I have a table named friends;
friends
id uid fid
1 1 2 (1 is a friend of 2 and vice versa)
2 1 3 (1 is a friend of 3 and vice versa)
3 4 1 (1 is a friend of 4 and vice versa)
4 5 1 (1 is a friend of 5 and vice versa)
And a corresponding table for users;
users
uid name
1 mike
2 john
3 karl
4 mary
5 tony
This doesn't seem to do the trick:
SELECT name FROM users LEFT JOIN friends ON friends.uid=users.uid WHERE friends.uid='1' OR friends.fid='1'
What should my query be to get all the names of mike's friends?
This should do it just fine with a single, easy to index, query;
SELECT name FROM users u
JOIN friends f
ON u.uid = f.uid OR u.uid = f.fid
WHERE (f.uid=1 OR f.fid=1)
AND u.uid<>1;
Demo here.
Untested:
SELECT name from friends LEFT JOIN users on users.uid=friends.fid where friends.uid=1 UNION
SELECT name from friends LEFT JOIN users on users.uid=friends.uid where friends.fid=1
This may look a little strange if anyone is ever friends with themselves.
try one of these:
SELECT a.uid as UserID,
a.`Name` as UserName,
c.`Name as FriendsName
FROM users a LEFT JOIN friends b on a.uid = b.uid
LEFT JOIN users c on b.fid = c.uid
OR
SELECT a.uid as UserID,
a.`Name` as UserName,
GROUP_CONCAT(c.`Name`) as FriendsList
FROM users a LEFT JOIN friends b on a.uid = b.uid
LEFT JOIN users c on b.fid = c.uid
GROUP BY a.uid
As in your prequel question, you need to cover both foreign keys to the user table to get all his friends:
SELECT users.*
FROM (
SELECT uid FROM friends WHERE fid = 1
UNION ALL
SELECT fid FROM friends WHERE uid = 1
) f
JOIN users USING (uid)
Switch friends and users in your query and I think you'll get what you want.
In other words:
SELECT name FROM friends LEFT JOIN users ON friends.uid=users.uid WHERE friends.uid='1' OR friends.fid='1'
I think this is right SELECT name FROM users LEFT JOIN friends ON friends.uid=users.uid WHERE friends.uid=1 OR friends.fid=1
Related
I have scenario in which I am storing user email in a table user_info. Its structure is given as below.
user_info(_id,email)
However one user can be friend of other users.So for that purpose i have another table user_friend. Its structure is given below
user_friend(_id,userA , userB)
Now suppose there are 5 user A,B,C,D and E. We got table values something like this.
user_info
_id email
1 A#...
2 B#...
3 C#...
4 D#...
5 E#...
user_friends
_id userA userB
1 1 2
2 1 3
3 1 4
4 5 1
it's mean A is friend of B,C,D and E. i want to get email of these but i am unable to think about any proper query.
i am using left join but it does not seems to work properly
Getting friends of 1(A#...). But it's not working properly
SELECT DISTINCT email FROM user_info AS UI
LEFT JOIN user_friends AS UF
ON UI._id = UF.userA
LEFT JOIN user_friends AS UF2
ON UI._id = UF2.userB
WHERE UF2.userA = '1' || UF2.userB ='1'
Any Suggestions?
Use UNION to get all friends email
(SELECT
u2.email
FROM user_info u
LEFT JOIN user_friends f ON f.userA = u._id
LEFT JOIN user_info u2 on u2._id = f.userB
WHERE u._id = 1)
UNION
(SELECT
u2.email
FROM user_info u
LEFT JOIN user_friends f ON f.userB = u._id
LEFT JOIN user_info u2 ON u2._id = f.userA
WHERE u._id = 1)
It will be Something like this.
SELECT email from user_info WHERE _id IN (select userA from user_friends where userB = '1' Union All select userB from user_friends where userA = '1')
I have three table group, user and belongs where belongs is relation between group and user.
Table group
group_id (pk)
group_name
Table user
user_id (pk)
user_name
Table belongs
user_id
group_id
I want to show a random groups that people in specified group are also in.
