I have a table of Users: id, type, name
and a table of Articles: id, writer_id, status
where articles.writer_id = users.id.
I'd like to display a table of each User's name WHERE type = 'writer' along with how many Articles are associated with them that have status = 'assigned'.
So far I have:
SELECT u.name, COUNT(a.id) as count
FROM users u LEFT OUTER JOIN articles a
ON a.writer_id = u.id
WHERE u.type = 'writer' AND a.status = 'assigned'
GROUP BY u.name
Problem is, this doesn't display writers with 0 'assigned'-status articles associated with them. I'm pretty sure I need a subquery but I'm not sure what to do. Thanks in advance!
Since you are using a LEFT JOIN, move the a.status = 'assigned' predicate from the WHERE clause to the JOIN clause.
SELECT u.name, COUNT(a.id) as count
FROM users u LEFT OUTER JOIN articles a
ON a.writer_id = u.id
AND a.status = 'assigned'
WHERE u.type = 'writer'
GROUP BY u.name
Explanation: For those users that do not have a article a.status will be NULL, Leaving the predicate in the WHERE defeats the purpose of a LEFT join, since NULL = 'assigned' will evaluate to false.
Related
This query is taking forever to finish in MySql 8, doing some research i found out that the "EXISTS" in this code can be extremely slow in some queries.
When i remove the "OR EXISTS" sub-query part, it runs in less than a second.
So i need to substitute the "OR EXISTS" in this query so i can get all the users i need:
SELECT u.name,
u.email,
u.cpf,
u.register,
r.name AS role_name,
s.name AS sector_name,
b.name AS branch_name,
u.status
FROM users u
INNER JOIN roles r ON r.id = u.role_id
INNER JOIN sectors s ON s.id = u.sector_id
INNER JOIN branches b ON b.id = u.branch_id
WHERE u.status = 2 OR EXISTS (
SELECT *
FROM user_recovery ur
WHERE ur.user_id = u.id
AND ur.status_recovery = 1
)
Is there a way to do it without the "OR EXISTS"?
Or can enforce a full scan
try
you can't get rid of the eXISTS clause because it increases the number of returned rows.
Add a INDEX on user status and user_recovery userid,status_recovery and on the on Clause columns.
SELECT u.name,
u.email,
u.cpf,
u.register,
r.name AS role_name,
s.name AS sector_name,
b.name AS branch_name,
u.status
FROM users u
INNER JOIN roles r ON r.id = u.role_id
INNER JOIN sectors s ON s.id = u.sector_id
INNER JOIN branches b ON b.id = u.branch_id
WHERE u.status = 2
UNION
SELECT u.name,
u.email,
u.cpf,
u.register,
r.name AS role_name,
s.name AS sector_name,
b.name AS branch_name,
u.status
FROM users u
INNER JOIN roles r ON r.id = u.role_id
INNER JOIN sectors s ON s.id = u.sector_id
INNER JOIN branches b ON b.id = u.branch_id
WHERE EXISTS (
SELECT 1
FROM user_recovery ur
WHERE ur.user_id = u.id
AND ur.status_recovery = 1
)
"I'll see your UNION; and raise you a derived table."
SELECT u.name,
u.email,
u.cpf,
u.register,
r.name AS role_name,
s.name AS sector_name,
b.name AS branch_name,
u.status
FROM ( SELECT id
FROM users
WHERE status = 2
UNION DISTINCT -- or UNION ALL; see below
SELECT user_id
FROM user_recovery
WHERE status_recovery = 1 -- see new index
) AS u1
JOIN users AS u USING(id) -- self-join to pick up other columns
JOIN roles r ON r.id = u.role_id
JOIN sectors s ON s.id = u.sector_id
JOIN branches b ON b.id = u.branch_id;
Indexes:
user_recovery: INDEX(status_recovery, user_id) -- in this order
users: INDEX(status, id) -- in this order
(I assume `id` is the PRIMARY KEY in each table)
The general rule here is... When you have a bunch of JOINs, but a single table that controls which rows, but that is messy or slow (eg UNION in this case, GROUP BY or LIMIT in other cases),
Optimize finding the ids (user.id aka user_id) is the optimal way.
Then JOIN back to the original table (if needed), plus the other tables.
In doing all that, it became apparent that a new index for user_recovery might be beneficial.
(If UNION ALL won't produce any dups, switch to it for a little more speed.)
