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How to filter SQL results in a has-many-through relation
(13 answers)
Closed 8 years ago.
I have a standard Twitter-style schema with users who can follow and be followed by other users. I'd like to select the users who follow two other specific users.
We have four users in the users table.
--------------
| id | name |
--------------
| 1 | Alan |
| 2 | Peter |
| 3 | Clare |
| 4 | Julia |
--------------
The relationships table describes who follows who. followed_id and follower_id are foreign keys for users.
Julia follows Alan and Peter. Peter follows Alan and Julia. Clare follows Alan.
----------------------------------
| id | followed_id | follower_id |
----------------------------------
| 1 | 1 | 4 |
| 2 | 2 | 4 |
| 3 | 1 | 2 |
| 4 | 4 | 2 |
| 5 | 1 | 3 |
----------------------------------
I'd like to select only the users who follow both Alan and Peter (i.e. the result should be Julia alone). How do I do this?
I think you need to join two times relationships table with person table like the following query
SELECT DISTINCT relationships.follower_id,followers.name from relationships
INNER JOIN users as followers on relationships.follower_id = followers.id
INNER JOIN users as followed on relationships.followed_id = followed.id
where followed.name IN('Alan','Peter');
Related
I'm using nextcloud to track data via the forms app, the table oc_forms_v2_submissions contains the entries:
SELECT * FROM `oc_forms_v2_submissions` WHERE `form_id` = 3;
+----+---------+--------------------------------------------+------------+
| id | form_id | user_id | timestamp |
+----+---------+--------------------------------------------+------------+
| 8 | 3 | anon-user-96684f301d22e7be44f07780a9bffe06 | 1663789158 |
| 9 | 3 | anon-user-a1eaa4f939b59e00b403c046410788aa | 1663835954 |
| 10 | 3 | anon-user-440d0dbe9c107492b6ec1a06d98004a8 | 1663942458 |
+----+---------+--------------------------------------------+------------+
the second table is oc_forms_v2_answers
SELECT * FROM `oc_forms_v2_answers`;
+----+---------------+-------------+-----------------------+
| id | submission_id | question_id | text |
+----+---------------+-------------+-----------------------+
| 10 | 8 | 7 | foo |
| 11 | 9 | 7 | bar |
| 12 | 10 | 7 | foo |
+----+---------------+-------------+-----------------------+
So basically i need to the take all the id entries from table submissions and match them with submission_id from answers and I want to have the data from the text column.
SELECT oc_forms_v2_submissions.id as submission_id
FROM `oc_forms_v2_submissions`
RIGHT JOIN `oc_forms_v2_answers` ON submission_id=oc_forms_v2_answers.submission_id;
This is all i could come up with so far but that returns only the submission_id field and everything triplicated :-D
+---------------+
| submission_id |
+---------------+
| 8 |
| 8 |
| 8 |
| 9 |
| 9 |
| 9 |
| 10 |
| 10 |
| 10 |
+---------------+
Edit:
The updated query still does not get me the field from oc_forms_answers:
SELECT oc_forms_v2_submissions.id as submission_id
FROM `oc_forms_v2_submissions`
RIGHT JOIN `oc_forms_v2_answers` ON oc_forms_v2_submissions.id=oc_forms_v2_answers.submission_id where form_id="3";
that is because you are comparing to identical columns, you need in the ON Clause, the link columns of both tables
Also you can use alias, to reduce the typing time
The RIGHT JOIN would also combine all answers with thes ubmission, but you you will never have more submission as answer, so a LEFT JOIN would gove ou all submissions even if there is no answer
SELECT oc_forms_v2_submissions.id as submission_id
FROM `oc_forms_v2_submissions`
LEFT JOIN `oc_forms_v2_answers` ON oc_forms_v2_submissions.id=oc_forms_v2_answers.submission_id;
This should do the trick (just update the correct naming of columns and tables)
SELECT s.id as submission_id, a.txt FROM submissions s
LEFT JOIN answers a
ON s.id=a.submission_id;
You can check this here in db-fiddle. I've used your info for creating a DB, so WHERE clause is missing but all the rest should give you results you're after.
I'm trying to merge 2 queries into 1 (cuts the number of daily queries in half): I have 2 tables, I want to do a query against 1 table, then the same query against the other table that has the same list just less entries.
