See, I've got this bulletin board. This query here works just fine.
SELECT bulletin.date,
bulletin.title,
bulletin.content,
bulletin.id,
bulletin.made_by,
users.ID,
users.name
FROM bulletin, users
WHERE bulletin.id = '12345'
AND bulletin.made_by = users.ID
LIMIT 1
Now, I'm running into troubles because when a user deletes his account there is no information about said person left, he is deleted from the users table. I know there may be other ways to deal with this but I can't fix that now.
So I ask: How can I make that query return bulletin.date, bulletin.title and bulletin.content where bulletin.id is equal to 12345 even though I've got "AND bulletin.made_by = users.ID"? You see, I need to have that for all the other posts where the user still exists.
Any suggestions?
Use a LEFT OUTER JOIN
SELECT b.date,
b.title,
b.content,
b.id,
b.made_by,
b.ID,
b.name
FROM bulletin b
LEFT OUTER JOIN users u on b.made_by = u.ID
WHERE b.id = '12345'
LIMIT 1
See this great explanation of joins
Currently you are using INNER JOIN, Use LEFT JOIN in order to solve your problem.
SELECT a.date,
a.title,
a.content,
a.id,
a.made_by,
a.ID,
a.name
FROM bulletin a
LEFT JOIN users c
ON a.made_by = c.ID
WHERE a.id = '12345'
LIMIT 1
LEFT JOIN basically fetches all the records from the left table whether it has a matching row in the second table or not.
Related
I want to get the total likes and total count of the every post in a single query with the help of joins.
I am using this query. but the result is wrong
SELECT blog.id, count(blog_comments.id) as likes , count(blog_likes.id) as comments
FROM blog LEFT JOIN
blog_comments
ON blog.id = blog_comments.blog_id LEFT JOIN
blog_likes
ON blog.id = blog_likes.blog_id
GROUP BY blog.id
Please check the image for table structure:
Your problem is that you are aggregating along two dimensions at the same time. The produces a Cartesian product -- a row with each like pairs with each comment, for a total of l * c rows.
The simplest way to fix this is to use the DISTINCT keyword:
SELECT b.id, count(DISTINCT bl.id) as likes , count(DISTINCT bc.id) as comments
FROM blog b LEFT JOIN
blog_comments bc
ON b.id = bc.blog_id LEFT JOIN
blog_likes
ON b.id = bl.blog_id
GROUP BY b.id;
If you have posts that have lots of likes and lots of comments, this is not recommended, because it creates a Cartesian product of the two.
There are several solutions for this, but I would recommend correlated subqueries:
select b.id,
(select count(*) from blog_likes bl where bl.blog_id = b.id) as likes,
(select count(*) from blog_comments bc where bc.blog_id = b.id) as comments
from blogs b;
This can take advantage of indexes on blog_likes(blog_id) and blog_comments(blog_id).
This is according to my table it will help you...
SELECT people.pe_name, COUNT(distinct orders.ord_id) AS num_orders, COUNT(items.item_id) AS num_items FROM people INNER JOIN orders ON orders.pe_id = people.pe_id INNER JOIN items ON items.ord_id = orders.ord_id GROUP BY people.pe_id;
I have a really simple table - follow - in which I store followers.
user | following
-----------------
1 | 2
The above means user 1 is following user 2.
I want to display all users on the home page and order them buy who has the most followers, and then return the rest of the users who have no followers. The below query is working as far as displaying the users, but I can't figure out how to retrieve the users who do not have any followers. I've tried RIGHT JOIN users u ON f.following=u.id but that gives me weird results.
This query returns user 2 who has a follower, but doesn't return users 1 and 3, who do not have followers.
Edit: this query is also checking to see if the user is following back, which is why I'm joining using the ID of 1 as a test.
