I have m:n relationship between users and tags. One user can have m tags, and one tag can belong to n users. Tables look something like this:
USER:
ID
USER_NAME
USER_HAS_TAG:
USER_ID
TAG_ID
TAG:
ID
TAG_NAME
Let's say that I need to select all users, who have tags "apple", "orange" AND "banana". What would be the most effective way to accomplish this using SQL (MySQL DB)?
SELECT u.*
FROM (
SELECT user_id
FROM tag t
JOIN user_has_tag uht
ON uht.tag_id = t.id
WHERE tag_name IN ('apple', 'orange', 'banana')
GROUP BY
user_id
HAVING COUNT(*) = 3
) q
JOIN user u
ON u.id = q.user_id
By removing HAVING COUNT(*), you get OR instead of AND (though it will not be the most efficient way)
By replacing 3 with 2, you get users that have exactly two of three tags defined.
By replacing = 3 with >= 2, you get users that have at least two of three tags defined.
In addition to the other good answers, it's also possible to check the condition in a WHERE clause:
select *
from user u
where 3 = (
select count(distinct t.id)
from user_has_tag uht
inner join tag t on t.id = uht.tag_id
where t.name in ('apple', 'orange', 'banana')
and uht.user_id = u.userid
)
The count(distinct ...) makes sure a tag is counted only once, even if the user has multiple 'banana' tags.
By the way, the site fruitoverflow.com is not yet registered :)
You can do it all with joins...
select u.*
from user u
inner join user_has_tag ut1 on u.id = ut1.user_id
inner join tag t1 on ut1.tag_id = t1.id and t1.tag_name = 'apple'
inner join user_has_tag ut2 on u.id = ut2.user_id
inner join tag t2 on ut2.tag_id = t2.id and t2.tag_name = 'orange'
inner join user_has_tag ut3 on u.id = ut3.user_id
inner join tag t3 on ut3.tag_id = t3.id and t3.tag_name = 'banana'
SELECT *
FROM USER u
INNER JOIN USER_HAS_TAG uht
ON u.id = uht.user_id
INNER JOIN TAG t
ON uht.TAG_ID = t.ID
WHERE t.TAG_NAME IN ('apple','orange','banana')
Related
I have a table of posts, post_likes, and I need a query that will give me both totals for likes for posts, and also a given specific user's likes for those posts. This means I need a good way of giving MY_USER_ID as input data.
Here's a query that works
create view post_view as
select post.id,
coalesce(sum(post_like.score),0) as score,
(
select score
from post_like
where post.id = post_like.post_id
and post_like.fedi_user_id = MY_USER_ID
) as my_vote,
from post
left join post_like on post.id = post_like.post_id
group by post.id;
BUT my ORM (rust diesel) doesn't allow me to set or query for that necessary MY_USER_ID field, since it's a subquery.