Basically its a list of (let's call it for obfuscation) people and hobby. One table is ALL people & hobby, the other shorter list is people & hobby that I've met. Table 2 would all be found in table 1. Table 1 includes entries (people I have yet to meet) not found in table 2
The tables are synced up from elsewhere, what I'm looking to do is print a list of ALL people in the first column then print the hobby ONLY of people that are on both lists. That way I can see the lists merged, and track the rate at which the gap between both lists is closing. I have tried a number of SQL combinations but they either filter out the first table and match only items that are true for both (i.e. just giving me table 2) or just adding table 2 to table 1.
Example of what I'm trying to do below:
+---------+----------+--+----------+---------+--+---------+----------+
| table1 | | | table2 | | | query | |
+---------+----------+--+----------+---------+--+---------+----------+
| name | hobby | | activity | person | | name | hobby |
| bob | fishing | | fishing | bob | | bob | fishing |
| bill | vidgames | | hiking | sarah | | bill | |
| sarah | hiking | | planking | sabrina | | sarah | hiking |
| mike | cooking | | | | | mike | |
| sabrina | planking | | | | | sabrina | planking |
+---------+----------+--+----------+---------+--+---------+----------+
Normally I'd just take the few days to learn SQL a bit better however I'm stretched pretty thin at work as it is!
I should mention the table 2 is flipped and the headings are all unique (don't think this matters)!
I think you just want a left join:
select t1.name, t2.activity as hobby
from table1 t1 left join
table2 t2
on t1.name = t2.person;
EDIT
I've edited this question to make it a little more concise, if you see my edit history you will see my effort and 'what I've tried' but it was adding a lot of unnecessary noise and causing confusion so here is a summary of input and output:
People:
ID | FullName
--------------------
1 | Jimmy
2 | John
3 | Becky
PeopleJobRequirements:
ID | PersonId | Title
--------------------
1 | 1 | Some Requirement
2 | 1 | Another Requirement
3 | 2 | Some Requirement
4 | 3 | Another Requirement
Output:
FullName | RequirementTitle
---------------------------
Jimmy | Some Requirement
Jimmy | Another Requirement
John | Some Requirement
John | null
Becky | null
Becky | Another Requirement
Each person has 2 records, because that's how many distinct requirements there are in the table (distinct based on 'Title').
Assume there is no third table - the 'PeopleJobRequirements' is unique to each person (one person to many requirements), but there will be duplicate Titles in there (some people have the same job requirements).
Sincere apologies for any confusion caused by the recent updates.
CROSS JOIN to get equal record for each person and LEFT JOIN for matching records.
Following query should work in your scenario
select p.Id, p.FullName,r.Title
FROM People p
cross join (select distinct title from PeopleJobRequirements ) pj
left join PeopleJobRequirements r on p.id=r.personid and pj.Title=r.Title
order by fullname
Online Demo
Output
+----+----------+---------------------+
| Id | FullName | Title |
+----+----------+---------------------+
| 3 | Becky | Another Requirement |
+----+----------+---------------------+
| 3 | Becky | NULL |
+----+----------+---------------------+
| 1 | Jimmy | Some Requirement |
+----+----------+---------------------+
| 1 | Jimmy | Another Requirement |
+----+----------+---------------------+
| 2 | John | NULL |
+----+----------+---------------------+
| 2 | John | Some Requirement |
+----+----------+---------------------+
use left join, no need any subquery
select p.*,jr.*,jrr.*
from People p left join
PeopleJobRequirements jr on p.Id=jrPersonId
left join JobRoleRequirements jrr p.id=jrr.PersonId
according the explanation, People and PeopleJobRequirements tables have many to many relationship (n to n).
so first of all you'll need another table to relate these to table.
first do this and then a full join will make it right.
Basically, I have 3 tables, titles, providers, and provider_titles.
Let's say they look like this:
| title_id | title_name |
|------------|----------------|
| 1 | San Andres |
| 2 |Human Centipede |
| 3 | Zoolander 2 |
| 4 | Hot Pursuit |
| provider_id| provider_name |
|------------|----------------|
| 1 | Hulu |
| 2 | Netflix |
| 3 | Amazon_Prime |
| 4 | HBO_GO |
| provider_id| title_id |
|------------|----------------|
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 3 | 1 |
| 3 | 3 |
| 4 | 4 |
So, clearly there are titles with multiple providers, yeah? Typical many-to-many so far.
