This query tries to get information about a company and its parent company:
select c.*, p.*
from companies c, companies p
where c.parent_id = p.id and c.name ilike '%google%'
but this seems to return data from the parent company (the one specified latter) only, and is missing the c.*.
Perhaps the reason is that because this is a self-join, the second one overrides the first ones?
I'm using this via the Sequal gem.
What you observe is not what Postgres does for this query. It returns all columns of the table companies twice, once for each instance, effectively duplicating column names, which can be a problem for some clients that would expect unique column names.
db<>fiddle here
Related
I want to count how many people are applied for each category in my database and put all of those counts in one table. I don't have exact idea how should I do that. I've done it already like this but I want to have all in one query, and not to do it like this for each category.
select
count(cc.fk_id_candidates) as 'category A'
from candidate_category cc, candidate c, category cat
where c.id=cc.fk_id_candidates and cc.fk_id_category=cat.id and category.name='A';
From that code I get number of people applied for category A as an output, which is correct, but I just need the same info for other categories too. I tried with case but it's not working right.
Thank you.
You would typically add a group by clause on the category name and/or id:
select cat.category.name, count(*) cnt
from candidate_category cc
inner join candidate c on c.id = cc.fk_id_candidates and
inner join category cat on cc.fk_id_category = cat.id
group by cat.id, cat.category.name;
Note that I changed your query to use standard joins (with the on keyword) rather than implicit joins (with commas in the from clause and conditions in the where clause) - this old syntax should not be used in new code.
As i understand you have three tables:
- candidate_category cc,
- candidate c,
- category cat
Also, you have Created below relationship using the REFERENCES:
where c.id=cc.fk_id_candidates
and cc.fk_id_category=cat.id
and category.name='A;
Now, you must have all the three tables in one table already and you can view it by:
and this is very important step to find a common column where you can filter it and find the proper data you want.
Select *
from candidate_category cc,
candidate c,
category cat
where c.id=cc.fk_id_candidates
and cc.fk_id_category=cat.id
I'm working on enhancing a query for a DB2 database and I'm having some problems getting acceptable performance due to the number of joins across large tables that need to be performed to get all of the data and I'm hoping that there's a SQL function or technique that can simplify and speed up the process.
To break it down, let's say there are two tables: People and Groups. Groups contain multiple people, and a person can be part of multiple groups. It's a many-to-many, but bear with me. Basically, there's a subquery that will return a set of groups. From that, I can join to People (which requires additional joins across other tables) to get all of the people from those groups. However, I also need to know all of the groups that those people are in, which means joining back to the Groups table again (several more joins) to get a superset of the original subquery. There are additional joins in the query as well to get other pieces of relevant data, and the cost is adding up in a very ugly way. I also need to return information from both tables, so that rules out a number of techniques.
What I'd like to do is be able to start with the People table, join it to Groups, and then compare Groups with the subquery. If the Groups attached to one person has one match in the subquery, it should return ALL Group items associated with that person.
Essentially, let's say that Bob is part of Group A, B, and C. Currently, I start with groups, and let's say that only Group A comes out of the subquery. Then I join A to Bob, but then I have to come back and join Bob to Group again to get B and C. SQL example:
SELECT p.*, g2.*
FROM GROUP g
JOIN LINKA link
ON link.GROUPID = g.GROUPID
JOIN LINKB link1
ON link1.LISTID = link.LISTID
JOIN PERSON p
ON link1.PERSONID = p.PERSONID
JOIN LINKB link2
ON link2.PERSONID = p.PERSONID
JOIN LINKA link3
ON link2.LISTID = link3.LISTID
JOIN GROUP g2
ON link3.GROUPID = g2.GROUPID
WHERE
g.GROUPID IN (subquery)
Yes, the linking tables aren't ideal, but they're basically normalized tables containing additional information that is not relevant to the query I'm running. We have to start with a filtered Group set, join to People, then come back to get all of the Groups that the People are associated to.
What I'd like to do is start with People, join to Group, and if ANY Group that Bob is in returns from the subquery, ALL should be returned, so if we have Bob joined to A, B, and C, and A is in the subquery, it will return three rows of Bob to A, B, and C as there was at least one match. In this way, it could be treated as a one-to-many relationship if we're only concerned with the Groups for each Person and not the other way around. SQL example:
SELECT p.*, g.*
FROM PEOPLE p
JOIN LINKB link
ON link.PERSONID = p.PERSONID
JOIN LINKA link1
ON link.LISTID = link1.LISTID
JOIN GROUP g
ON link1.GROUPID = g.GROUPID
WHERE
--SQL function, expression, or other method to return
--all groups for any person who is part of any group contained in the subquery
The number of joins in the first query make it largely unusable as these are some pretty big tables. The second would be far more ideal if this sort of thing is possible.
From the question, I think you are querying hierarchical data. DB2 provides facility to deal with such data. There are two clauses Start with and Connect by in DB2 which will be useful. They are explained here.
So, I once again seem to have an issue with MS Access being finicky, although it seems to also be an issue when trying similar queries in SSMS (SQL Server Management Studio).