I'd really love to be able to do something like:
select *
from post_view
where my_user_id = 'X';
Expose my_user_id on select clause of view
-- get all of the user's score on all post, regardless of the user liking the post or not
create view post_view as
select
u.id as my_user_id,
p.id as post_id,
sum(pl.score) over (partition by p.id) as score,
coalesce(pl.score, 0) as my_vote -- each u.id's vote
from user u
cross join post p
left join post_like pl on u.id = pl.fedi_user_id and p.id = pl.post_id;
select * from post_view where my_user_id = 'X';
UPDATE
This can obtain post's scores even when no user is given
create view post_view as
with all_post as
(
select
p.id as post_id,
sum(pl.score) as score
from post p
left join post_like pl on p.id = pl.post_id
group by p.id
)
select
u.id as my_user_id,
ap.post_id,
ap.score,
coalesce(pl.score, 0) as my_vote
from user u
cross join all_post ap
left join post_like pl on u.id = pl.fedi_user_id and ap.post_id = pl.post_id
union all
select
'' as my_user_id,
ap.post_id,
ap.score,
0 as my_vote
from all_post ap
;
select * from post_view where my_user_id = 'X';
When no user is passed, select the query denoted by my_user_id of ''
select * from post_view where my_user_id = '';
you can move that condition in on clause
create view post_view as
select post.id,
coalesce(sum(post_like.score),0) as score
from post
left join post_like
on post.id = post_like.post_id and post_like.fedi_user_id = MY_USER_ID
group by post.id;
You can move the logic to the select using conditional aggregation:
select p.id,
coalesce(sum(pl.score), 0) as score,
sum( (pl.fedi_user_id = MY_USER_ID)::int ) as my_vote
from post p left join
post_like pl
on p.id = pl.post_id
group by p.id;
I have a simple database schema composed of 3 tables
User
id
name
matricule
Document
id
serial
user_id(owner of document : foreign key to User(id))
User_Document (join table)
user_id
document_id
I want all document serial from both user sources : owner of document and join table . The query is filtered by a list of users matricule
I managed to achieve the desired goal with union query :
select d.serial from Document d
INNER JOIN users u ON u.id = d.user_id
where u.matricule IN ('1234')
UNION
select d.serial from Document d
inner join User_Document ud on d.id = ud.document_id
inner join users u on ud.user_id = u.id
where u.matricule IN ('1234')
How to arrive to the same result with only a join query ? I need as well skip document with no serial ( this case is possible)
Thank you very much
Try this:
SELECT d.serial
FROM Document d
LEFT JOIN User_Document ud
ON d.id = ud.document_id
LEFT JOIN users u
on ud.user_id = u.id
LEFT JOIN users u2
ON d.user_id = u2.id
WHERE d.serial IS NOT NULL
AND
(
ISNULL(u.matricule,'') IN ('1234')
OR ISNULL(u2.matricule,'') IN ('1234')
)
Why do you want to remove the joins? Probably the most efficient query would use exists:
select d.*
from documents d
where d.serial is not null and
(exists (select 1
from users u
where u.id = d.user_id and u.matricule = '1234'
) or
exists (select 1
from user_document ud join
users u
on u.id = d.user_id
where ud.document_id = d.id and u.matricule = '1234'
)
);
For this, you would then want indexes on users(id, matricule), user_documents(document_id, user_id), and documents(serial, user_id, document_id).
The use of indexes saves the elimination of duplicates -- which should be a big win for this type of query.
I have four tables Level, Tag, Level_Tag and Tag_hierarchy. How can select all tags of a level which have this condition id_tag = id_parent which means the Tag is the root. I can select from join table (Maybe not a good performance?) but I don't know how to add the other self join here.
SELECT level.name, tag.id, tag.name
FROM level INNER JOIN
tag_level ON level.id = tag_level.id_level INNER JOIN
tag ON tag_level.id_tag = tag.id
WHERE (level.Id = #id)
Tag Table contains thousands of rows and I'm really worry about memory and performance issues.
Could you please help me on this? Here is the schema
Try this:
;with cte as
(select id_tag
from tag_hierarchy where id_tag = id_parent)
select l.name, t.id, t.name
from cte c
inner join tag t on t.id = c.id_tag
inner join tag_level tl on t.id = tl.id_tag
inner join level l on tl.id_level = l.id
where l.lid = #id
Maybe you can add another exists. Like this:
SELECT
level.name,
tag.id,
tag.name
FROM
level
INNER JOIN tag_level
ON level.id = tag_level.id_level
INNER JOIN tag
ON tag_level.id_tag = tag.id
WHERE
(level.Id = #id)
AND EXISTS
(
SELECT NULL
FROM Tag_hierarchy
WHERE Tag_hierarchy.id_tag=tag.id
AND Tag_hierarchy.id_tag=Tag_hierarchy.id_parent
)
I have the below table structure
Users (PK - UserId)
System (PK - SystemId)
SystemRoles (PK-SystemRoleId, FK - SystemId)
UserRoles (PK-UserId & SystemRoleId, FK-SystemRoleId, FK-UserId)
Users can have access to different Systems and one System can have different SystemRoles defined.
