Kind of new to pgsql, what i'm looking for is a bit of common need, I think, but I still wasn't able to find a solution.
I have 3 tables:
users
user_id|username|password
-------|--------|--------
1 | guest | blabla
2 | admin | blabla
roles
role_id|name | descr
-------|-----|--------
1 |role1|role one
2 |role2|role two
user_roles
user_id|role_id
-------|-------
2 | 1
2 | 2
I want to display a table of user along with all its' roles, my guess (feel free to correct me), the way to do that would be:
Group user roles into array:
select user_id,array_agg(role_id) from user_roles group by user_id where user_id = 2;
Somehow join the array of array_agg(role_id) into roles to select the role names.
As you can see, I'm a bit confused on how to do #2. Is this the best way to do what I want? Is there something wrong with how I've build my db tables?
Thanks!
To achieve your query, you would first do the join between the tables and at last aggregate on the name column.
Note that it may be prettier using string_agg(r.name,',')
select ur.user_id,array_agg(r.name)
from user_roles ur,
roles r
where ur.role_id = r.role_id
and ur.user_id = 2
group by ur.user_id;
Related
I'm dealing with a lot of unique data that has the same type of columns, but each group of rows have different attributes about them and I'm trying to see if PostgreSQL has a way of storing metadata about groups of rows in a database or if I would be better off adding custom columns to my current list of columns to track these different attributes. Microsoft Excel for instance has a way you can merge multiple columns into a super-column to group multiple columns into one, but I don't know how this would translate over to a PostgreSQL database. Thoughts anyone?
Right, can't upload files. Hope this turns out well.
Section 1 | Section 2 | Section 3
=================================
Num1|Num2 | Num1|Num2 | Num1|Num2
=================================
132 | 163 | 334 | 1345| 343 | 433
......
......
......
have a "super group" of columns (In SQL in general, not just postgreSQL), the easiest approach is to use multiple tables.
Example:
Person table can have columns of
person_ID, first_name, last_name
employee table can have columns of
person_id, department, manager_person_id, salary
customer table can have columns of
person_id, addr, city, state, zip
That way, you can join them together to do whatever you like..
Example:
select *
from person p
left outer join student s on s.person_id=p.person_id
left outer join employee e on e.person_id=p.person_id
Or any variation, while separating the data into different types and PERHAPS save a little disk space in the process (example if most "people" are "customers", they don't need a bunch of employee data floating around or have nullable columns)
That's how I normally handle this type of situation, but without a practical example, it's hard to say what's best in your scenario.
I'm trying to create a pivot table from related data. I am planning on using this data to better understand which tags are correlated to high frequency users.
I have a users table, a tag_users and a tags table.
EDIT
The non-pivoted query is
SELECT users.id, tags.title
FROM users
INNER JOIN tag_users ON tag_users.user_id = users.id
INNER JOIN tags ON user_tags.tag_id = tags.id
I would like to create an output with the following format:
user_id | tag_1_name | tag_2_name | ...tag_{n}_name
1 1 0 1
I have ~ 196 tags and ~ 40,000 users and downloading tags a a comma separated field and then doing a lookup on excel breaks my computer.
I am aware of crosstab but I can't find any examples where it uses related data.
What do you suggest as a suitable solution?
Sorry, I can't find an example anywhere, mainly because I can't think of any other way to explain it that doesn't include DISTINCT or UNIQUE (which I've found to be misleading terms in SQL).
I need to select unique values AND null values from one table.
FLAVOURS:
id | name | flavour
--------------------------
1 | mark | chocolate
2 | cindy | chocolate
3 | rick |
4 | dave |
5 | jenn | vanilla
6 | sammy | strawberry
7 | cindy | chocolate
8 | rick |
9 | dave |
10 | jenn | caramel
11 | sammy | strawberry
I want the kids who have a unique flavour (vanilla, caramel) and the kids who don't have any flavour.
I don't want the kids with duplicate flavours (chocolate, strawberry).
My searches for help always return an answer for how to GROUP BY, UNIQUE and DISTINCT for chocolate and strawberry. That's not what I want. I don't want any repeated terms in a field - I want everything else.
What is the proper MySQL select statement for this?
