SQLite3 user.
I have read thru numerous books on relational DBs and SQL and not one shows how to maintain the linking tables for many-to-many relationships. I just went through a book that went into the details of SELECT and JOINS with examples, but then glosses over the same when many-to-many relationships are covered. The author just showed some pseudo code for a table, no data, and then a pseudo code query--WTF? I am probably missing something, but it has become quite maddening.
Anyways, say I have a table like [People] with 3 columns: pID (primary), name and age. A table [Groups] with 3 columns: gID (primary), groupname and years. Since people can belong to multiple groups and groups can have multiple people, I set up a linking table called [peoplegroups] with two columns: pID, and gID both of which come from their respective tables.
So ,how do I efficiently get data into the linking table when INSERTING on the others and how do I get data out using the linking table?
Example: I want to INSERT "Jane" into [people] and make her a member of group gID 2, "bowlers" and update the linking table {peoplegroups] at the same time.
Later I want to go back and pull out a list of all of the bowlers or all the groups a person is part of.
If you already don't use primary and foreign keys (which you should!) I think you may need to consider using triggers in your design as well? So if you have a specific set of rules (e.g. if you want to create Jane with id = 1 and choose existing group 2, then after insert jane into people automatically create an entry pair personid=1,groupid=2 in the table peoplegroups. You can also create views with specific selects to see the data you want, for example if you want a query where you only show the peoples names and groups names you could create a view 'PeopleView':
SELECT P.PersonName, G.GroupName
FROM People P
INNER JOIN PeopleGroup PG ON P.PersonID = PG.PersonID
INNER JOIN Group G ON G.GroupId = PG.GroupID
then you can query 'PeopleView' saying
SELECT * FROM PeopleView WHERE GroupName = 'bowlers'
When inserting new data into the tables mentioned, the "linking" table that you are referring to needs to contain both primary keys from the other tables as foreign keys. So basically The [People] tables (pID) and the [Groups] table (gID) should both be foreign keys in the [PeopleGroups] table. In order to create a new "link" in [PeopleGroups] the record has to already exist in the [People] table as well as the [Groups] table BEFORE you try and create the link in the [PeopleGroups] table. I hope this helps
Related
How to remove duplicate values a = b and b = a?
with a as(select w.id , w.doc, w.org
, d.name_s, d.name_f, d.name_p, d.spec
, o.name, o.extid
from crm_s_workplaces w
join crm_s_docs d on d.id=w.doc
join crm_s_orgs o on o.id=w.org
where d.active=1 and d.cst='NY' and w.active=1 and w.cst='NY' and o.active=1
and
o.cst='NY')
select a1.doc, a2.doc,
a1.org,a1.name_s,a1.name_f,a1.name_p,a2.name_s,a2.name_f,a2.name_p from a a1
join a a2 on
a1.name_s=a2.name_s and
substr(a1.name_f,1,1)=substr(a2.name_f,1,1) and
substr(a1.name_p,1,1)=substr(a2.name_p,1,1) and
a1.org=a2.org and
a1.spec<>a2.spec
order by a1.name_s `enter code here`
ER model diagram:
Repeat example:
Sometimes comes across a1.spec > a2.spec:
What you are calling "duplicates" are actually not duplicates in your database.
You basically have multiple doc records for what could be the same person or not. See that even their names do not always match. For instace,
doc_id 1131385 has NAME_F = "Gabr" while
doc_id 1447530 has NAME_F = "Gabor"
In your database these are two different entities, and you cannot match them using primary key. You can try to join on the first, middle and last names, but as you can see in the above example with Gabor/Gabr, even that would not work.
Can you change the schema of the db? If so you need to separate the docs in one table - 1 record per doctor. And have the specialization in a separate table with the folloing columns:
spec_id (int, PK)
doc_id (foreign key to Doc table)
specialization
that way, if you have 1 doctor with 3 specs, he/she will show up only once in doc table, and multiple times in spec table.
I just notice something else. You have spec field in table workplaces. why? If you meant to say that Doc Gabor works as admin in hospital 1 but as a Therapist in hospital 2, you can do that. However, you have to remove the spec field from the doc table and only use the spec in workplaces table.
I have two tables named "users" and "userReviews". I want to fetch all the reviews of a specific user. but i don't know how my two foreign keys "reviewForUserId" and "reviewFromUserId" will give me the exact user's "fullName". i have attached photos of my tables
It seems like you want to join users twice. Once for reviewforuserid and once for reviewfromuserid from userreviews.
SELECT *
FROM userreviews ur
INNER JOIN users ufor
ON ufor.userid = ur.reviewforuserid
INNER JOIN users ufro
ON ufro.userid = ur.reviewfromuserid;
And next time please don't post images. Use CREATE TABLE and INSERT INTO statements to show how your tables look like.
In MS Access, I am trying to implement a many-to-many table that will store 2-way relationships, similar to Association between two entries in SQL table. This table stores info such as "Person A and Person B are coworkers" "C and D are friends", etc. The table is like this:
ConstitRelationships
LeftId (number, primary key, foreign key to Constituents.ConstitId)
RightId (number, primary key, foreign key to Constituents.ConstitId)
Description (text)
Note that the primary key is a composite of the two Id fields.
