I have the following database structure:
users:
id
name
user_activities:
user_id
activity_id
activities:
id
name
I'm trying to write a Select query that will output the following:
- user1.id, activities: {[activity.id, activity.name], ['activity.id, activity.name]};
- user2.id, activities: {[activity.id, activity.name], ['activity.id, activity.name]};
I know how to do it in two steps, but I have many users and selecting activities for each one is heavy. Is there way to do it with one MySQL query?
Solution using GROUP_CONCAT function with SEPARATOR option, which lets you get all activities for a certain user:
SELECT u.id, GROUP_CONCAT( a.id, "|", a.name
SEPARATOR ', ' ) AS useractivities
FROM `users` u
INNER JOIN `user_activities` ua ON u.id = ua.user_id
INNER JOIN `activities` a ON a.id = ua.activity_id
GROUP BY u.id
This query will return a resultset in such state:
id | useractivities
------------------------------------
79 | 17|sport
18 | 3|music, 19|outing, 17|sport
.. | ....
Then, while fetching each row from resultset you can easily explode(split) the useractivities field value by comma(as delimiter) and thus, get all activities for a certain user
Related
User Table:
ID InstructionSets
1 123,124
Instruction Set Table:
ID Name
123 Learning SQL
124 Learning More SQL
Desired Query Result:
UserID SetID SetNames
1 123,124 Learning SQL,Learning More SQL
Current SQL:
SELECT U1.ID AS UserID, U1.InstructionSets AS SetID, (
SELECT GROUP_CONCAT(Name ORDER BY FIELD(I1.ID, U1.InstructionSets))
FROM Instructions I1
WHERE I1.ID IN (U1.InstructionSets)
) AS SetName
FROM Users U1
WHERE `ID` = 1
RESULT
UserID SetID SetNames
1 123,124 Learning SQL
As expected, if I remove the WHERE clause in the sub-query, all of the SetNames appear; but if I specify the required IDs, I only get the name associated with the first ID. Obviously, I also need to fetch the SetNames in the same order as the IDs. Hence ORDER BY in GROUP_CONCAT.
Also:
Is there a better approach (other than storing the user instruction set assignments in a separate table — overkill for this application)? Couldn't see how to use JOIN in this
situation.
Is there a better title for this question?
Thanks.
Instead of IN use LIKE operator like this:
SELECT U1.ID AS UserID, U1.InstructionSets AS SetID, (
SELECT GROUP_CONCAT(Name ORDER BY (I1.ID))
FROM Instructions I1
WHERE CONCAT(',', U1.InstructionSets, ',') LIKE concat('%,', I1.ID, ',%')
) AS SetName
FROM Users U1
WHERE `ID` = 1
See the demo.
Results:
| UserID | SetID | SetName |
| ------ | ------- | ------------------------------ |
| 1 | 123,124 | Learning SQL,Learning More SQL |
We can use FIND_IN_SET(). In this context, using FIELD() function doesn't make sense.
We can also use FIND_IN_SET() in the WHERE clause. (Function returns 0 when the string isn't found in the string list.)
e.g.
SELECT u.id AS userid
, u.instructionsets AS setid
, ( SELECT GROUP_CONCAT(i.name ORDER BY FIND_IN_SET(i.id, u.instructionsets))
FROM `Instructions` i
WHERE FIND_IN_SET(i.id, u.instructionsets))
) AS setname
FROM `Users` u
WHERE u.id = 1
Storing comma separated lists is an anti-pattern; a separate table isn't overkill.
Assuming id is unique in Users table, we could do a join operation with a GROUP BY
SELECT u.id AS userid
, MIN(u.instructionsets) AS setid
, GROUP_CONCAT(i.name ORDER BY FIND_IN_SET(i.id, u.instructionsets))) AS setname
FROM `Users` u
LEFT
JOIN `Instructions` i
ON FIND_IN_SET(i.id, u.instructionsets)
WHERE u.id = 1
GROUP BY u.id
I have this table:
users
id | name
---------------------
1 | Lisa
2 | John
and i have this users list:
(1,2,2,2,1,2)
and i want to get this result:
Lisa,John,John,John,Lisa,John
how i can get this result just using SQL query in my Phpmyadmin without using PHP?
i must use Left Join ?