For example specified group is where group_id = 1, Of course, i can get people in this group by use
"SELECT user_id FROM user u JOIN belongs b ON u.user_id = b.user_id and b.group_id = '1'"
I have been trying, and now, i have no idea how to select group that these people are in too.
"SELECT g.* FROM group g WHERE ..."
Thanks in advance.
You can get all the groups by using an extra join:
SELECT DISTINCT b2.group_id
FROM user u JOIN
belongs b
ON u.user_id = b.user_id and b.group_id = '1' JOIN
belongs b2
ON b2.user_id = b.user_id;
One way to choose a random such group is to use order by rand() limit 1. You might want to add where b2.group_id <> '1'.
Assuming that you want to count the members of group_id = 1 in these groups, you would just use group by:
SELECT b2.group_id, count(distinct b2.user_id) as numusers
FROM user u JOIN
belongs b
ON u.user_id = b.user_id and b.group_id = '1' JOIN
belongs b2
ON b2.user_id = b.user_id
GROUP BY b2.group_id;
The join to user is superfluous, but I have left it in assuming that it might be used for some other conditions not in the original question.
You can get a list of all the other groups associated with people in group 1 by using a sub-query:
SELECT g.group_name, COUNT(g.group_name)
FROM group g
INNER JOIN belongs b ON b.group_id = g.group_id
WHERE b.user_id IN ( SELECT u.user_id FROM user u JOIN belongs b ON u.user_id = b.user_id and b.group_id = '1' )
GROUP BY g.group_name
ORDER BY COUNT(g.group_name) DESC
I want to show the conclusion of all users.
I have 3 tables.
table post
post_id(index) user_id
1 1
2 3
3 3
4 4
table photo
photo_id(index) user_id
1 2
2 4
3 1
4 1
table video
photo_id(index) user_id
1 4
2 4
3 3
4 3
and in table user
user_id(index) user_name
1 mark
2 tommy
3 john
4 james
in fact, it has more than 4 rows for every tables.
I want the result like this.
id name post photo videos
1 mark 1 2 0
2 tommy 0 1 0
3 john 2 0 2
4 james 1 1 2
5 .. .. .. ..
Code below is SQL that can work correctly but very slow, I will be true appreciated if you help me how it using LEFT JOIN for it. Thanks.
SQL
"select user.*,
(select count(*) from post where post.userid = user.userid) postCount,
(select count(*) from photo where photo.userid = user.userid) photoCount,
(select count(*) from video where video .userid = user.userid) videoCount
from user order by user.id"
(or ORDER BY postCount, photoCount or videoCount ASC or DESC as i want )
I done researched before but no any helped me.
SELECT u.user_id,
u.user_name,
COUNT(DISTINCT p.post_id) AS `postCount`,
COUNT(DISTINCT ph.photo_id) AS `photoCount`,
COUNT(DISTINCT v.video_id) AS `videoCount`
FROM user u
LEFT JOIN post p
ON p.user_id = u.user_id
LEFT JOIN photo ph
ON ph.user_id = u.user_id
LEFT JOIN video v
ON v.user_id = u.user_id
GROUP BY u.user_id
ORDER BY postCount;
Live DEMO
Your method of doing this is quite reasonable. Here is your query:
select user.*,
(select count(*) from post where post.userid = user.userid) as postCount,
(select count(*) from photo where photo.userid = user.userid) as photoCount,
(select count(*) from video where video.userid = user.userid) as videoCount
from user
order by user.id;
For this query, you want the following indexes:
post(userid)
photo(userid)
video(userid)
user(id)
You probably already have the last one, because user.id is probably the primary key of the table.
Note that a left join approach is a bad idea in this case. The three tables -- posts, photos, and videos -- are independent of each other. If a user has five of each, then joining them together would produce 125 intermediate rows. If a user has fifty of each, it would be 125,000 -- a lot of extra processing.
Your answer is probably slow as it is using a correlated sub-query i.e. the sub query is running once for each user_id (unless the optimizer is doing something smart - which shouldn't be counted on).
