I have two tables, users and contestants. I'm trying to select the max contestant ID that has a profile picture(which is on the user table)
Heres my terrible SQL:
SELECT u.thumbnail, u.id FROM users AS u
INNER JOIN
(
SELECT c.id, c.user_id FROM contestants AS c
WHERE u.id = c.users_id
AND c.id = (select max(c.id))
) WHERE u.thumbnail IS NOT NULL
The error currently is: #1248 - Every derived table must have its own alias.
This confuses me since Users has an alias of u, and contestants has an alias of c..
What am I doing wrong here? I'm guessing a lot so some help would be really appreciated!
Whenever you are performing a join operation, you are actually joining two table. The subquery you wrote here, for instance, is working as a separate table. Hence, you have to use an alias to this table. That's the reason behind your error message.
Your query:
SELECT u.thumbnail, u.id FROM users AS u
INNER JOIN
(
SELECT c.id, c.user_id FROM contestants AS c
WHERE u.id = c.users_id
AND c.id = (select max(c.id))
) WHERE u.thumbnail IS NOT NULL
It should contain an alias for the subquery:
SELECT c.id, c.user_id FROM contestants AS c
WHERE u.id = c.users_id
AND c.id = (select max(c.id))
Let's say, it's T.
So, your query now becomes:
SELECT u.thumbnail, u.id FROM users AS u
INNER JOIN
(
SELECT c.id, c.user_id FROM contestants AS c
WHERE u.id = c.users_id
AND c.id = (select max(c.id))
) AS T
WHERE u.thumbnail IS NOT NULL
But what you are trying to achieve, can actually be done in a neater way:
SELECT u.thumbnail, u.id, max(c.id),
FROM users as u
LEFT JOIN contestants as c
on u.id = c.user_id
WHERE u.thumbnail IS NOT NULL
Why make all the fuss when you have a better and neater approach at your disposal?
try this:
SELECT u.thumbnail, u.id
FROM users AS u
INNER JOIN
(
SELECT c.id, c.user_id FROM contestants AS c
WHERE u.id = c.users_id
AND c.id = (select max(c.id))
)A
WHERE u.thumbnail IS NOT NULL
i think this should be simple,
SELECT u.thumbnail, u.id
FROM users u
INNER JOIN contestants c
ON u.id = c.users_id
WHERE u.thumbnail IS NOT NULL
ORDER BY c.id DESC
LIMIT 1
This is very simple.
SELECT user.thumbnail, user.id
FROM users user
INNER JOIN contestants cont ON cont.id = cont.users_id
WHERE cont.thumbnail IS NOT NULL
ORDER BY user.id DESC
Related
I have a query which selects all comments for each post. Here is my query:
SELECT c.id, c.content, u.name, u.reputation, SUM(v.value) AS total_vote_comments
FROM comments c
INNER JOIN users u ON c.user_id = u.id
LEFT JOIN votes_comments v ON c.id = v.comment_id
WHERE c.post_id = :id;
Now I want to add ORDER BY c.id to that query. How?
An order by is irrelevant because this query returns one row:
SELECT c.id, c.content, u.name, u.reputation, SUM(v.value) AS total_vote_comments
FROM comments c INNER JOIN
users u
ON c.user_id = u.id LEFT JOIN
votes_comments v
ON c.id = v.comment_id
WHERE c.post_id = :id;
This is an aggregation query (because of the SUM()) without a GROUP BY. Such a query always returns one row, even when no rows match the join.
You probably want a GROUP BY. My best guess is:
SELECT c.id, c.content, u.name, u.reputation, SUM(v.value) AS total_vote_comments
FROM comments c INNER JOIN
users u
ON c.user_id = u.id LEFT JOIN
votes_comments v
ON c.id = v.comment_id
WHERE c.post_id = :id
GROUP BY c.id, c.content, u.name, u.reputation
ORDER BY c.id;
You can just add the ORDER BY clause at the end:
SELECT c.id, c.content, u.name, u.reputation, SUM(v.value) AS total_vote_comments
FROM comments c
INNER JOIN users u ON c.user_id = u.id
LEFT JOIN votes_comments v ON c.id = v.comment_id
WHERE c.post_id = :id
ORDER BY c.id;
By using this query you get only one row which id you put in where clause.
If you want to get one post ordered then you write order by in last.
If you want to get last's comment first so use desc with order by.
