categories
FROM posts
INNER JOIN users ON users.id = posts.author