(Assuming orders table is on the same server) You should use inner join here. Because, there should be an user to have an entry in orders table.
SELECT abc = STUFF(( Select user.user_name + ',' from
orders inner join users on users.id = orders.user_id where orders.user_id in (your string of user ids) for
xml_path(' ')), 1, 1, '') FROM temp1
Working example: http://sqlfiddle.com/#!9/80995/20
I have three tables, a user table, a user_group table, and a link table.
The link table contains the dates that users were added to user groups. I need a query that returns the count of users currently in each group. The most recent date determines the group that the user is currently in.
SELECT
user_groups.name,
COUNT(l.name) AS ct,
GROUP_CONCAT(l.`name` separator ", ") AS members
FROM user_groups
LEFT JOIN
(SELECT MAX(added), group_id, name FROM link LEFT JOIN users ON users.id = link.user_id GROUP BY user_id) l
ON l.group_id = user_groups.id
GROUP BY user_groups.id
My question is if the query I have written could be optimized, or written better.
Thanks!
Ben
You actual query is not giving you the answer you want; at least, as far as I understand your question. John actually joined group 2 on 2017-01-05, yet it appears on group 1 (that he joined on 2017-01-01) on your results. Note also you're missing one Group 4.
Using standard SQL, I think the next query is what you're looking for. The comments in the query should clarify what each part is doing:
SELECT
user_groups.name AS group_name,
COUNT(u.name) AS member_count,
group_concat(u.name separator ', ') AS members
FROM
user_groups
LEFT JOIN
(
SELECT * FROM
(-- For each user, find most recent date s/he got into a group
SELECT
user_id AS the_user_id, MAX(added) AS last_added
FROM
link
GROUP BY
the_user_id
) AS u_a
-- Join back to the link table, so that the `group_id` can be retrieved
JOIN link l2 ON l2.user_id = u_a.the_user_id AND l2.added = u_a.last_added
) AS most_recent_group ON most_recent_group.group_id = user_groups.id
-- And get the users...
LEFT JOIN users u ON u.id = most_recent_group.the_user_id
GROUP BY
user_groups.id, user_groups.name
ORDER BY
user_groups.name ;
This can be written in a more compact way in MySQL (abusing the fact that, in older versions of MySQL, it doesn't follow the SQL standard for the GROUP BY restrictions).
That's what you'll get:
group_name | member_count | members
:--------- | -----------: | :-------------
Group 1 | 2 | Mikie, Dominic
Group 2 | 2 | John, Paddy
Group 3 | 0 | null
Group 4 | 1 | Nellie
dbfiddle here
Note that this query can be simplified if you use a database with window functions (such as MariaDB 10.2). Then, you can use:
SELECT
user_groups.name AS group_name,
COUNT(u.name) AS member_count,
group_concat(u.name separator ', ') AS members
FROM
user_groups
LEFT JOIN
(
SELECT
user_id AS the_user_id,
last_value(group_id) OVER (PARTITION BY user_id ORDER BY added) AS group_id
FROM
link
GROUP BY
user_id
) AS most_recent_group ON most_recent_group.group_id = user_groups.id
-- And get the users...
LEFT JOIN users u ON u.id = most_recent_group.the_user_id
GROUP BY
user_groups.id, user_groups.name
ORDER BY
user_groups.name ;
dbfiddle here
I need help with a sql query.
I have these 2 tables:
player_locations:
ID | playerid | location <- unqiue key
---|-----------------------
1 | 1 | DOWNTOWN
and users:
ID | playername | [..]
----|--------------------
1 | example1 | ...
I need a select to get the users.playername from the player_locations.playerid. I have the unique location to get the player_locations.playerid.
Pseudo query:
SELECT playername
FROM users
WHERE id = player_locations.playerid
AND player_locations.location = "DOWNTOWN";
The output should be example1.
