How can I add the following code example 1 to example 2 without messing up my query.
Example 1
INNER JOIN users ON users_articles.user_id = users.user_id
Example 2.
SELECT users.*
FROM users_articles
INNER JOIN articles_comments ON users_articles.id = articles_comments.article_id
INNER JOIN users ON articles_comments.user_id = users.user_id
WHERE users.active IS NULL
AND users.deletion = 0
ORDER BY articles_comments.date_created DESC
LIMIT 50
If I understand you correctly, you want to join table users twice, once for comments, and once for articles? In that case, you need to alias the table. I usually use single- or two-letter aliases for brevity even when I do not double tables, but it is not important.
SELECT ...
FROM users_articles UA
INNER JOIN articles_comments AC ON UA.id = AC.article_id
INNER JOIN users UC ON AC.user_id = UC.user_id
AND UC.active IS NULL
AND UC.deletion = 0
INNER JOIN users UA ON UA.user_id = users.user_id
AND UA.active IS NULL
AND UA.deletion = 0
ORDER BY AC.date_created DESC
LIMIT 50
BTW, Don't use SELECT *, it is almost always better to list specifically what you want.
Disclaimer: I might have misunderstood what you are trying to do; posting a bit of context to your code is usually a good idea. In this case, the table names threw me a bit (if it's what I think it is, I'd've just gone with users, articles and comments).
Related
I am stuck with some SQL query.
I have four tables. Which are connected:
user =>user_account=>acount_profile_entries=>profile_entries
From left to right they are one to many.
user_account has a user_id field as FK.
account_profile_field has user_account_id and profile_entry_id.
Profile_entries has a text field that I need to show for each user (account).
I need to write a query that will show me, all accounts for every user, and its profile entries.
I am sorry if this is confusing, I tried to make it simple
This is what I have done so far. I can show all accounts for every user and this is the point I am stuck with. Last two commented out Joins are not working properly. I believe I am close somewhat, I just need a push :)
SELECT
u.email AS Email,
u.id AS UserId,
ua.id AS UserAccountId,
ua.app_id AS Application
FROM users AS u
INNER JOIN user_accounts ua ON ua.user_id = u.id
-- INNER JOIN account_profile_entries ape ON ape.user_account_id = ua.id
-- INNER JOIN profile_entries as pe ON pe.id = ape.profile_entry_id
limit 10
Try this SQL Query with using LEFT JOIN
Description :- The MySQL LEFT JOIN joins two tables and fetches rows based on a condition, which is matching in both the tables and the unmatched rows will also be available from the table written before the JOIN clause.
SYNTAX
SELECT column_name(s)
FROM table1
LEFT JOIN table2 ON table1.column_name = table2.column_name;
SELECT u.*,
u.id AS UserId,
ua.id AS UserAccountId,
ua.app_id AS Application,pe.* FROM `users` u
LEFT JOIN user_accounts ua ON ua.user_id = u.id
LEFT JOIN account_profile_entries ape ON ape.user_account_id = ua.id
LEFT JOIN profile_entries as pe ON pe.id = ape.profile_entry_id LIMIT 10
My query looks like:
SELECT *
FROM users U
LEFT JOIN posts P ON P.userId = U.id AND P.userId IS NOT NULL;
Why the query also return result where userId is null ?
I know that for my needs I can use INNER JOIN to get only posts related to user but is so strange that LEFT JOIN support multiple conditions, but not work with NOT NULL conditions.
This is because "posts" does not contain the null-values and hence they can´t be filtered at that stage. The Null-values are only generated trough the join, when the server can´t find a corresponding row on the right table. So just put the not null in the where clause and it will work:
SELECT * FROM users U LEFT JOIN posts P ON P.userId = U.id WHERE userId IS NOT NULL;
(EDIT: You should use an inner join for productive work though, as it is the proper way and will give you much greater performance.)
You can also see all users who don´t have posts by inverting that:
SELECT * FROM users U LEFT JOIN posts P ON P.userId = U.id WHERE userId IS NULL;
You are outer joining the posts table. This means for every users record that has no match in posts you still get this record with all posts columns null.
So say you have a users record with userid = 5 and there is no posts record with id = 5.
ON P.userId = U.id AND P.userId IS NOT NULL
The two combined conditions are not met (there is no record with userid 5), so you get the users record with all posts columns set to null in your results.
Maybe you are simply looking for an inner join? All users records with their posts data?
This query:
SELECT *
FROM users U LEFT JOIN
posts P
ON P.userId = U.id AND P.userId IS NOT NULL;
Returns all rows in the users as well as all columns from posts, regardless of whether or not they match. This is true, regardless of whether the ON clause evaluates to TRUE or FALSE.
What you want is a WHERE. In addition, you should only select the columns from users:
SELECT u.*
FROM users U LEFT JOIN
posts P
ON P.userId = U.id
WHERE P.userId IS NOT NULL;
Note that you can also accomplish this using NOT IN or NOT EXISTS.
Because the LEFT JOIN must return every row from the left table by it's definition. The raw may be augmented with the data of the right table depending on the ON clause evaluation. So the following code must return a row.
select u.*, p.*
from (
select 1 as id
) u
left join (
-- no data at all
select 2 as id where 1=2
) p on 3 = 4 -- never is true
Try this
SELECT * FROM users U LEFT JOIN posts P ON P.userId = U.id
SELECT * FROM users U LEFT JOIN posts P ON P.userId = U.id where P.userId IS NOT NULL;
IS NOT NULL WITH JOINS
SELECT * FROM users
LEFT JOIN posts ON post.user_id = users.id
WHERE user_id IS NOT NULL;
I'm trying to select results from two different unrelated tables, showcase and questions to appear in a feed. They should be ordered by the common column hotness which is a float value.
