The following query does what I want. It returns all the resuls in the users table and then if there is a match in the details tble, returns the relevant data
users
id|username
details
id|userid|firstname|lastname
$sql = "SELECT u.*, d.*
FROM `users` u
LEFT JOIN `details` d on
u.id = d.userid
ORDER BY $strorder";
However, when I try to join an additonal table where I want to do the same thing--return all the results of the users table and if there is a match in the third table, return the relevant data (total followers of this user)--it only returns one record.
3rd table
follow
id|followerid|followedid
$sql = "SELECT u.*, d.*, COUNT(f.id)
FROM `users` u
LEFT JOIN `details` d on
u.id = d.userid
LEFT JOIN `follow` f on
u.id = f.followedid
ORDER BY $strorder";
Can anyone see what I am doing wrong? Thanks in advance for any suggestions.
Many thanks.
Try to avoid * to select fields, it will be clearer to group your datas (even if mysql is quite permissive with groupings).
When you have an aggregate function (like COUNT, SUM), the other "non aggregated" requested fields should be in a GROUP BY clause.
Mysql don't force you to GROUP BY all the fields, but... I think it's quite a good habit to be "as ANSI as possible" (usefull when you use another DBMS)
SELECT u.id, u.username, d.firstname, d.lastname, count(*) as numberfollowers
FROM user u
LEFT JOIN details d on u.id = d.userid
LEFT JOIN follow f on u.id = f.followedid
GROUP BY u.id, u.username, d.firstname, d.lastname --or just GROUP BY u.id with Mysql
ORDER BY count(*) desc
COUNT being an aggregate function, when selected with other columns, requires you to group your results by those other columns in the select list.
You should rewrite your query with columns that you want to select from users and details and group by those columns.
Related
I am presently in a DBS course and I am working on an inline view:
SELECT userId, firstname, lastname, gender
FROM
(SELECT COUNT(dvdId) dvdId, u.userId
FROM userDVD
JOIN users u ON userDVD.userId = u.userId
GROUP BY userId) as T
WHERE gender = 'F';
When I run the Query it returns the error unknown column in field list. If I try to specify
u.firstname, u.lastname, u.gender
I return the same error. Any thoughts?
SELECT T.userId, T.firstname, T.lastname, T.gender
FROM (
SELECT users.userId, users.firstname, users.lastname, users.gender
FROM userDVD
JOIN users ON userDVD.userId = users.userId
WHERE gender = 'F' GROUP BY userId
) as T;
I worked through it and it turns out I didn't realize that because I had to alias my inline view that I needed to specify it in the original select statement. This concept was sadly not covered in my course. I appreciate those who gave helpful tips versus rude comments. So, Thank you Drew and Matt
Drew is right that you have a derived table T, but I'll attempt to add some details which might be useful to you as you are in a course.
you have a derived table T
an outer query (first select)
an inner query (second select in brackets)
the inner query uses a table alias of u for users.
the final columns of the inner query are dvdId which is a count of the dvdIds in userDvd table and userId
Because an outer query is constrained to use the final recordset of the inner query only those 2 columns are available to be selected and seeing they don't include firstname, lastname, or gender you are recieving the error.
If you want to keep your query the same but to use those columns you could join your derived table back to the user table and get the columns you desire. Such as this:
SELECT userId, firstname, lastname, gender
FROM
(SELECT COUNT(dvdId) dvdId, u.userId
FROM userDVD
JOIN users u ON userDVD.userId = u.userId
GROUP BY userId) as T
INNER JOIN users u2
ON t.user_id = u2.user_id
WHERE gender = 'F';
That technique to join back to the original table is great for when you have to optimize an aggregation that is on a large table or look for duplicates or something like that. However in your case you really just need a single query aggregating on all of the columns you want in the result set such as this:
SELECT
u.userId, u.firstname, u.lastname, u.gender, COUNT(d.dvdId) as DVDCount
FROM
userDVD d
JOIN users u
ON d.userId = u.userId
WHERE u.gender = 'F'
GROUP BY
u.userId, u.firstname, u.lastname, u.gender
;
I am trying to use column alias in the left join but I get sql error with uknown field.
select *, (select SenderId from messages where messageId = 5) as senderId from threads
Left join users where users.id = senderId
This query is quite simple but why is not it working or what is the best way to achieve this ?
I'll really appreciate any contribution.
Thanks
You can't use column aliases in ON or WHERE clauses, because column aliases are assigned to values in the result set, but ON and WHERE are used to create the result set. You need to use another JOIN.