This is just a simple INNER JOIN. The general syntax for a JOIN is:
SELECT stuff
FROM table1
JOIN table2 ON table1.relatedColumn = table2.relatedColumn
In your case, you can relate the two tables using the id column from users and playerid column from player_locations. You can also include your 'DOWNTOWN' requirement in the JOIN statement. Try this:
SELECT u.playername
FROM users u
JOIN player_locations pl ON pl.playerid = u.id AND pl.location = 'DOWNTOWN';
EDIT
While I personally prefer the above syntax, I would like you to be aware of another way to write this which is similar to what you have now.
You can also select from multiple tables by using a comma in your FROM clause to separate them. Then, in your WHERE clause you can insert your conditions:
SELECT u.playername
FROM users u, player_locations pl
WHERE u.id = pl.playerid AND pl.location = 'DOWNTOWN';
Here is the solution.
SELECT
playername
FROM users
WHERE id = (SELECT id FROM player_locations WHERE location='DOWNTOWN')
I have a idea, try this:
SELECT playername
FROM users
WHERE id IN (SELECT DISTINCT playerid FROM player_location WHERE location LIKE "DOWNTOWN");
I'm having trouble figuring out how to structure a SQL query. Let's say we have a User table and a Pet table. Each user can have many pets and Pet has a breed column.
User:
id | name
______|________________
1 | Foo
2 | Bar
Pet:
id | owner_id | name | breed |
______|________________|____________|_____________|
1 | 1 | Fido | poodle |
2 | 2 | Fluffy | siamese |
The end goal is to provide a query that will give me all the pets for each user that match the given where clause while allowing sort and limit parameters to be used. So the ability to limit each user's pets to say 5 and sorted by name.
I'm working on building these queries dynamically for an ORM so I need a solution that works in MySQL and Postgresql (though it can be two different queries).
I've tried something like this which doesn't work:
SELECT "user"."id", "user"."name", "pet"."id", "pet"."owner_id", "pet"."name",
"pet"."breed"
FROM "user"
LEFT JOIN "pet" ON "user"."id" = "pet"."owner_id"
WHERE "pet"."id" IN
(SELECT "pet"."id" FROM "pet" WHERE "pet"."breed" = 'poodle' LIMIT 5)
In Postgres (8.4 or later), use the window function row_number() in a subquery:
SELECT user_id, user_name, pet_id, owner_id, pet_name, breed
FROM (
SELECT u.id AS user_id, u.name AS user_name
, p.id AS pet_id, owner_id, p.name AS pet_name, breed
, row_number() OVER (PARTITION BY u.id ORDER BY p.name, pet_id) AS rn
FROM "user" u
LEFT JOIN pet p ON p.owner_id = u.id
AND p.breed = 'poodle'
) sub
WHERE rn <= 5
ORDER BY user_name, user_id, pet_name, pet_id;
When using a LEFT JOIN, you can't combine that with WHERE conditions on the left table. That forcibly converts the LEFT JOIN to a plain [INNER] JOIN (and possibly removes rows from the result you did not want removed). Pull such conditions up into the join clause.
The way I have it, users without pets are included in the result - as opposed to your query stub.
The additional id columns in the ORDER BY clauses are supposed to break possible ties between non-unique names.
Never use a reserved word like user as identifier.
Work on your naming convention. id or name are terrible, non-descriptive choices, even if some ORMs suggest this nonsense. As you can see in the query, it leads to complications when joining a couple of tables, which is what you do in SQL.
Should be something like pet_id, pet, user_id, username etc. to begin with.
With a proper naming convention we could just SELECT * in the subquery.
MySQL does not support window functions, there are fidgety substitutes ...
SELECT user.id, user.name, pet.id, pet.name, pet.breed, pet.owner_id,
SUBSTRING_INDEX(group_concat(pet.owner_id order by pet.owner_id DESC), ',', 5)
FROM user
LEFT JOIN pet on user.id = pet.owner_id GROUP BY user.id
Above is rough/untested, but this source has exactly what you need, see step 4. also you don't need any of those " 's.