i'm not an mysqlologist but i have to deal with the following problem:
given a following table:
+-------+-----------+-------------+------+
| id | articleID | img | main |
+-------+-----------+-------------+------+
| 48350 | 4325 | scr426872xa | 1 |
| 48351 | 4325 | scr426872ih | 2 |
| 48352 | 4325 | scr426872jk | 2 |
| 48353 | 4326 | scr426882vs | 1 |
| 48354 | 4326 | scr426882ss | 2 |
| 48355 | 4326 | scr426882nf | 2 |
+-------+-----------+-------------+------+
each set of images of one distinct articleID should have one image set as main=1 and an unspecified number of images with main value of 2
Due to processing issues it can happen that there is no main=1 set for an image and i need to find the articleID where images with main=2 exist, but not with main=1.
By explaining it backwards it is easier to fomulate what my thinking process for the query is. My idea was to create a result set (subquery) by querying the table for articleID where main is "1". Then use that result to check which distinct articleID of a query where main=2 is not in the results of aforementioned (sub-)query. Basically "substracting" all matching articleID lines.
This should give basically the leftover of all main=2 lines which have no line with the same articleID where main=1
SELECT DISTINCT articleID
FROM img_table WHERE main = 2
AND articleID
NOT IN (SELECT articleID FROM img_table WHERE main = 1 );
I get no result when I know for a fact that there are some. There is surely something I'm doing wrong. I hope my problem is explained in a way that not only me know what I want :)
Given your problem description, it looks like you're actually looking for NOT EXISTS to check for rows that don't have a matching row in the subselect. Note that you do have to add the article id to the where clause in the subselect:
SELECT DISTINCT articleID
FROM img_table t1
WHERE main = 2
AND NOT EXISTS
(SELECT articleID
FROM img_table t2
WHERE main = 1
AND t2.articleID = t1.articleID);
I think your current solution should work too, but maybe you didn't show all the data. For the data you specified, the query would indeed return 0 rows, because all articleIDs have at least one main=1 and a main=2 image.
One important thing to remember: the subquery must not return any NULL value, otherwise NOT IN won't work properly. So if articleID is nullable, make sure your subselect looks like this:
(SELECT articleID FROM img_table WHERE main = 1 and articleID IS NOT NULL)
I didn't find any issue in your query, Please add some data where article id having only main 2. Your query checking both article ID contains main 1,2. ie why you not getting any result.
Related
I have two columns displaying the same type of information but not necessarily the same data. Although some of the data overlaps each column may/may not contain information that will also include NULL values. Like so:
Company ID | Company Name | Company ID | Company Name
-----------+--------------+------------+-------------
1 | A | 1 | A
2 | B | NULL | NULL
NULL | NULL | 3 | C
I am trying to merge columns 1 and 2 to columns 3 and 4, respectively, so that I have two columns that look like this:
Company ID | Company Name
-----------+-------------
1 | A
2 | B
3 | C
Looking at similar stackoverflow questions, I have doubt this may be done easily. Is this possible? Please, let me know!
Anything helps.
As you don't seem to be around to answer questions for clarification right now, let's go ahead.
It seems, you do actually have the four columns in question in a single table - but than, there should be no duplicate column names. Once they are unique, the following should work:
UPDATE SomeTable
SET company_ID_1 = IFNULL(company_ID_1, company_ID_2)
, company_Name_1 = IFNULL(company_Name_1, company_Name_2)
WHERE
company_ID_1 IS NULL
OR
company_Name_1 IS NULL
;
If the presented is actually the output of a join, you could replace the same by:
SELECT
IFNULL(SomeTable1.company_ID, SomeTable2.company_ID) company_ID
, IFNULL(SomeTable1.company_Name, SomeTable2.company_Name) company_Name
FROM SomeTable1
LEFT JOIN SomeTable2
ON SomeTable1.company_ID = SomeTable2.company_ID
UNION ALL
SELECT
IFNULL(SomeTable1.company_ID, SomeTable2.company_ID) company_ID
, IFNULL(SomeTable1.company_Name, SomeTable2.company_Name) company_Name
FROM SomeTable1
RIGHT JOIN SomeTable2
ON SomeTable1.company_ID = SomeTable2.company_ID
WHERE SomeTable1.company_ID IS NULL
ORDER BY company_ID
;
See it in action: SQL Fiddle
Please comment, if and as this requires adjustment / further detail.