So what I'm doing to query it is with a JOIN like the following:
SELECT * FROM provider_title JOIN provider ON provider_title.provider_id = provider.provider_id JOIN title ON title.title_id = provider_title.title_id WHERE provider.name IN ('Netflix', 'HBO_GO', 'Hulu', 'Amazon_Prime')
Ok, now to the actual issue. I don't want repeated title names back, but I do want all of the providers associated with the title. Let me explain with another table. Here is what I am getting back with the current query, as is:
| provider_id| provider_name | title_id | title_name |
|------------|---------------|----------|---------------|
| 1 | Hulu | 1|San Andreas |
| 1 | Hulu | 2|Human Centipede|
| 2 | Netflix | 1|San Andreas |
| 3 | Amazon_Prime | 1|San Andreas |
| 3 | Amazon_prime | 3|Zoolander 2 |
| 4 | HBO_GO | 4|Hot Pursuit |
But what I really want would be something more like
| provider_id| provider_name |title_id| title_name|
|------------|-----------------------------|--------|-----------|
| [1, 2, 3] |[Hulu, Netflix, Amazon_Prime]| 1|San Andreas|
Meaning I only want distinct titles back, but I still want each title's associated providers. Is this only possible to do post-sql query with logic iterating through the returned rows?
Depending on your database engine, there may be an aggregation function to help achieve this.
For example, this SQLfiddle demonstrates the postgres array_agg function:
SELECT t.title_id,
t.title_name,
array_agg( p.provider_id ),
array_agg( p.provider_name )
FROM provider_title as pt
JOIN
provider as p
ON pt.provider_id = p.provider_id
JOIN title as t
ON t.title_id = pt.title_id
GROUP BY t.title_id,
t.title_name
Other database engines have equivalents. For example:
mySQL has group_concat
Oracle has listagg
sqlite has group_concat (as well!)
If your database isn't covered by the above, you can google '[Your database engine] aggregate comma delimited string'
I'm generating test data for a new database, and I'm having trouble populating one of the foreign key fields. I need to create a relatively large number (1000) of entries in a table (SurveyResponses) that has a foreign key to a table with only 6 entries (Surveys)
The database already has a Schools table that has a few thousand records. For arguments sake lets say it looks like this
Schools
+----+-------------+
| Id | School Name |
+----+-------------+
| 1 | PS 1 |
| 2 | PS 2 |
| 3 | PS 3 |
| 4 | PS 4 |
| 5 | PS 5 |
+----+-------------+
I'm creating a new Survey table. It will only have about 3 rows.
Survey
+----+-------------+
| Id | Col2 |
+----+-------------+
| 1 | 2014 Survey |
| 2 | 2015 Survey |
| 3 | 2016 Survey |
+----+-------------+
SurveyResponses simply ties a school to a survey.
Survey Responses
+----+----------+----------+
| Id | SchoolId | SurveyId |
+----+----------+----------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | 3 | 1 |
| 4 | 4 | 3 |
| 5 | 5 | 2 |
+----+----------+----------+
Populating the SurveyId field is what's giving me the most trouble. I can randomly select 1000 Schools, but I haven't figured out a way to generate 1000 random SurveyIds. I've been trying to avoid a while loop, but maybe that's the only option?
I've been using Red Gate SQL Data Generator to generate some of my test data, but in this case I'd really like to understand how this can be done with raw SQL.
Here is one way, using a correlated subquery to get a random survey associated with each school:
select s.schoolid,
(select top 1 surveyid
from surveys
order by newid()
) as surveyid
from schools s;
Note: This doesn't seem to work. Here is a SQL Fiddle showing the non-workingness. I am quite surprised it doesn't work, because newid() should be a
EDIT:
If you know the survey ids have no gaps and start with 1, you can do:
select 1 + abs(checksum(newid()) % 3) as surveyid
I did check that this does work.
EDIT II:
This appears to be overly aggressive optimization (in my opinion). Correlating the query appears to fix the problem. So, something like this should work:
select s.schoolid,
(select top 1 surveyid
from surveys s2
where s2.surveyid = s.schoolid or s2.surveyid <> s.schoolid -- nonsensical condition to prevent over optimization
order by newid()
) as surveyid
from schools s;
Here is a SQL Fiddle demonstrating this.