SELECT c.id, c.content, u.name, u.reputation, SUM(v.value) AS total_vote_commentFROM comments c INNER JOIN users u ON c.user_id = u.id LEFT JOIN votes_comments v ON c.id = v.comment_id WHERE c.post_id = :id order by id desc;
desc = descending order
asc = ascending order
I have total 6 tables in which different info has been saved
Now i need a result in which get count from 5 tables and select all info from main table but if record does not exist than it must be need to return 0 instead of no row found that's the problem here
I have tried below query but didn't get success
SELECT
u.*,
COUNT(DISTINCT c.id) as comments,
COUNT(DISTINCT d.id) as dislikes,
COUNT(DISTINCT l.id) as likes,
COUNT(DISTINCT s.id) as shares,
COUNT(DISTINCT t.id) as tags
FROM
job_details as u
JOIN job_comments as c ON u.id = c.job_id
JOIN job_dislike as d ON u.id = d.job_id
JOIN job_like as l ON u.id = l.job_id
JOIN job_share as s ON u.id = s.job_id
JOIN job_tags as t ON u.id = t.job_id
WHERE
u.id = c.job_id AND
u.id = d.job_id AND
u.id = l.job_id AND
u.id = s.job_id AND
u.id = t.job_id
GROUP BY
u.id
This query is executed, but didn't get exact result.
I don't quite understand why.
I was hoping somebody here could help me out?
Thanks!
You probably didn't get the exact result because some tables may be missing values.
Although you can solve this problem with a LEFT JOIN, the safer solution is to pre-aggregate the data:
SELECT u.*, c.comments, d.dislikes, l.likes, s.shares, t.tags
FROM job_details as u LEFT JOIN
(select c.job_id, count(*) as comments from job_comments group by c.job_id
) c
ON u.id = c.job_id LEFT JOIN
(select d.job_id, count(*) as dislikes from job_dislike d group by d.job_id
) d
ON u.id = d.job_id LEFT JOIN
(select l.job_id, count(*) as likes from job_like l group by l.job_id
) l
ON u.id = l.job_id LEFT JOIN
(select s.job_id, count(*) as shares from job_share s group by s.job_id
) s
ON u.id = s.job_id LEFT JOIN
(select t.job_id, count(*) as tags from job_tags t group by t.job_id
) t
ON u.id = t.job_id;
Why is this better? Consider an id that has 5 comments, likes, dislikes, shares and tags. The JOIN approach produces an intermediate result with 5*5*5*5*5 = 3,125 intermediate rows. Things can really get out of hand for popular ids.
Use LEFT JOIN instead of JOIN. and you don't need WHERE clause since you have joined those tables. And, use IFNULL function to return 0 for null values. You need to modify you query like this :
SELECT u.id,
IFNULL(COUNT(DISTINCT c.id),0) as comments,
IFNULL(COUNT(DISTINCT d.id),0) as dislikes,
IFNULL(COUNT(DISTINCT l.id),0) as likes,
IFNULL(COUNT(DISTINCT s.id),0) as shares,
IFNULL(COUNT(DISTINCT t.id),0) as tags
FROM job_details as u
LEFT JOIN job_comments as c ON u.id = c.job_id
LEFT JOIN job_dislike as d ON u.id = d.job_id
LEFT JOIN job_like as l ON u.id = l.job_id
LEFT JOIN job_share as s ON u.id = s.job_id
LEFT JOIN job_tags as t ON u.id = t.job_id
GROUP BY u.id
I have a query like that:
SELECT DISTINCT r.name AS role, r.id AS role_id, u.id AS user_id, concat( u.firstname, '', u.lastname ) AS name
FROM users u
INNER JOIN members m ON u.id = m.user_id
INNER JOIN member_roles mr ON m.id = mr.member_id
INNER JOIN roles r ON r.id = mr.role_id
WHERE m.project_id =11
My goal is to get unique u.id(user_id). Unfortunately, the result show two duplicate u.id from this multiple join. If I remove the last one join(INNER JOIN roles r ON r.id = mr.role_id), this query run correctly. But I didn't know why it was wrong if I add this line to my query. The last one join line is necessary to get the role's name, so I can ignore it. Can someone tell me where I did incorrectly in my query?
Use GROUP BY in your query to get rows with unique user_id.
Then the updated query will be look like the following.
