I have a list of users, and a junction table where both fields are user_id, which represents a friendship.
So the database is
users
id | name
----------------------
1 Julia
2 Davos
3 Michael
4 Anthony
friendships
user1_id | user2_id
--------------------------
3 1
2 4
I want to select a list of the friendships, like
Michael | Julia
Davos | Anthony
But this one is just totally stumping me, and Google isn't helping. Can anyone offer a tip?
For get that information you should join twice to user table using friendship like pivot
SELECT friend_1.name,friend_2.name
FROM friendships AS relation_friend
JOIN users AS friend_1 ON friend_1.id=relation_friend.user1_id
JOIN users AS friend_2 ON friend_1.id=relation_friend.user1_id;
Regards.
It is dynamically working fine for any number of data:
select tbl.name, tb2.name from
(select *
from
users as a
inner join
friendships as b
on a.id=b.user1_id
or a.id=b.user2_id) as tbl
inner join
(select *
from
users as a
inner join
friendships as b
on a.id=b.user1_id
or a.id=b.user2_id) as tb2
on tbl.user2_id=tb2.id
where tbl.name <> tb2.name
Related
I am having 2 tables in my DB
1) Report table
AppNo AppName AddedBy AssignedTo ModifiedBy
-------------------------------------------
1 App1 1 2 1
2 App2 1 2 2
3 App3 2 2 2
4 App4 1 2 3
2) Users table
UserId UserName Role
----------------------
1 Raj Manager
2 Sid Lead
3 KK Rep
So i want to join two tables so that i can get names in place of Id's
Result Needed:
AppNo AppName AddedBy AssignedTo ModifiedBy
-------------------------------------------
1 App1 Raj Sid Raj
2 App2 Raj Sid Sid
3 App3 Sid Sid Sid
4 App4 Raj Sid KK
My Query:
SELECT
R.AppNo, R.AppName,
u1.UserName as AddedBy,
u2.UserName as AssignedTo,
u3.UserName as ModifiedBy
FROM Report R
LEFT OUTER JOIN Users u1 on u1.UserID = R.AddedBy
LEFT OUTER JOIN Users u2 on u2.UserID = R.AssignedTo
LEFT OUTER JOIN Users u3 on u3.UserID = R.ModifiedBy
But i dont want to join multiple time with User table..
As in my original report table there are nearly 8 UserId columns are there so i cant join 8 times it reduces performance.
Can anyone suggest the best way.
Note: I cant change table schema
Thanks in advance
Joining multiple times is the only way; that is what the data and structure is.
There should be no performance issue, as the database is typically well designed to handle this. Just because a JOIN looks complex to humans doesn't mean it is complex or expensive for the optimizer or the database.
You could use a cartesian join for the columns, although performance will depend heavily on how well indexed the tables are...
select R.AppNo,
R.AppName,
(select UserName from Users where UserID = R.AddedBy) as AddedBy,
(select UserName from Users where UserID = R.AssignedTo) as AssignedTo,
(select UserName from Users where UserID = R.ModifiedBy) as ModifiedBy
from Report R
I have 2 tables - user, region and a join/connecting table that is used to join both user and region.
I need to insert into the join table all the region values that the user does not already have and i am unsure how to go about this.
I have attempted this numerous ways but i am not entirely sure how to only place the values that do already exist within the table into the join table. Does anyone have any ideas or suggestions?
SELECT
CONVERT( CONCAT('INSERT INTO user_region VALUES(',
user.id,
',',
reg.id,
');') USING UTF8)
FROM
user user
JOIN
user_region user_reg ON user_reg.id = user.id
JOIN
region reg ON reg.id = user_reg.id
WHERE
(user.email_address LIKE '%gmail%'
OR user.email_address LIKE '%hotmail%');
User Table User Region Region
----------- ----------- ------
1 1 2 1
2 3 2 2
3 3 4 3
4 4 3 4
Kind of
INSERT INTO user_region (userID, regionID)
SELECT u.userID, r.regionID
FROM
(SELECT DISTINCT userId
FROM user
WHERE user.email_address LIKE '%gmail%'
OR user.email_address LIKE '%hotmail%') u
JOIN region r ON NOT EXISTS (
SELECT 1
FROM user_region ur
WHERE ur.userID = u.userID AND ur.regionID = r.regionID )
I have tables users and topics. Every user can have from 0 to several topics (one-to-many relationship).
How I can get only those users which have at least one topic?
I need all columns from users (without columns from topics) and without duplicates in table users. In last column I need number of topics.
UPDATED:
Should be like this:
SELECT user.*, count(topic.id)
FROM ad
LEFT JOIN topic ON user.id = topic.ad
GROUP BY user.id
HAVING count(topic.id) > 0;
but it takes 0 result. But it should not be 0.
Firstly you need to have your two tables, because you have left limited information about your table structure I will use an example to explain how this works, you should then be able to easily apply this to your own tables.
Firstly you need to have two tables (which you do)
Table "user"
id | name
1 | Joe Bloggs
2 | Eddy Ready
Table "topic"
topicid | userid | topic
1 | 1 | Breakfast
2 | 1 | Lunch
3 | 1 | Dinner
Now asking for a count against each user is done using the follwing;
SELECT user.name, count(topic.topicid)
FROM user
INNER JOIN topic ON user.id = topic.userid
GROUP BY user.name
If you use a left join, this will include records from the "user" table which does not have any rows in the "topic" table, however if you use an INNER JOIN this will ONLY include users who have a matching value in both tables.