i faced a unique problem by accident
But before that i want to show you a table structure
td_category
|---------------------------------------------------------------------|
| category_id | category_title | category_slug | p_cid |
|---------------------------------------------------------------------|
| 1 | Shirts | 1-Shirts | 0 |
|---------------------------------------------------------------------|
| 2 | Jeans | 2-Jeans | 0 |
|---------------------------------------------------------------------|
Now,
category_id is INT and auto-increment value
category_title is VARCHAR
category_slug is VARCHAR
Now what i amdoing is that, by mistake i wrote a query
SELECT * FROM td_category WHERE category_id = '2-Jeans'
and instead of giving me any error it displayed the 2nd tuple
Isn't it supposed to throw an error??
please can anybody clarify?
mysql performs implicit conversion for int datatype due to which '2-Jeans' is treated as 2-0 (since Jeans is not an int type and is defaulted to 0 for compatibility as described in the docs here)
Hence the final query as the parser interprets is as below:
SELECT * FROM td_category WHERE category_id = 2;
The following query will take id as 2 which is your first character and display second record
SELECT * FROM td_category WHERE category_id = '2-Jeans'
Try this query which will return first record
SELECT * FROM td_category WHERE category_id = '1-Jeans'
2-jeans is treated as 2 so return second record and 1-jeans is treated as 1 so return first record.
Check Manual for auto casting in mysql.
so im making a file hub nothing huge or fancy just to store some files that may be shared by others for download. and it just occured to me in the way that i originally intended to count the amount of upvotes or downvotes the query could be server heavy.the query to get the files is something along the lines of
select*from files;
and in such i would recieve an array of my files that i could loop over and get specifics on each file now with the inclusion of voting a file that same foreach loop would include a further query that would get the count the amount votes a file would get (the file id in the where clause) like so
select*from votes where upvoted=true and file.id=?
and i was thinking of using pdo::rowCount to get my answer. now evey bone in my body just says this is bad very bad as imagine im getting 10,000 files i just ran 10,000 extra queries one on each file and i havent looked at the downvotes yet which i was think could go in a similar fasion. any optimization adviece here is a small rep of the structure of a few tables. the upvoted and downvoted columbs are of type bool or tinyint if you will
table: file table: user table: votes
+----+-------------+ +----+-------------+ +--------+--------+--------+--------+
| id |storedname | | id | username | |file_id | user_id| upvoted | downvoted
+----+-------------+ +----+-------------+ +--------+--------+--------+--------+
| 1 | 45tfvb.txt | | 1 | matthew | | 1 | 2 | 1 | 0
| 2 |jj7fnfddf.pdf| | 2 | mark | | 2 | 1 | 1 | 1
| .. | .. | | .. | .. | | .. | .. | .. | ..
there are two ways to do this. the better way to do this (aka faster) is to write separate queries and build into one variable in your programming language (like php, python.. etc.)
SELECT
d.id as doc_id,
COUNT(v.document_id) as num_upvotes
FROM votes v
JOIN document d on d.id = v.document_id
WHERE v.upvoted IS TRUE
GROUP BY doc_id
);
that will return your list of upvoted documents. you can do the same for your downvotes.
then after your select from document do a for loop to compare the votes with the document by ID and build into a dictionary or list.
The second way to do this which can take a lot longer at runtime if you have a bunch of records in the table (its less efficient, but easier to write) is to add subquery selects in your select statement like this...
SELECT
logical_name ,
document.id ,
file_type ,
physical_name ,
uploader_notes ,
views ,
downloads ,
user.name ,
category.name AS category_name,
(Select count(1) from votes where upvoted=true and document_id=document.id )as upvoted,
(select count(1) from votes where upvoted=false and document_id=document.id) as downvoted
FROM document
INNER JOIN category ON document.category_id = category.id
INNER JOIN user ON document.uploader_id = user.id
ORDER BY category.id
Two advices:
Avoid SELECT * especially if you're going to count. Replace it, with something like that:
SELECT COUNT(1) AS total WHERE upvoted=true AND file.id=?
Maybe you want to create a TRIGGER to keep update a counter in the file table.
I hope it will be helpfull to you.
I need a single query. Delete all rows from the table except the top N rows. The table has only one column. Like,
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
| etc... |
This column may contain repeated names as well.
Contains repeated names
Only one column.