SELECT s.id,s.date,s.title,s.views,s.image,s.hidpi,s.width,s.description,u.display_name,u.avatar
FROM showcase AS s
INNER JOIN users AS u ON s.user_id = u.id
UNION
SELECT q.id,q.date,q.title,q.views,q.text,u.display_name,u.avatar,0,0,0
FROM questions AS q
INNER JOIN users AS u ON q.user_id = u.id
ORDER BY hotness DESC
LIMIT 10
I've tried UNION, but I have no idea how I should be using it here and get this error unknown column hotness
You need to select the value in order for the ORDER BY to recognize it:
SELECT s.id,s.date,s.title,s.views,s.image,s.hidpi,s.width,s.description,u.display_name,u.avatar, s.hotness
FROM showcase AS s
INNER JOIN users AS u ON s.user_id = u.id
UNION ALL
SELECT q.id,q.date,q.title,q.views,q.text,u.display_name,u.avatar,0,0,0, q.hotness
FROM questions AS q
INNER JOIN users AS u ON q.user_id = u.id
ORDER BY hotness DESC;
Note that I also changed the UNION to UNION ALL. Unless you intend to remove duplicates, there is no reason to incur the extra processing for doing that.
You can try this query:
SELECT r.* FROM (
SELECT s.id,s.date,s.title,s.views,s.image,s.hidpi,s.width,s.description,u.display_name,u.avatar, s.hotness
FROM showcase AS s
INNER JOIN users AS u ON s.user_id = u.id
UNION
SELECT q.id,q.date,q.title,q.views,q.text,u.display_name,u.avatar,0,0,0, q.hotness
FROM questions AS q
INNER JOIN users AS u ON q.user_id = u.id
) as r
ORDER BY r.hotness DESC
LIMIT 10
You need to merge Union result in subquery to apply Order by on the result. I also added hotness in select clause, please check I take field from good table.
I have a query
select c.CommentId
,c.CommentText
, c.CommenterId
, c.CommentDate
, u.first_name
, u.last_name
, i.ImageName
, i.Format
from comment c
join users u
on c.CommenterId = u.user_id
join user_profile_image i
on u.user_id = i.UserId
where PostId = 76
order
by CommentDate desc
limit 10
This query returns empty results when i.ImageName field is empty in the table. I want to return the row if the ImageName field is emty. How should I do this?
JOIN defaults to INNER JOIN for MySQL - try changing
join user_profile_image i
to
LEFT join user_profile_image i
The accepted answer here has a good visual explanation: Difference in MySQL JOIN vs LEFT JOIN
To include the rows when the ImageName field is empty, use LEFT JOIN, like this:
SELECT c.CommentId,c.CommentText, c.CommenterId, c.CommentDate, u.first_name,
u.last_name,i.ImageName,i.Format
FROM comment c
INNER JOIN users u ON c.CommenterId=u.user_id
LEFT JOIN user_profile_image i ON u.user_id=i.UserId
WHERE PostId = 76
ORDER BY CommentDate DESC
LIMIT 10;
The issue isn't exactly that i.ImageName is empty. The issue is that there is no image associated with the user. The join doesn't find an image, and without a match, the user isn't returned.
The solution is to use left join. My inclination is to write the query entirely with left join:
select c.CommentId, c.CommentText, c.CommenterId, c.CommentDate,
u.first_name, u.last_name,
i.ImageName, i.Format
from comment c left join
users u
on c.CommenterId = u.user_id left join
user_profile_image i
on u.user_id = i.UserId
where PostId = 76
order by c.CommentDate desc
limit 10;
Note: This assumes that PostId is in the comment table, which seems reasonable given the table names.
i have a problem with sql query.
I need to get list of users. The problem is the left join i use.
this is my query
SELECT
u.*
FROM
users u LEFT JOIN game g on g.user_id = u.user_id
LEFT JOIN game_actions ga on ga.game_id = g.id
LEFT JOIN emails e on e.id = ga.email_id
WHERE
u.user_id = 0
AND u.is_contact_by_email = 1
AND email.type = 2
this query returns the same user more then once because of the join with other tables
i want this query to return each user just one time.
i'm using sql developer.
thanks in advanced.
Given that you're not using game, game_actions, or emails for any purpose (they're not filtering users, and you're not enriching the results with data from either of these tables), there's no need to join those tables at all:
SELECT
u.*
FROM users u
WHERE u.user_id = 0
AND u.is_contact_by_email = 1
You may group result by user_id and use aggregate functions to return other fields, for eaxmple -
SELECT u.user_id, GROUP_CONCAT(g.game_id) games FROM users u
LEFT JOIN game g
ON g.user_id = u.user_id
LEFT JOIN game_actions ga
ON ga.game_id = g.id
LEFT JOIN emails e
ON e.id = ga.email_id
WHERE
u.user_id = 0 AND u.is_contact_by_email = 1
GROUP BY
u.user_id
I suppose there are more games, actions, and emails for each user. How do you imagine the result table to look like? Because when you are selecting only from user table (SELECT u.*), then there is no point in joining others. If you need the other tables, use GROUP BY (u.user_id) to group rows by user.
Try to use keyword distinct with your query