Sorry for the vague topic but I'm having a hard time explaining this. What I'm trying to do is fetching an ID representing a post which can be posted in different categories, and I want my post to belong to all three categories to match the criteria. The table looks like this
id category_id
1 3
1 4
1 8
What I wanna do is fetch an id that belongs to all 3 of these categories, but since they're on different rows I can't use
SELECT id FROM table WHERE category_id = '3' AND category_id = '4' AND category_id = '8';
This will of course return nothing at all, since no row matches that criteria. I've also tried with the WHERE IN clause
SELECT id from table WHERE category_id IN (3, 4, 8);
This returns any post in any of these categories, the post I want returned has to be in all three of these categories.
So the question becomes, is there any good way to look for an id that belongs to all three of these categories, or do I have to use the WHERE IN clause and see if I get 3 rows with the id 1, then I'll know that it occured three times, therefor belongs to all three categories, seems like a bad solution.
Thanks in advance. I appreciate the help. :)
You could use group_concat to get a comma separated string of your category id's and check if that contains all the categories you're filtering
SELECT id, GROUP_CONCAT(t2.category_id) as categories
FROM table AS t1
INNER JOIN table AS t2 ON t1.id = t2.id
WHERE
FIND_IN_SET('3', categories) IS NOT NULL AND
FIND_IN_SET('4', categories) IS NOT NULL AND
FIND_IN_SET('8', categories) IS NOT NULL
Update
SELECT t1.id, GROUP_CONCAT(t2.category_id) as `categories`
FROM `table` AS t1
INNER JOIN `table` AS t2 ON t1.id = t2.id
HAVING
FIND_IN_SET('3', `categories`) IS NOT NULL AND
FIND_IN_SET('4', `categories`) IS NOT NULL AND
FIND_IN_SET('8', `categories`) IS NOT NULL
The last query would not have worked, since it is a group function the value cannot be used in the WHERE clause but can be used in the HAVING clause.
I think you need to say GROUP BY id like this:
SELECT id FROM table WHERE category_id = '3' OR category_id = '4' OR category_id = '8'; GROUP BY id;
Hope that works for you.
i believe you need to establish the amount of categories a post appears in.
this can be done by applying COUNT and HAVING to the SQL query as follows:
SELECT id,
COUNT(category_id) AS categories
FROM `table`
GROUP BY id
HAVING categories > 3
and if you wish the number of categories in your site to change dynamically you can always have an inner SELECT statement like so:
SELECT id,
COUNT(category_id) AS categories
FROM `table`
GROUP BY id
HAVING categories > (
SELECT COUNT(category_id)
FROM `categories`
)
where categories is the table you store all your categories information
Related
I often have a situation with two tables in MySQL where I need one record for each foreign key. For example:
table post {id, ...}
table comment {id, post_id, ...}
SELECT * FROM comment GROUP BY post_id ORDER BY id ASC
-- Oldest comment for each post
or
table client {id, ...}
table payment {id, client_id, ...}
SELECT * FROM payment GROUP BY client_id ORDER BY id DESC
-- Most recent payment from each client
These queries often fail because the "SELECT list is not in GROUP BY clause" and contains nonaggregated columns.
Failed Solutions
I can usually work around this with a min()/max() but that creates a very slow query with mis-matched results (row with min(id) isn't equal to row with min(textfield))
SELECT min(id), min(textfield), ... FROM table GROUP BY fk_id
Adding all the columns to GROUP BY results in duplicate records (from the fk_id) which defeats the purpose of GROUP BY.
SELECT id, textfield, ... FROM table GROUP BY fk_id, id, textfield
Same idea as #GurV but using a join instead of a correlated subquery. The basic idea here is that the subquery finds, for each post which has comments, the oldest post and its corresponding id in the comments table. We then join back to comments again to restrict to the records we want.