SELECT DISTINCT r.name AS role, r.id AS role_id, u.id AS user_id,
concat( u.firstname, '', u.lastname ) AS name
FROM users u
INNER JOIN members m ON u.id = m.user_id
INNER JOIN member_roles mr ON m.id = mr.member_id
INNER JOIN roles r ON r.id = mr.role_id
WHERE m.project_id =11 GROUP BY user_id
You are missing the fact that users can have multiple roles on a project. Fortunately, MySQL has group_concat() which can bring the multiple values together:
SELECT u.id AS user_id, concat( u.firstname, '', u.lastname ) AS name,
GROUP_CONCAT(r.name) as roles,
GROUP_CONCAT(r.id) as role_ids
FROM users u INNER JOIN
members m
ON u.id = m.user_id INNER JOIN
member_roles mr ON m.id = mr.member_id INNER JOIN
roles r
ON r.id = mr.role_id
WHERE m.project_id = 11
GROUP BY u.id;
I have four tables are users(id,name), roles(id,name), members(project_id,user_id,id), and member_roles(member_id, role_id). So how can I select users.name and roles.name from this tables simultaneously(project_id is the condition to select exactly the names what we need)?. I can select users.name and roles.name separately but when I connect two queries, my code was failed. This is the query to get users.name
select users.name from users
inner join members on members.user_id = users.id
where project_id = 1
and here is the query to get roles.name
select roles.name from roles
inner join member_roles on member_roles.role_id = roles.id
join members on members.id = member_roles.member_id
where project_id = 1
You're almost there. Also, to avoid ambiguity, supply an Alias on column Name.
SELECT DISTINCT
u.Name AS member_name,
r.Name AS role_name
FROM users u
INNER JOIN members m
ON u.id = m.user_id
INNER JOIN member_roles mr
ON m.id = mr.member_id
INNER JOIN roles r
ON r.id = mr.role_id
WHERE m.project_id = 1
How do you connect 2 queries? How about something like this:
select u.name as user_name, r.name as role_name from users u join members m on m.user_id = u.id join member_roles mr on mr.member_id = m.id join roles r on r.id = mr.role_id where m.project_id = 1;
How would I do following in MySQL:
I have 3 tables:
user: id
communication: id, creation_date
user_communication: user_id, communication_id
Now I want to select all users that have had no communication since a given date.
Following is what I have now, but I'm stuck on how to get what I described above.
SELECT DISTINCT u.id FROM user u
LEFT JOIN user_communication uc ON uc.user_id = u.id
LEFT JOIN communication c ON c.id = uc.communication_id
WHERE c.creation_date < '2013-8-1';
The where condition is undoing the left join. The initial solution would be to move it to the on clause:
SELECT DISTINCT u.id FROM user u
LEFT JOIN user_communication uc ON uc.user_id = u.id
LEFT JOIN communication c ON c.id = uc.communication_id and c.creation_date < '2013-8-1';
But this doesn't do what you want. This retrieves all records. If you had a creation date field in the select clause, it would be NULL when there is record before that date.
For no communication since that date, you can do a "double" negative" query. Look for records that are since that date, and return the mismatches:
SELECT DISTINCT u.id
FROM user u LEFT JOIN
user_communication uc
ON uc.user_id = u.id LEFT JOIN
communication c
ON c.id = uc.communication_id and c.creation_date >= '2013-08-01'
WHERE c.creation_date is NULL;
EDIT:
I see. The problem is a little more subtle than my answer above. Each user has multiple communications, so none can be later. The following query tests this by grouping by u.id and then checking that there are no non-NULL values from the above join:
SELECT u.id
FROM user u LEFT JOIN
user_communication uc
ON uc.user_id = u.id LEFT JOIN
communication c
ON c.id = uc.communication_id and c.creation_date >= '2012-08-01'
group by u.id
having min(c.creation_date is null) = 1;
SELECT DISTINCT u.id FROM user u
LEFT JOIN user_communication uc ON uc.user_id = u.id
LEFT JOIN (SELECT * FROM communication WHERE creation_date < '2013-8-1') c
ON c.id = uc.communication_id
WHERE c.id is NULL;
After some research and help I have following query, which seems to work:
SELECT DISTINCT(u.id)
FROM user u
WHERE (SELECT coalesce(max(c.creation_date), '1900-01-01 00:00:00') last_creation_date
FROM user inneru
LEFT JOIN user_communication uc ON uc.user_id = inneru.id
LEFT JOIN communication c ON c.id = uc.communication_id
WHERE inneru.id = u.id) < '2012-08-01'
SQLFiddle: http://sqlfiddle.com/#!2/5dfad/10