SELECT
u.id
,u.username
,u.avatar
,COUNT(1) AS followers
,ul.*
,fo.*
FROM follow f
LEFT JOIN users u ON f.following=u.id
LEFT JOIN follow fo ON fo.following=u.id AND fo.user=1
LEFT JOIN users_likes ul ON ul.likes=u.id AND ul.user=1
GROUP BY f.following
ORDER BY COUNT(1) DESC
SQL Fiddle: http://sqlfiddle.com/#!2/98f65/1
The problem with your query in the question is that you are left-joining to the follow table. That means that all rows in the follow table are included regardless of their connection to another table. What you want is to show all users, so that is the table that should be on the outer end of the join.
I also think you're trying to do too many things at once here, which is why you're having trouble figuring it out. You want to know who has followers and who doesn't, who's following back, order them, consider the users_likes and so on. I recommend taking a step back and breaking them down into individual queries, and then building those into one result set as needed.
To get the users and number of followers, you can outer join the users table with the follow table like this:
SELECT u.id, u.username, u.avatar, (IFNULL(COUNT(f.following), 0)) AS numFollowers
FROM users u
LEFT JOIN follow f ON f.following = u.id
GROUP BY u.id
ORDER BY numfollowers DESC;
IFNULL is used to check the cases when there are no followers, and no link is made in the outer join so a null value appears.
If you want to work in the users_likes table, you should add it in as another left join. The problem this causes, is that it will return null values for all columns if there are no likes. (Example, if I left join the users_likes table here, I will see null for users 1 and 3 because nobody 'likes' them.) To make the result set a little more understandable, I recommend you don't collect all rows of the users_likes table. Perhaps this query would make more sense:
SELECT u.id, u.username, u.avatar, (IFNULL(COUNT(f.following), 0)) AS numFollowers, ul.user AS likedByUser, ul.created_at
FROM users u
LEFT JOIN follow f ON f.following = u.id
LEFT JOIN users_likes ul ON ul.likes = u.id
GROUP BY u.id
ORDER BY numfollowers DESC;
As far as whether or not a user is following back, I think this would change a bit, as the above only shows the number of followers, and doesn't produce a row for each follower.
Let me know if you have any more questions, here is an SQL Fiddle for the above. I will leave it up to you for handling the null values that occur right now.
You can use an outer join (left or right) from Users to your current query in any number of ways. An easy example that should get you started. This isn't a clean-up up solution, just a dmeo of a way that will work.
SELECT a.*
,b.*
FROM users a
LEFT JOIN (
SELECT
u.id
,u.username
,u.avatar
,COUNT(1) AS followers
FROM follow f
LEFT JOIN users u ON f.following=u.id
LEFT JOIN follow fo ON fo.following=u.id AND fo.user=1
LEFT JOIN users_likes ul ON ul.likes=u.id AND ul.user=1
GROUP BY f.following
) b
ON a.id = b.id
ORDER BY followers DESC
You can do this:
SELECT * FROM (
SELECT u.id, u.username, u.avatar, COUNT(f.user) as followers
FROM users AS u
LEFT JOIN follow AS f ON u.id = f.following
GROUP BY u.id
) AS subselect ORDER BY subselect.followers DESC
Ok... well I have to put the subquery in a JOIN clause since it selects more than one column and putting it in the SELECT clause does not allow that as it gives me an error of an operand.
Anywho, this is my query:
SELECT
c.id,
c.title,
c.description,
c.icon,
p.id as topic_id,
p.title AS topic_title,
p.date,
p.username
FROM forum_cat c
LEFT JOIN (
SELECT
ft.id,
ft.cat_id,
ft.title,
fp.date,
u.username
FROM forum_topic ft
JOIN forum_post fp ON fp.topic_id = ft.id
JOIN user u ON u.user_id = fp.author_id
WHERE ft.cat_id = c.id
ORDER BY fp.date DESC
LIMIT 1
) p ON p.cat_id = c.id
WHERE c.main_cat = ?
ORDER BY c.list_no
Now the important thing I need here... FOR EACH category, I want to show the latest post and topic title in each category.