You could use a left outer join and count or use something temporary like:
SELECT u.user_id,
u.user_name,
ph.user_count AS 'photoCount',
p.user_count AS 'postCount',
v.user_count AS 'videoCount'
FROM user u
INNER JOIN ( SELECT user_id,
COUNT(*) AS user_count
FROM photo
GROUP BY user_id
) ph
ON ph.user_id=u.user_id
INNER JOIN ( SELECT user_id,
COUNT(*) AS user_count
FROM post
GROUP BY user_id
) p
ON p.user_id=u.user_id
INNER JOIN ( SELECT user_id,
COUNT(*) AS user_count
FROM video
GROUP BY user_id
) v
ON v.user_id=u.user_id
There are pros and cons for both (depending on indexes). Always have a look at the query plan (using EXPLAIN for MySQL).
Suppose this:
users
id | name | address
partners
id | name | company | address
Even though the tables are distinct sometimes happens having to associate the users or partners id to the same function ..
For example, the access table
acl
uri | uid | group | operations
Here "uid" can be both user and partner.
How can I read the data of 2 tables with the same alias?
something like:
$selectQuery = <<<QUERY
SELECT A. *,
U.name P.name AS username,
G.name AS groupname
FROM [acl] A
LEFT JOIN [users] U ON A.uid = U.id
LEFT JOIN [partner] P ON A.uid = P.id
LEFT JOIN [groups] G ON A.gid = G.id
WHERE A.id =: id
LIMIT 0.1
QUERY;
You probably want to use COALESCE or IFNULL
IFNULL(U.name, P.name) AS username,
COALESCE(U.name, P.name) AS username,
Both of those will do the exact same thing in this situation. If U.name is not NULL then username will be U.name, otherwise it'll be P.name.
USE UNION ALL with the condition......it should help hopefully
say i had the following tables
user_table
id username
1 abc
2 def
3 ghij
courses_table
id title
1 csc
2 math
3 syn
user_courses
user_id course_id
2 1
1 3
2 3
i want to select the username whos taking course 1 AND 3 ,
not at least 1 or 3 , i mean both 1 and 3
i've tried the following mysql queries
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id = 1 AND uc.course_id=3;
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id IN (1,3);
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id IN (1,3) AND uc.user_id = u.id ;
the first and third queries executed with no results shown , and the second one show all users who had at least course_id 1 or 3
if you are wondering why am i using the LEFT JOIN , this is because i need to join table's results , and the above line of code is just an example , and im using to get data from about 9 tables using the LEFT join .
any help please ? thanks
SELECT DISTINCT u.* FROM user_table as u LEFT JOIN user_courses as uc ON uc.user_id = u.id WHERE uc.course_id IN( 1,3) AND uc.user_id = 2 ";
this show me the result i want , its output "def" ,
but i can't use the user_id as a static value ( number 2 in this example )
This problem is called Relational Division
SELECT a.id, a.username
FROM user_table a
INNER JOIN user_courses b
ON a.id = b.user_ID
WHERE b.course_ID IN (1,3)
GROUP BY a.id, a.username
HAVING COUNT(*) = 2
SQL of Relational Division
If course_ID is not unique for every users considering that the user have retake the course, a DISTINCT keyword is needed to coung unique courses,
SELECT a.id, a.username
FROM user_table a
INNER JOIN user_courses b
ON a.id = b.user_ID
WHERE b.course_ID IN (1,3)
GROUP BY a.id, a.username
HAVING COUNT(DISTINCT b.course_ID) = 2
SQLFiddle Demo
OUTPUT
╔════╦══════════╗
║ ID ║ USERNAME ║
╠════╬══════════╣
║ 2 ║ def ║
╚════╩══════════╝
please try this:
SELECT
U.id,
U.username
FROM
user_courses UC
INNER JOIN user_table U
ON UC.`user_id` = U.`id`
WHERE UC.`course_id` = 1
OR UC.`course_id` = 3
GROUP BY U.`id`
HAVING COUNT(*) > 1