Now, I need to delete Users who have SystemRoles assigned to them ONLY for a specific System(s). If they have SystemRoles defined for other Systems, they should not be deleted.
I have come up the below query to identify the records that are eligible for delete but think this can surely be optimized. Any suggestions?
SELECT U.*
FROM
(
SELECT
distinct UR.UserID
FROM
dbo.UserRole UR
INNER JOIN dbo.SystemRole SR ON (SR.SystemRoleID = UR.SystemRoleID)
INNER JOIN dbo.[System] S ON (S.SystemID = SR.SystemID)
WHERE
S.SystemName = 'ABC' OR S.SystemName = 'XYZ'
) T
INNER JOIN dbo.[User] U ON (U.UserID = T.UserID)
WHERE T.UserID NOT IN
(
select
distinct UR.UserID
from
dbo.[UserRole] UR
INNER JOIN dbo.SystemRole SR ON (SR.SystemRoleID = UR.SystemRoleID)
INNER JOIN dbo.[System] S ON (S.SystemID = SR.SystemID)
WHERE
S.SystemName <> 'ABC'
AND S.SystemName <> 'XYZ'
)
something like this?
select userid from (
SELECT
UR.UserID,
max(case when (S.SystemName = 'ABC' OR S.SystemName = 'XYZ')
then 1 else 0 end) as kill,
max(case when (S.SystemName <> 'ABC' AND S.SystemName <> 'XYZ')
then 1 else 0 end) as keep
FROM
dbo.UserRole UR
INNER JOIN dbo.SystemRole SR ON (SR.SystemRoleID = UR.SystemRoleID)
INNER JOIN dbo.[System] S ON (S.SystemID = SR.SystemID)
group by UR.UserID
) u where kill = 1 and keep = 0
This sort of structure will get you the records you need.
select yourfields -- or delete
from userroles
where userid in
(select userid
from userroles join etc
where system.name = the one you want
except
select userid
from userroles join etc
where system.name <> the one you want
)
I have three database tables: users, user_profiles and friends:
users
id
username
password
user_profiles
id
user_id
full_name
friends
id
usera_id
userb_id
What would be a query which finds the friends list of any user and also joins the table users and user_profiles to get the profile and user information of that friend?
Use:
SELECT f.username,
up.*
FROM USERS f
JOIN USER_PROFILES up ON up.user_id = f.id
JOIN FRIENDS fr ON fr.userb_id = f.id
JOIN USERS u ON u.id = fr.usera_id
WHERE u.username = ?
...assuming userb_id is the friend id.
This may not be the best way to do it, but this felt like the logical way:
select a.id , a.friend_id ,
Users.username
from
( SELECT id , IF(usera_id = 1, userb_id , usera_id) friend_id
FROM friends
where usera_id = 1 OR userb_id = 1 ) a
left join Users on a.friend_id = Users.id
this uses a mySQL function so probably wont work in Oracle/MSSQL
Falling back on bad habits (not using JOIN notation in the FROM clause):
SELECT a.id, a.username, a.full_name,
b.id, b.username, b.full_name
FROM friends AS f, users AS ua, users AS ub,
user_profiles AS a, user_profiles AS b
WHERE f.usera_id = ua.id
AND f.userb_id = ub.id
AND a.user_id = ua.id
AND b.user_id = ub.id
The key point is using table aliases (all those 'AS' clauses) and referencing the same table more than once when necessary.
Someone could write this with JOIN instead.
Some modification to eugene y's answer, will this work?
SELECT * FROM users u
JOIN friends f ON (f.userb_id = u.id OR f.usera_id = u.id)
JOIN user_profiles p ON u.id = p.user_id
WHERE u.id = ?