I have a collection of tables, loosely defined as follows:
table widget_mfg { id (int), name (nvarchar) }
table widget { id (int), name (nvarchar), mfg_id (int) }
table widget_component { id (int), name (nvarchar), widget_id (int), component_id }
table component { id (int), name (nvarchar), ... } -- There are ~25 columns in this table
What I'd like to do is query the database and get a list of all components that a specific manufacturer uses. I've tried some of these queries:
SELECT c.*, wc.widget_id, w.mfg_id
FROM ((widget_component wc INNER JOIN widget w ON wc.widget_id = w.id)
INNER JOIN widget_manufacturer wm on w.mfg_id = wm.id)
INNER JOIN component c on c.id = wc.component_id
WHERE wm.id = 1
The previous example displays duplicates of any part that is contained in multiple widget_component lists for different widgets.
I've also tried doing:
SELECT DISTINCT c.id, c.name, wc.widget_id, w.mfg_id
FROM component c, widget_component wc, widget w, widget_manufacturer wm
WHERE wm.id=w.mfg_id AND wm.id = 1
This doesn't display anything at all. I was reading about sub-queries, but I do not understand how they work or how they would apply to my current application.
Any assistance in this would be beneficial.
As an aside, I am not very good with either MS Access or SQL in general. I know the basics, but not a lot beyond that.
Edit:
I just tried this code, and it works to get all the component.id's while limiting them to a single entry each. How do I go about using the results of this to get a list of all the rest of the component data (component.*) where the id's from the first part are used to select this data?
SELECT DISTINCT c.part_no
FROM component c, widget w, widget_component wc, widget_manufacturer wm
WHERE(((c.id=wc.component_id AND wc.widget_id=w.id AND w.mfg_id=wm.id AND wm.id=1)))
(P.S. this is probably not the best way to do this, but I am still learning SQL.)
What I'd like to do is query the database and get a list of all
components that a specific manufacturer uses
There are several ways to do this. IN is probably the easiest to write
SELECT c.*
FROM component c
WHERE c.id IN (SELECT c.component_id
FROM widget w
INNER JOIN widget_component c
ON w.id = c.widget_id
WHERE w.mfg_id = 123)
The IN sub query finds all the component ids that a specific manufacturer uses. The outer query then selects any component.id that is that result. It doesn't matter if its in there once or 1000 times it will only get the component record once.
The other ways of doing this are using an EXISTS sub query or using a join to the query (but then you do need to de-dup it)
It sounds like your component -to- widget relationship is one-to-many. Hence the duplicates. (i.e., the same component is used by more than one widget).
Your Select is almost OK --
SELECT c.*, wc.widget_id, w.mfg_id
but the wc.widget_id is causing the duplicates (per the assumption above).
So remove wc.widget_id from the SELECT, or else aggregate it (min, max, count, etc.). Removing is easier. If you agregate, remember to add a group by clause.
Try this:
SELECT DISTINCT c.*, w.mfg_id
Also -- FWIW, it's generally a better practice to use field names, instead of the *
I have two tables. One called "members" and another called "homes" (should be household, but I suck in english). Those have a many-one relation (i.e. several members belong to one household). Those are linked together by members.homefk and homes.Id
Now, how can I find homes that don't belong to any members? I want this for house cleaning purposes.
SELECT homes.*
FROM homes
LEFT JOIN members ON (members.home_id = home.id)
WHERE members.home_id IS NULL
Use a subquery to return all the homefk values, then select from homes where id not in the subquery,
In Oracle would look something like
SELECT h.id
FROM homes h
WHERE h.id NOT IN(
SELECT DISTINCT(m.homefk)
FROM members m)
I'm trying to implement something similar to Ruby on Rails' polymorphic relationships.
I have the following three tables :
Events
Users
Organisations
An event can be owner by either a user or an organisation, so my Events table includes the columns: owner_type and owner_id.
I can easily list all events that belong to either users or organisations through an inner join and where clause, however, is there a way to make the join table conditional based on the value of the owner_type column, allowing all events to be listed together, regardless of owner_type?
I hope that makes sense.
Any advice appreciated.
Thanks.
You can't make the join table conditional, so in this case you would have to join events to both users and organisations and use coalesce to merge the common fields (eg. name) together.
select e.id, coalesce(u.name, o.name) owner_name
from events e
left join users u on e.owner_id = u.id and e.owner_type = 'user'
left join organisations o on e.owner_id = o.id and e.owner_type = 'org'
However, you may consider creating an owners table, which contains both users and organisations, with a structure like (id, type, org_id, name, ...). This would only require a single join, but may complicate other areas of your schema, eg. user membership of an organisation.
An alternative method would be to union the users and organisations tables together and then join once from events.
Owner has columns common to all owner-subtypes.
Person and Organization have columns specific to each one.
How about moving ownership information out of Events into two join tables, EventsUsers and EventsOrganisations (each of which has just two columns, FKs to Events and the apppropriate owning-object table) ? Then you can UNION two queries each of which joins through the join table to the owning-object table.
A bit old, but I think it would be useful, this version performs better in my scenario than multiple joins
select e.id,
case when e.owner_type = 'person' then
(
select p.name from person p
where p.id=e.owner_id
)
else
(
select o.name from organization o
where o.id=e.owner_id
)
end entityName,
e.owner_type
from events e
in postgres you could even build a json of entire related entity