If you can order your records by friends_name, and if there are no duplicates, you could use this:
DELETE FROM names
WHERE
friends_name NOT IN (
SELECT * FROM (
SELECT friends_name
FROM names
ORDER BY friends_name
LIMIT 10) s
)
Please see fiddle here.
Or you can use this:
DELETE FROM names ORDER BY friends_name DESC
LIMIT total_records-10
where total_records is (SELECT COUNT(*) FROM names), but you have to do this by code, you can't put a count in the LIMIT clause of your query.
If you don't have an id field, i suppose you use an alphabetic order.
MYSQL
DELETE FROM friends
WHERE friends_name
NOT IN (
SELECT * FROM (
SELECT friends_name
FROM friends
ORDER BY friends_name ASC
LIMIT 10) r
)
You delete all rows exept the 10 firsts (alphabetic order)
I just wanted to follow up on this relatively old question because the existing answers don't capture the requirement and/or are incorrect. The question states the names can be repeated, but only the top N must be preserved. Other answers will delete incorrect rows and/or incorrect number of them.
For example, if we have this table:
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
| Misbahu |
| Roshit |
| Misbahu |
| Rohan |
And we want to delete all but top 3 rows (N = 3), the expected result would be:
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
The DELETE statement from the currently selected answer will result in:
|friends_name|
==============
| Arunji |
| Misbahu |
| Misbahu |
| Misbahu |
See this sqlfiddle. The reason for this is that it first sorts names alphabetically, then takes top 3, then deletes all that don't equal that. But since they are sorted by name they may not be the top 3 we want, and there's no guarantee that we'll end up with only 3.
In the absence of unique indexes and other fields to determine what "top N" means, we go by the order returned by the database. We could be tempted to do something like this (substitute 99999 with however high number):
DELETE FROM names LIMIT 99999 OFFSET 3
But according to MySQL docs, while the DELETE supports the LIMIT clause, it does not support OFFSET. So, doing this in a single query, as requested, does not seem to be possible; we must perform the steps manually.
Solution 1 - temporary table to hold top 3
CREATE TEMPORARY TABLE temp_names LIKE names;
INSERT INTO temp_names SELECT * FROM names LIMIT 3;
DELETE FROM names;
INSERT INTO names SELECT * FROM temp_names;
Here's the sqlfiddle for reference.
Solution 2 - new table with rename
CREATE TABLE new_names LIKE names;
INSERT INTO new_names SELECT * FROM names LIMIT 3;
RENAME TABLE names TO old_names, new_names TO names;
DROP TABLE old_names;
Here's the sqlfiddle for this one.
In either case, we end up with top 3 rows in our original table:
|friends_name|
==============
| Arunji |
| Roshit |
| Misbahu |
Sorry for the confusing question, I will try to clarify.
I have an SQL database ( that I did not create ) that I would like to write a query for. I know very little about SQL, so it is hard for me to even know what to search for to see if this question has already been asked, so sorry if it has. It should be an easy solution for those in the know.
The query I need is for a search I would like to perform on an existing data management system. I want to return all the documents that a given user has NOT signed-off on, as indicated by rows in a signoffs_table. The data is stored similarly to as follows: (this is actually a simplification of the actual schema and hides several LEFT JOINS and columns)
signoffs_table:
| id | user_id | document_id | signers_list |
The naive solution I had was to do something like the following:
SELECT document_id from signoffs_table WHERE (user_id <> $BobsID) AND signers_list LIKE "%Bob%";
This works if ONLY Bob signs the document. The problem is that if Bob and Mary have signed the document then the table looks like this:
signoffs_table:
-----------------------------------------------
| id | user_id | document_id | signers_list |
-----------------------------------------------
| 1 | 10 | 100 | "Bob,Mary,Jim" |
| 2 | 20 | 100 | "Bob,Mary,Jim" |
-----------------------------------------------
(assume Bob's ID = 10 and mary's ID = 20).
and then when I do the query then I get back document_id 100 (in row #2) because there is a row that Bob should have signed, but did not.
Is what I am trying to do possible with the given database structure? I can provide more details if needed. I am not sure how much details are needed.
I guess this query is what you mean:
SELECT document_id FROM signoffs_table AS t1
WHERE signers_list LIKE "%Bob%"
AND NOT EXISTS (
SELECT 1 FROM signoffs_table AS t2
WHERE (t2.user_id = $BobsID) AND t2.document_id = t1.document_id )
I believe your design is incorrect. You have a many-to-many relationship between documents and signers. You should have a junction table, something like:
ID DocumentID SignerID