Thanks!
You can use HAVING to select just some of the groups, so to select the groups where there is only one flavor, you use:
SELECT * from my_table GROUP BY flavour HAVING COUNT(*) = 1
If you then want to select those users that have NULL entries, you use
SELECT * FROM my_table WHERE flavour IS NULL
and if you combine them, you get all entries that either have a unique flavor, or NULL.
SELECT * from my_table GROUP BY flavour HAVING COUNT(*) = 1 AND flavour IS NOT NULL
UNION
SELECT * FROM my_table WHERE flavour IS NULL
I added the "flavour IS NOT NULL" just to ensure that a flavour that is NULL is not picked if it's the single one, which would generate a duplicate.
I don't have a database to hand, but you should be able to use a query along the lines of.
SELECT name from FLAVOURS WHERE flavour IN ( SELECT flavour, count(Flavour) from FLAVOURS where count(Flavour) = 1 GROUP BY flavour) OR flavour IS NULL;
I apologise if this isn't quite right, but hopefully is a good start.
You need a self-join that looks for duplicates, and then you need to veto those duplicates by looking for cases where there was no match (that's the WHERE t2.flavor IS NULL). Then your're doing something completely different, looking for nulls in the original table, with the second line in the WHERE clause (OR t1.flavor IS NULL)
SELECT DISTINCT t1.name, t1.flavor
FROM tablename t1
LEFT JOIN tablename t2
ON t2.flavor = t1.flavor AND t2.ID <> t1.ID
WHERE t2.flavor IS NULL
OR t1.flavor IS NULL
I hope this helps.
I have two tables like this:
Table Name: users
emx | userid
---------------
1 | 1
2 | 2
and another table called bodies
id | emx | text
--------------------------
1 | 1 | Hello
2 | 2 | How are you?
As you can see, bodies table has emx which is id numbers of users table. Now, when i want to fetch message that contains Hello i just search it on bodies and get the emx numbers and after that i fetch users table with these emx numbers. So, i am doing 2 sql queries to find it.
So, all i want to do is make this happen in 1 SQL query.
I tried some queries which is not correct and also i tried JOIN too. No luck yet. I just want to fetch users table with message contains 'Hello' in bodies table.
Note: I am using PostgreSQL 9.1.3.
Any idea / help is appreciated.
Read docs on how to join tables.
Try this:
SELECT u.emx, u.userid, b.id, b.text
FROM bodies b
JOIN users u USING (emx)
WHERE b.text ~ 'Hello';
This is how I'd do the join. I've left out the exact containment test.
SELECT users.userid
FROM users JOIN bodies ON (users.emx = bodies.emx)
WHERE ⌜true if bodies.text contains ?⌟
(I don't think I have titled this question correctly - but I don't know how to describe it)
Here is what I am trying to do:
Let's say I have a Person table that has a PersonID field. And let's say that a Person can belong to many Groups. So there is a Group table with a GroupID field and a GroupMembership table that is a many-to-many join between the two tables and the GroupMembership table has a PersonID field and a GroupID field. So far, it is a simple many to many join.
Given a list of GroupIDs I would like to be able to write a query that returns all of the people that are in ALL of those groups (not any one of those groups). And the query should be able to handle any number of GroupIDs. I would like to avoid dynamic SQL.
Is there some simple way of doing this that I am missing?
Thanks,
Corey
select person_id, count(*) from groupmembership
where group_id in ([your list of group ids])
group by person_id
having count(*) = [size of your list of group ids]
Edited: thank you dotjoe!
Basically you are looking for Persons for whom there is no group he is not a member of, so
select *
from Person p
where not exists (
select 1
from Group g
where not exists (
select 1
from GroupMembership gm
where gm.PersonID = p.ID
and gm.GroupID = g.ID
)
)
You're basically not going to avoid "dynamic" SQL in the sense of dynamically generating the query at query time. There's no way to hand a list around in SQL (well, there is, table variables, but getting them into the system from C# is either impossible (2005 & below) or else annoying (2008)).
One way that you could do it with multiple queries is to insert your list into a work table (probably a process-keyed table) and join against that table. The only other option would be to use a dynamic query such as the ones specified by Jonathan and hongliang.