SELECT t1.*
FROM comments t1
INNER JOIN
(
SELECT post_id, MIN(id) AS min_id
FROM comments
GROUP BY post_id
) t2
ON t1.post_id = t2.post_id AND
t1.id = t2.min_id
You can use a correlated query with aggregation to find out the earliest comment for each post:
select *
from comments c1
where id = (
select min(id)
from comments c2
where c1.post_id = c2.post_id
)
Compound index - comments(id, post_id) should be helpful.
If you are querying the whole table with many rows, then it will. This query is more useful and performant if you are querying for a small subset of posts. If you are querying the whole table, then #Tim's answer is better suited I think.
I currently use the following query which works perfectly:
SELECT * FROM `items` WHERE `id` IN
(SELECT `item_id` FROM `categories_items`
WHERE `category_id` IN (1, 2) GROUP BY `item_id`
HAVING COUNT(`item_id`) = 2);
It selects all the items that are in all the selected (checkboxes) categories.
The problem is that most items are in many categories and a few items are only in two categories and when I only check those two, I still get a list of hundreds of items, making it nearly impossible to find the items that are in few categories.
My first idea was to add an ORDER BY "number_of_total_categories_that_the_selected_item_is_in" ASC somewhere in the query, but since I even got help with the current one and there would probably be a lot of calculations/subqueries for it to work, I thought of an extra column in the items table that would hold the number of categories it's in!
Is it possible to add an effective ORDER BY clause to the query and if so, what would it look like?
Do you have any other ideas? Would "an inverted" foreign key solution work here? Not a chance, right? :p
If not, all I can think of is to manually update a category_count column in items whenever it's needed.
Edit: Table field that holds row count from another table looks interesting, but I have no idea whether it would work in MySQL.
You want to use join rather than in. The following query filters for items that only have the two categories you want. It also counts the total number of categories, which can be used for the order by:
SELECT i.*
FROM `items` i JOIN
(SELECT `item_id`, COUNT(*) as cnt
FROM `categories_items`
WHERE `category_id` IN (1, 2)
GROUP BY `item_id`
HAVING COUNT(DISTINCT CASE WHEN category_id IN (1, 2) THEN category_id END) = 2
) c
ON i.id = c.item_id
ORDER BY cnt ASC;
EDIT:
If you want to count all the categories, then get rid of the where. It is not really doing anything:
SELECT i.*
FROM `items` i JOIN
(SELECT `item_id`, COUNT(*) as cnt
FROM `categories_items`
GROUP BY `item_id`
HAVING COUNT(DISTINCT CASE WHEN category_id IN (1, 2) THEN category_id END) = 2
) c
ON i.id = c.item_id
ORDER BY cnt ASC;
I have an ordering system that can have multiple receipts related to one order. I recently ran into a query as follows that produced an undesirable result.
SELECT info FROM orders WHERE id IN (1, 2, 2) ORDER BY FIELD (id, 1, 2, 2);
Is there a way to return the row for order #2 twice? As of right now the query returns row one then row two as expected; however, in this particular instance returning row #2 twice is needed.
The tables are roughly as follows (I know it isnt totally valid MySQL, just for illustration):
CREATE TABLE orders (
id int(),
info VARCHAR(),
)
CREATE TABLE links (
orderid int(),
receiptid int()
)
CREATE TABLE receipts (
id int(),
otherinfo VARCHAR(),
)
If I'm understanding the situation correctly, you have two entries in the orders table
but orderId 2 is listed twice in the links table. If that is correct, then what you want is:
select o.info from orders o
inner join links l on o.id = l.orderid
If you need to return the row twice, then filtering in the where clause is not what you want. You can do this by filtering using a join:
SELECT o.info
FROM orders o join
(select 1 as id union all select 2 union all select 2
) ids
on o.id = ids.id
ORDER BY FIELD (o.id, 1, 2, 2);
Well, you coul make use of a UNION ALL
Something like
SELECT info FROM orders WHERE id IN (1, 2)
UNION ALL
SELECT info FROM orders WHERE id IN (2)
I appologise for the NOOB question, but I just can't get my head around JOINING results from two tables.
I have one table (tableA) that links entries with categories has the columns entry_id and cat_id
I have another table (tableB) that counts the number of times entry_id has been viewed against a timestamp
I want to get a list of entry_id's from tableB sorted by the number of times they have been viewed and filtered by cat_id from tableA.