SELECT t.*, m.senderId, u.*
FROM threads AS t
CROSS JOIN messages AS m
LEFT JOIN users AS u ON u.id = m.SenderId
WHERE m.messageId = 5
select t.*, mu.*
from threads as t
Left Join
(select SenderId,users. *
from messages, users
where messages.messageId
= users.id and messsageId = 5) as mu
You need to join threads and mu tables with some id
I have the following query:
SELECT u.user_id, u.username, u.email
hp.homepage_id
FROM table_u u
LEFT JOIN table_hp hp
ON (u.user_id = hp.user_id)
WHERE u.blocked = 'N'
AND u.email LIKE 'someemailaddress'
I am joining on the user_id column, for the given emailadress I know both properties are the same so I should get a result but still I don't get any result... so what is wrong with this query?
Put the '%' before and after in the LIKE clause - see below
SELECT u.user_id, u.username, u.email
hp.homepage_id
FROM table_u u
LEFT JOIN table_hp hp
ON (u.user_id = hp.user_id)
WHERE u.blocked = 'N'
AND u.email LIKE '%someemailaddress%'
Did you try the query without LEFT JOIN ??
SELECT u.user_id, u.username, u.email
FROM table_u u
WHERE u.blocked ='N'
AND u.email LIKE 'someemailaddress'
Does it return any result? Cause LEFT JOIN just append data for the existing data from your "base table" .. so if the LEFT JOIN didn't find any user_id in table_hp there should be some data returned, if there are some from the query above
The point was how some data was stored in the database... Seems that in an old (buggy) version of the application data was stored without decent trimming etc. The issue was the field was stored in with a whitespace...
Stupid... should/must had checked the actual data that was stored...
I've never been all that great with much more then regular select queries. I have a new project that has users, roles and assigned_roles (lookup table for users with roles).
I want to group_concat the roles.name so that my result shows me what roles each user has assigned.
I've tried several things:
select users.id, users.displayname,users.email, rolenames from `users`
left join `assigned_roles` on `assigned_roles`.`user_id` = `users`.`id`
left join (SELECT `id`, group_concat(`roles`.`name`) as `rolenames` FROM `roles`) as uroles ON `assigned_roles`.`role_id` = `uroles`.`id`
This gives me the grouped role names but shows me duplicate entries if a user has two roles, so the second row in the result shows the same user but no role names.
select users.id, users.displayname,users.email, rolenames from `users`
join `assigned_roles` on `assigned_roles`.`user_id` = `users`.`id`
join (SELECT `id`, group_concat(`roles`.`name`) as `rolenames` FROM `roles`) as uroles ON `assigned_roles`.`role_id` = `uroles`.`id`
Just regular joins shows me what I want but wont lists users who do not have any assigned.roles, so its not complete.
I'll keep plugging away but I thought stack could help, hopefully I'll learn a bit more about joins today.
Thank you.
For GROUP CONCAT to work in this scenario, you'll need a GROUP BY to get the group info per user, something like;
SELECT u.id, u.displayname, u.email, GROUP_CONCAT(r.name) rolenames
FROM users u
LEFT JOIN assigned_roles ar ON ar.user_id = u.id
LEFT JOIN roles r ON r.id = ar.role_id
GROUP BY u.id, u.displayname, u.email
I am trying to count users that are NOT referenced in another table... Right now, I have something along the lines of this:
SELECT COUNT(DISTINCT u.id) FROM users u INNER JOIN orders o ON o.assigned!=u.id;
However, it's returning an invalid value. Any suggestions?
Thank you!
I would suggest using a LEFT JOIN between the two tables and filter the rows without a matching id in the orders table:
select count(u.id)
from users u
left join orders o
on o.assigned = u.id
where o.assigned is null
See SQL Fiddle with Demo
Use a left join and count the rows with no match:
SELECT COUNT(*)
FROM users u
LEFT JOIN orders o
ON o.assigned = u.id
WHERE o.assigned IS NULL
An alternative is to use a NOT IN check:
SELECT COUNT(*)
FROM users
WHERE id NOT IN (SELECT distinct(assigned) FROM orders)
However, in my experience the left join performs better (assuming appropriate indexes).
Simply use this query, assuming that the id is unique in users table:
select count(*) From Users as u where u.id not in (select assigned from orders)
an inner join explicitly looks for rows that match so that isn't the way to go if you are looking for non matched records
assuming that ORDERS.ASSIGNED is matched with USER.ID an outer join could return values from both and show when there aren't matches like so
select
u.id,
o.*
from users u
full outer join orders o
on o.assigned = u.id;
if you only want to know which USER.ID don't have an ORDERS record you could also INTERSECT or use NOT IN () eg
select u.id from users u where id not in (select o.assigned from orders.o);
SELECT COUNT(1) FROM users u
WHERE NOT EXISTS (SELECT * FROM orders o WHERE o.assigned=u.id);
Are you wanting a straight count (like you mentioned), or do you need values returned? This will give you the count; if you want other values, you should take one of the other approaches listed above.