I want to optimize this query becouse it takes to much time to return records
SELECT
u.*,
s.legal_name AS structure_name,
ui.id AS userinfo_id,
ui.structure_id AS structure_id,
ui.lrn_user,
ui.gender,
ui.fiscal_code,
ui.prov,
ui.phone,
ui.school_name,
ui.school_codice_meccanografico,
us.status, us.date AS status_date,
CONCAT(u.lastname,' ',u.firstname) AS fullname,
CONCAT(u.firstname,' ',u.lastname) AS display_name,
uu.username AS created_by_name,
g.group_names,
IF(u.website_id = 0,'Sito Web principale', w.name) AS website_name
FROM fcf_users AS u
LEFT JOIN (
SELECT
gu.user_id,
GROUP_CONCAT(gg.name SEPARATOR ', ') AS group_names
FROM fcf_user_user_groups gu
JOIN fcf_user_groups gg ON gg.id = gu.group_id
GROUP BY user_id
) g ON g.user_id = u.id
LEFT JOIN fcf_users_userinfo AS ui ON ui.user_id = u.id
LEFT JOIN fcf_users_user_statuses AS us ON us.user_id = u.id
LEFT JOIN fcf_structures_structures AS s ON s.id = ui.structure_id
LEFT JOIN fcf_users AS uu ON uu.id = u.created_by
LEFT JOIN fcf_websites AS w ON w.id = u.website_id
WHERE
u.id IN (SELECT user_id FROM fcf_user_user_groups WHERE group_id = '8')
AND u.id IN (SELECT user_id FROM fcf_user_user_groups WHERE group_id = '8')
AND ui.lrn_user = '0'
ORDER BY fullname ASC
LIMIT 0,25
If anyone can help, thanks
Turn it inside-out. That is, first use a 'derived' table to locate 25 users you want. Then gather the rest of the info.
What you have gathers all the info (including all the JOIN work) for all the users, then sorts and peels off 25.
It will be something like:
SELECT -- lots of stuff
FROM ( SELECT u.id,
CONCAT(u.lastname,' ',u.firstname) AS fullname
FROM fcf_users AS u
JOIN fcf_user_user_groups AS ug ON ...
JOIN fcf_users_userinfo AS ui ON ui.user_id = u.id
WHERE ug.group_id = '8'
AND ui.lrn_user = '0'
ORDER BY u.lastname, u.firstname -- now sargeable
LIMIT 25
) AS u25
JOIN .... -- whatever tables are needed to get the rest of the columns
ORDER BY u25.fullname -- yes, again, but now using the CONCAT
-- no limit here
Also:
u: INDEX(lastname, firstname, id)
user_user_group is a "many-t0=many mapping" table? If so, follow the indexing advice here: http://mysql.rjweb.org/doc.php/index_cookbook_mysql#many_to_many_mapping_table
Ditto for any other many:many tables.
Note how I put into the derived table only the tables needed to achieve the LIMIT.
I have a table of ratings for comments, when I fetch comments, I also fetch the ratings and I also want to be able to display which comments the logged user has already voted on. This is what I am doing now
SELECT
c.id,
c.text,
c.datetime,
c.author,
u.email AS author_name,
SUM(cr.vote) AS rating,
cr2.vote AS voted
FROM comments c
LEFT JOIN users u ON u.id = c.author
LEFT JOIN comments_ratings cr ON c.id = cr.comment
LEFT JOIN comments_ratings cr2 ON c.id = cr2.comment AND cr2.user = :logged_user_id
GROUP BY c.id ORDER BY c.id DESC
But I don't like how I'm performing a second join on the same table. I know it is perfectly valid but if I could get the information I want from the first join, which is there anyway, why perform a second one?
Is it possible to figure out if a row with column user equal to :logged_user_id exists on table comments_ratings cr before executing the aggregate function(s)?
P.S.: If someone could come up with a better title, people can find in future, I'd also appreciate that.
You can do what you want with conditional aggregation:
SELECT c.id, c.text, c.datetime, c.author, u.email AS author_name,
SUM(cr.vote) AS rating,
MAX(cr.user = :logged_user_id) as voted
FROM comments c LEFT JOIN
users u
ON u.id = c.author LEFT JOIN
comments_ratings cr
ON c.id = cr.comment
GROUP BY c.id
ORDER BY c.id DESC;
I have 2 tables.
users(id,username) and links(id,usernameORid).
Example of rows: users{ [1,test] , [2,stack] } and links{ [1,overflow] , [2, 1] }
So, table links may contain username or id from table users. As you can see in the example,
usernameORid from links may not contain the id or username from users.
I hope you understood my example.
Now, i have this query:
SELECT l.usernameORid, u.username, u.id
FROM links l
LEFT JOIN users u
ON l.usernameORid= u.id
LEFT JOIN user_roles ur
ON ur.userID = u.id
WHERE ur.roleID < 4
group by u.id
But this query does not return rows from links if usernameORid is not an actual username or id from users.
In the previous example, will not return row [1,overflow]. I want that row too.
How can i achieve that?
EDIT: The problem is partialy related to
LEFT JOIN user_roles ur
ON ur.userID = u.id
WHERE ur.roleID < 4
but still, how can i achieve that?
user_roles ( id,userID,roleID)
Change your final WHERE condition to:
WHERE ur.roleID < 4 OR u.id IS NULL
This will allow it to return rows that didn't have a match in users. Normally a LEFT JOIN does that by itself, but since you're doing an additional join on that table, the WHERE clause is filtering those non-matching rows out because they don't have a roleID.