I have the two SQL statements, but can't figure out how to put them together.
Statement 1 - Get entry_id's within a particular category:
SELECT entry_id FROM tableA WHERE cat_id = ""
Statement 2 - Get a sorted list of entry_id's from tableB:
SELECT entry_id FROM tableB GROUP BY entry_id ORDER BY COUNT(*) DESC
Thanks!
SELECT
tableb.entry_id,
FROM
tableb
INNER JOIN tablea
ON tableb.entry_id = tablea.entry_id
AND tablea.cat_id = ""
GROUP BY
tableb.entry_id
ORDER BY
COUNT(*) DESC
Try this:
SELECT
A.entry_id, Count(B.entry_id) Viewed
FROM TableA A
LEFT JOIN TableB B
ON A.entry_id = B.entry_id
WHERE A.Cat_Id = 5 -- your cat_id
GROUP BY A.entry_id
ORDER BY Count(B.entry_id) DESC
Problem is as follows. I have a product that can be in one of three categories (defined by category_id). Each category table has category_id field related to category_id in product table. So I have 3 cases. I'm checking If my product.category_id is in table one. If yes, I take some values. If not I check in tables that are left. What can I write In the ELSE section? Can anyone correct my query ?
CASE
WHEN IF EXISTS(SELECT * FROM table1 WHERE category_id='category_id') THEN SELECT type_id FROM table1 WHERE category_id='category_id';
WHEN IF EXISTS(SELECT * FROM table2 WHERE category_id='category_id') THEN SELECT value_id FROM table2 WHERE category_id='category_id';
WHEN IF EXISTS(SELECT * FROM table3 WHERE category_id='category_id') THEN SELECT group_id FROM table3 WHERE category_id='category_id';
ELSE "dont know what here";
END;
In the else you would put whatever you want as default value, for example null.
I think that it would be much more efficient to make three left joins instead of several subqueries for each product in the result, and use coalesce to get the first existing value. Example:
select coalesce(t1.type_id, t2.value_id, t3.group_id)
from product p
left join table1 t1 on t1.category_id = p.category_id
left join table2 t2 on t2.category_id = p.category_id
left join table3 t3 on t3.category_id = p.category_id
example
SELECT CompanyName,
Fax,
CASE WHEN IF(Fax='', 'No Fax', 'Got Fax')='No Fax' THEN NULL
ELSE IF(Fax='', 'No Fax', 'Got Fax')
END AS Note
FROM Customers;
You can possibly include this...
SELECT "Unknown type" FROM table1;
You do not need to use ELSE if there is nothing left to do.
or something like this
CASE
WHEN IF EXISTS(SELECT * FROM table1 WHERE category_id='category_id') THEN SELECT type_id FROM table1 WHERE category_id='category_id';
WHEN IF EXISTS(SELECT * FROM table2 WHERE category_id='category_id') THEN SELECT value_id FROM table2 WHERE category_id='category_id';
ELSE SELECT group_id FROM table3 WHERE category_id='category_id';
In addition to Guffa's answer here is another approach - assuming #category_id is
SET #category_id = 'some_category_id_value'
then
SELECT t1.type_id
WHERE category_id = #category_id
UNION ALL
SELECT t2.value_id
WHERE category_id = #category_id
UNION ALL
SELECT t3.group_id
WHERE category_id = #category_id
should return what you ask for (and performance is not bad either).
If you have certain category_id in more then one table you will get multiple records (you can get out of that by limiting the number of results to 1; you might need to make it the whole union a subquery and order it, but not sure, consult the docs)
However, your question looks like you have a problem with a design of your tables
why do you keep three category tables and not one?
what is the relationship between type_id, value_id and group_id and why does it make sense to select them as if they were the same thing (what is the meaning/semantics of each table/column)?
how do you guarantee that you don't have entries in multiple tables that correspond to one product (and implement other business rules that you might have)?
These questions could have valid answers, but you should know them :)