However, this select statement is going INSIDE a foreach loop looping around the general categories which is found my main_cat.
So there are 5 main categories with 3-8 subcategories.. this is the subcategory query. BUT FOR EACH subcategory, I need to grab the latest post.. However, it only runs this SELECT query for each main category so it's only select THE LATEST post between all subcategories combined... I want to get the latest post of EACH subcategory, but I rather not run this query for each subcategory... since I want the page load to be fast.
BUT REMEMBER, some subcategories WILL NOT have a latest post since some of them may not even contain a topic yet! So hence the left join.
Does anyone know how to go about doing this?
AND BTW, there is an error it gives me (WHERE ft.cat_id = c.id) in the subquery because c.id is an unknown column. But I'm trying to reference it from the outer query so can someone help me on that issue as well?
Thank you!
All tables:
forum_cat (Subcategories)
-----------------------------------------------
ID, Title, Description, Icon, Main_cat, List_no
forum_topic (Topics in each subcategory)
--------------------------------------------
ID, Author_id, Cat_id, Title, Sticky, Locked
forum_post (Posts in each topic)
--------------------------------------------
ID, Topic_id, Author_id, Body, Date, Hidden'
The main categories are listed in a function. I didn't store them in the database since it was a waste of space since they never change. There are 7 main categories though.
It's hard to tell without seeing DDL of your tables, relevant sample data and desired output.
I could've got your requirements wrong, but try this:
SELECT *
FROM forum_cat c LEFT JOIN
(SELECT t.cat_id,
p.topic_id,
t.title,
p.id,
p.body,
MAX(p.`date`) AS `date`,
p.author_id,
u.username
FROM forum_post p INNER JOIN
forum_topic t ON t.id = p.topic_id INNER JOIN
`user` u ON u.user_id = p.author_id
GROUP BY t.cat_id) d ON d.cat_id = c.id
WHERE c.main_cat = 1
ORDER BY c.list_no
i have a MySQL SELECT query which fetches data from 6 tables using Mysql JOIN. here is the MySQL query i am using.
SELECT
u.id,u.password,
u.registerDate,
u.lastVisitDate,
u.lastVisitIp,
u.activationString,
u.active,
u.block,
u.gender,
u.contact_id,
c.name,
c.email,
c.pPhone,
c.sPhone,
c.area_id,
a.name as areaName,
a.city_id,
ct.name as cityName,
ct.state_id,
s.name as stateName,
s.country_id,
cn.name as countryName
FROM users u
LEFT JOIN contacts c ON (u.contact_id = c.id)
LEFT JOIN areas a ON (c.area_id = a.id)
LEFT JOIN cities ct ON (a.city_id = ct.id)
LEFT JOIN states s ON (ct.state_id = s.id)
LEFT JOIN countries cn ON (s.country_id = c.id)
although query works perfectly fine it sometimes returns duplicate results if it finds any duplicate values when using LEFT JOIN. for example in contacts table there exist two rows with area id '2' which results in returning another duplicated row. how do i make a query to select only the required result without any duplicate row. is there any different type of MySQL Join i should be using?
thank you
UPDATE :
here is the contacts table, the column area_id may have several duplicate values.
ANSWER :
there was an error in my condition in last LEFT JOIN where i have used (s.country_id = c.id) instead it should be (s.country_id = cn.id) after splitting the query and testing individually i got to track the error. thank you for your response. it works perfectly fine now.
Duplicating the rows like you mentioned seems to indicate a data problem.
If users is your most granular table this shouldn't happen.
I'd guess, then, that it's possible for a single user to have multiple entries in contacts
You could use DISTINCT as mentioned by #dxprog but I think that GROUP BY is more appropriate here. GROUP BY whichever datapoint could potentially be duplicated....
After all, if a user has corresponding contact records, which one are you intending to JOIN to?