You can use an OR statement in your join between links and users. This will allow you to pick up users records where the link.usernameORid is equal to either the users.id or the users.username
SELECT l.usernameORid,
u.username,
u.id
FROM links l
LEFT JOIN users u ON
l.usernameORid = u.id OR
lusernameORid = u.username
LEFT JOIN user_roles ur
ON ur.userID = u.id
WHERE ur.roleID < 4
GROUP BY u.id
This will still cause records to drop if the found users->user_roles.roleID is less than 4. If you wanted to have link records maintained regardless of whether of a user was found by username or ID then you would need to subquery the users and user_roles table joins and apply your WHERE statement there instead. This query is below:
SELECT
l.usernameORid,
u.username,
u.id
FROM links l
LEFT JOIN
(
SELECT
users.username,
users.idusers
FROM
users
LEFT JOIN user_roles ON
user_roles.userID = users.id
WHERE
user_roles.roleID < 4
) u ON
l.usernameORid= u.id OR
l.usernameORid = u.username
group by u.id
Furthermore, if you wish the 2nd or 3rd column of your return to hold the value that is in l.usernameORid when the users table lacks a match... if your users.id is always numeric you could do some trickery with a CASE statement:
SELECT
l.usernameORid,
Coalesce(u.username, CASE WHEN .lusernameORid REGEXP '^[0-9]+$' THEN NULL ELSE l.usernameORid END) as username,
Coalesce(u.username, CASE WHEN .lusernameORid REGEXP '^[0-9]+$' THEN l.usernameORid ELSE NULL END) as userid
FROM links l
LEFT JOIN
(
SELECT
users.username,
users.idusers
FROM
users
LEFT JOIN user_roles ON
user_roles.userID = users.id
WHERE
user_roles.roleID < 4
) u ON
l.usernameORid= u.id OR
l.usernameORid = u.username
group by u.id
Keep in mind though, that if the users table doesn't have a match for the links.usernameORid then only the username OR the id could be determined, so you will have a NULL in one of the two fields.
I have 3 MySQL tables namely chat_comments, chat_friends and user_details and I want to display a friend list.
My tables:
chat_comments(comment_id,comment,user_id,user_id2,date_added)
chat_friends(user_id,user_id2,approved)
user_details(user_id, mainimage_id, fullname)
To do this, I need a query that will return the needed fields (u.mainimage_id, u.fullname, b.comment, b.user_id) so I can loop through the list to display a table.
SQL so far (help from #Andriy M):
SELECT
cc.comment,
cc.date_added,
u.fullname,
u.mainimage_id
FROM
user_details u
LEFT JOIN
chat_comments cc
INNER JOIN (
SELECT
user_id,
MAX(comment_id) AS maxcomment
FROM chat_comments WHERE user_id=2020 OR user_id2=2020
GROUP BY user_id
) a ON a.user_id = cc.user_id
AND a.maxcomment = cc.comment_id
ON a.user_id = u.user_id
WHERE u.user_id IN (
SELECT user_id2
FROM chat_friends
WHERE user_id = 2020
AND approved = 1
)
The above query returns the last comment made by the logged-in user's friends in conversation not the last comment between the logged-in user and his/her friend regardless of who made it.
I would like it to return the last comment between the logged-in user and their friend individually regardless of who made it. In the chat_messages table, user_id is the sender and user_id2 is the receiver. Hope it makes sense?
Like #imm said in a comment, you need to use an outer join. In case of a left join, the user_details table should become the left side of the join, the right side being the result of your inner join of chat_comments with your a derived table. You'll also need to remove the user_id IN (…) condition from inside the a subselect and re-apply it to the user_details table. Here:
SELECT
cc.comment,
cc.date_added,
u.fullname,
u.mainimage_id
FROM
user_details u
LEFT JOIN
chat_comments cc
INNER JOIN (
SELECT
user_id,
MAX(comment_id) AS maxcomment
FROM chat_comments
GROUP BY user_id
) a ON a.user_id = cc.user_id
AND a.maxcomment = cc.comment_id
ON a.user_id = u.user_id
WHERE u.user_id IN (
SELECT user_id2
FROM chat_friends
WHERE user_id = 2020
AND approved = 1
)
;
Alternatively, you could use a right join. In this case you would just need to move the user_id IN (…) condition, similarly to the LEFT JOIN solution above, and replace the second INNER JOIN with RIGHT JOIN:
SELECT
cc.comment, cc.date_added, u.fullname, u.mainimage_id
FROM
(
SELECT user_id, MAX(comment_id) AS maxcomment
FROM chat_comments
GROUP BY user_id
) a
INNER JOIN
chat_comments cc ON
a.user_id = cc.user_id AND
a.maxcomment = cc.comment_id
RIGHT JOIN
user_details u ON
a.user_id = u.user_id
WHERE u.user_id IN (select user_id2 from chat_friends where user_id=2020 AND approved=1)