I.e. because the user id "2" (which we use to join) is not listed in the topic table you will not get any results for this user.
Hope that helps!
use inner join and distinct
select distinct user_table.id
from user_table
inner join topics_table on topic_table.user_id = user_table.id
select u.id
, u.name
, count(b.topicName)
from user u
left join topic t on t.userid = u.id
group by u.id, u.name
You can select topic number per user and then join it with user data. Something like this:
with t as
(
select userid, count(*) as n
from topic
group by userid
)
SELECT user.*, t.n
FROM user
JOIN t ON user.id = t.userid
Today I have a final Exam. I approved, happily :D but one of the problems is really blowing my mind.
I need help, so I can rest in peace.
THE PROBLEM
We have a table "People"
(PK)id | name | fatherID
---------------------
1 | gon | 2
2 | cesar| 6
3 | luz | 2
4 | maria| 5
5 | diego| 6
6 | john | -
this is only an example of data.
This table has a relation with itself, on table fatherId(FK) with table id(PK)
I need to do a query that show me 2 columns, in one the name of a person, and in the another one, his/her cousin.
Pretty simple until here, right?
The problem is that I have some restrictions
ONLY ANSI allowed. NO T-sql, or another one. Also, ANSI 99 standard, not 2003 or higher
subquerys are not allowed. And the worst:
NO relations repeated.
For example, considering in this example, gon and maria are cousins.
If I show, gon | maria in the results, I can't show maria | gon.
SO, how I can do this?
Is really burning my head.
What I tried?
Well, the big problem was in the last requisite, the repetition of data. Ignoring that, I put this on my exam (knowing is wrong..)
select p3.name as OnePerson, p4.name as Cousin
from
people p1
inner join people p2 on p1.fatherid = p2.fatherid and p1.id != p2.id
inner join people p3 on p1.id = p3.fatherid
inner join people p4 on p1.id = p4.fatherid
of course, this is not solving the last requeriment, and I have a 4 in the test(we pass with 4) but anyway, my head is burning. So please, help me!
Another options explored
one of my friends, that also had the same exam said me
"Well, considering every relation is duplicated, I can use top
count(*) and an order by and get the half correct"
but.. Top is not ANSI!
You can add to your query WHERE p3.id < p4.id. This will eliminate duplicate results like gon | maria and maria | gon.
SELECT T1.id , T2.id FROM
(
SELECT A.id,A.fid FROM family A
WHERE a.fid IN
(
SELECT id FROM family
WHERE fid IN (SELECT id FROM family WHERE fid IS NULL)
)
)T1
JOIN
(
SELECT A.id,A.fid FROM family A
WHERE a.fid IN
(
SELECT id FROM family
WHERE fid IN (SELECT id FROM family WHERE fid IS NULL)
)
)T2
ON t1.fid<>t2.fid
AND t1.id<t2.id
This will give you the results in format you want.
SELECT TAB1.ID,TAB2.ID
FROM
(
SELECT * FROM people T1
WHERE fatherID IN ( SEL T1.ID FROM people T1 INNER JOIN people T2
ON( T1.id=T2.fatherID) WHERE T1.fatherID IS NOT NULL GROUP BY 1) ) TAB1
INNER JOIN
(
SELECT * FROM people T1
WHERE fatherID IN ( SEL T1.ID FROM people T1 INNER JOIN people T2
ON( T1.id=T2.fatherID)WHERE T1.fatherID IS NOT NULL GROUP BY 1) ) TAB2
ON( TAB1.fatherID<>TAB2.fatherID)
GROUP BY 1,2
WHERE TAB1.ID <TAB2.ID;
Here is the case:
There is a user table
id email orders_counter
=================================
1 a#a.com 5
2 b#b.com 3
3 c#c.com 0
And a user data table for user's other data
id user_id title value
=======================================
1 1 Name Peter Johnson
2 1 Tel 31546988
3 2 Name Alan Johnson
4 2 Tel 56984887
If I want to fetch all user that
1, orders_counter greater then 3
2, Name contain 'Johnson'
3, Tel contain '88'
AT THE SAME TIME
What is my sql, and if I want to do it rubyonrails way
what is the ActiveRecord code
I know I can it one by one and then join all of them together, but it waste too much resource while conditions build up
Select * From `user` u
inner join `userdata` d on d.user_id=u.id and d.title='Name' and d.value like '%Johnson%'
inner join `userdata` c on c.user_id=u.id and c.title='Tel' and c.value like '%88%'
where orders_counter > 3
the way that you've got your user data table structured, you'll almost always have to join on that table several times in order to "pivot" those values into columns. I'd recommend just creating a table that has name and tel as columns. then the query becomes a lot more simple
select * from `user` u
inner join `user_data` d on d.Tel like '%88%' and d.Name like '%johnson%'
where u.orders_counter > 3
try this one up,
SELECT a.*, b.*
FROM user a
INNER JOIN data b
ON a.id = b.user_ID
WHERE a.orders_counter > 3 AND
(b.title = 'NAME' AND b.value like '%johnson%') AND
(b.title = 'TEL' AND b.tel like '%88%')
try this:
select *
from user U join user_data D
on U.id=D.user_id
where U.orders_counter>3
and D.title='Name' and value like '%Johnson%'
and D.title='Tel' and value like '%88%'