You must specify this if you want to remove "duplicates" because, as far as the RDBMS is concerned, the two rows matching
LEFT JOIN contacts c ON (u.contact_id = c.id)
Are, in fact, distinct already
I think a DISTINCT may be what you're looking for:
SELECT DISTINCT
u.id,u.password,
u.registerDate,
u.lastVisitDate,
u.lastVisitIp,
u.activationString,
u.active,
u.block,
u.gender,
u.contact_id,
c.name,
c.email,
c.pPhone,
c.sPhone,
c.area_id,
a.name as areaName,
a.city_id,
ct.name as cityName,
ct.state_id,
s.name as stateName,
s.country_id,
cn.name as countryName
FROM users u
LEFT JOIN contacts c ON (u.contact_id = c.id)
LEFT JOIN areas a ON (c.area_id = a.id)
LEFT JOIN cities ct ON (a.city_id = ct.id)
LEFT JOIN states s ON (ct.state_id = s.id)
LEFT JOIN countries cn ON (s.country_id = c.id)
This should only return rows where the user ID is distinct, though you may not get all the joined data you'd hoped for.
I have three tables: stories, story_types, and comments
The following query retrieves all of the records in the stories table, gets their story_types, and the number of comments associated with each story:
SELECT s.id AS id,
s.story_date AS datetime,
s.story_content AS content,
t.story_type_label AS type_label,
t.story_type_slug AS type_slug,
COUNT(c.id) AS comment_count
FROM stories AS s
LEFT OUTER JOIN story_types AS t ON s.story_type_id = t.id
LEFT OUTER JOIN comments AS c ON s.id = c.story_id
GROUP BY s.id;
Now what I want to do is only retrieve a record from stories WHERE s.id = 1 (that's the primary key). I have tried the following, but it still returns all of the records:
SELECT s.id AS id,
s.story_date AS datetime,
s.story_content AS content,
t.story_type_label AS type_label,
t.story_type_slug AS type_slug,
COUNT(c.id) AS comment_count
FROM stories AS s
LEFT OUTER JOIN story_types AS t ON s.story_type_id = t.id
AND s.id = 1
LEFT OUTER JOIN comments AS c ON s.id = c.story_id
GROUP BY s.id;
I have also tried a WHERE clause at the end, which throws an error.
Can someone point out the correct syntax for a condition like this in this situation?
I'm using MySQL 5.1.47. Thanks.
I'm guessing you put the WHERE after the GROUP BY, which is illegal. See this reference on the SELECT syntax in MySQL.
Try this:
SELECT
s.id AS id,
s.story_date AS datetime,
s.story_content AS content,
t.story_type_label AS type_label,
t.story_type_slug AS type_slug,
COUNT(c.id) AS comment_count
FROM
stories AS s
LEFT JOIN story_types AS t ON s.story_type_id = t.id
LEFT JOIN comments AS c ON s.id = c.story_id
WHERE
s.id = 1
GROUP BY
s.id;
editor's note: I reformatted the code to highlight the query structure
Following up this comment on the accepted answer:
It is not intuitive to me that this WHERE would go in the second JOIN
This is just to outline how proper code formatting enhances understanding. Here is how I usually format SQL:
SELECT
s.id AS id,
s.story_date AS datetime,
s.story_content AS content,
t.story_type_label AS type_label,
t.story_type_slug AS type_slug,
COUNT(c.id) AS comment_count
FROM
stories AS s
LEFT JOIN story_types AS t ON t.id = s.story_type_id
LEFT OUTER JOIN comments AS c ON s.id = c.story_id
WHERE
s.id = 1
GROUP BY
s.id;
The WHERE is not on the second join. There is only one WHERE clause allowed in a SELECT statement, and it always is top level.
PS: Also note that in many database engines (apart from MySQL) it is illegal to use a GROUP BY clause and then selecting columns without aggregating them via functions like MIN(), MAX(), or COUNT(). IMHO this is bad style and a bad habit to get into.