Also the table has constraints:
[LeftId]<>[RightId] AND [LeftId]<[RightId]
The table is working ok in my Access project, except that I cannot figure out how to make an updateable query that I want to use as a datasheet subform so users can easily add/delete records and change the descriptions. I currently have a non-updatable query:
SELECT Constituents.ConstituentId, Constituents.FirstName,
Constituents.MiddleName, Constituents.LastName,
ConstitRelationships.Description, ConstitRelationships.LeftId,
ConstitRelationships.RightId
FROM ConstitRelationships INNER JOIN Constituents ON
(Constituents.ConstituentId =
ConstitRelationships.RightId) OR (Constituents.ConstituentId =
ConstitRelationships.LeftId);
If I ignore the possibility that the constituentId I want is in the leftId column, I can do this, which is updatable. So the OR condition in the inner join above is what's messing it up.
SELECT Constituents.ConstituentId, Constituents.FirstName,
Constituents.MiddleName, Constituents.LastName,
ConstitRelationships.Description, ConstitRelationships.LeftId,
ConstitRelationships.RightId
FROM ConstitRelationships INNER JOIN Constituents ON
(Constituents.ConstituentId =
ConstitRelationships.RightId) ;
I also tried this wacky iif thing to collapse the two LeftId and RightId fields into FriendId, but it was not updateable either.
SELECT Constituents.ConstituentId, Constituents.FirstName,
Constituents.MiddleName,
Constituents.LastName, subQ.Description
FROM Constituents
INNER JOIN (
SELECT Description, Iif([Forms]![Constituents Form]![ConstituentId] <>
ConstitRelationships.LeftId, ConstitRelationships.LeftId,
ConstitRelationships.RightId) AS FriendId
FROM ConstitRelationships
WHERE ([Forms]![Constituents Form]![ConstituentId] =
ConstitRelationships.RightId)
OR ([Forms]![Constituents Form]![ConstituentId] =
ConstitRelationships.LeftId)
) subQ
ON (subQ.FriendId = Constituents.ConstituentId)
;
How can I make an updatable query on ConstitRelationships, including a JOIN with the Constituent.FirstName MiddleName LastName fields?
I am afraid that is not possible. Because you use joins in your query over three tables it is not updatable. There is no way around this.
Here some detailed information about the topic: http://www.fmsinc.com/Microsoftaccess/query/non-updateable/index.html
As mentioned in the linked article one possible solution and in my opinion best solution for you would be the temporary table. It is a load of work compared to the easy "bind-form-to-a-query"-approach but it works best.
The alternative would be to alter your datascheme in that way that you do not need joins. But then denormalized data and duplicates would go rampage which makes the temporary table a favorable choice.
I have 2 tables with a many-to-any relationship. For the example we will call the tables "Guys" and Girls" There is a junction table that contains the related primary keys...who has dated who.
If I want to find all the girls that Guy 1 has dated, I do a select on the junction table selecting all girls with guys.ID. This give me a RecordSet. Now to find the names of the girls, I need to select from the girls table a row using the key from each RecordSet row.
Isn't there an easier way? Since I've defined the relationships in Access I would think that there must be a way to build a single query. How do I do that?
SELECT girls.name
FROM (guys
INNER JOIN junct ON guys.guyID = junct.guyID)
INNER JOIN girls ON junct.girlID = girls.girlID
WHERE guys.guyID = [whatever id you're looking for]
I use three tables to get to the final result. They are called project_board_members, users and project_team.
This is the query:
SELECT `project_board_members`.`member_id`,
`users`.`name`,
`users`.`surname`,
`users`.`country`,
`project_team`.`tasks_completed`
FROM `project_board_members`
JOIN `users`
ON (`users`.`id` = `project_board_members`.`member_id`)
JOIN `project_team`
ON (`project_team`.`user_id` = `project_board_members`.`member_id`)
WHERE `project_board_members`.`project_id` = '5'
You can ignore last line because it just points to the project I'm using.
Table project_board_members holds three entries and have structure like:
id,
member_id,
project_id,
created_at;
I need to get member_id from that table. Then I join to users table to get name, surname and country. No problems. All works! :)
After that, I needed to get tasks_completed for each user. That is stored in project_team table. The big unexpected thing is that I got four entries returned and the big what-the-f*ck is that in the project_board_members table are only three entries.
Why is that so? Thanks in advice!
A SQL join creates a result set that contains one row for each combination of the left and right tables that matches the join conditions. Without seeing the data or a little more information it's hard to say what exactly is wrong from what you expect, but I'm guessing it's one of the following:
1) You have two entries in project_team with the same user_id.
2) Your entries in project_team store both user_id and project_id and you need to be joining on both of them rather than just user_id.
The table project_board_members represent what is called in the Entity-Relationship modelling world an "associative entity". It exists to implement a many-to-many relationship (in this case, between the project and user entities. As such it is a dependent entity, which is to say that the existence of an instance of it is predicated on the existence of an instance of each of the entities to which it refers (a user and a project).
As a result, the columnns comprising the foreign keys relating to those entities (member_id and project_id) must be form part or all of the primary key.
Normally, instances of an associative entity are unique WRT the entities to which it relates. In your case the relationship definitions would be:
Each user is seated on the board of 0-to-many projects;
Each project's board is comprise of 0-to-many users
which is to say that a particular user may not be on the board of a particular project more than once. The only reason for adding other columns (such as your id column) to the primary key would be if the user:project relationship is non-unique.
To enforce this rule -- a user may sit on the board a particular project just once -- the table schema should look like this:
create table project_board_member
(
member_id int not null foreign key references user ( user_id ) ,
project_Id int not null foreign key references project ( project_id ) ,
created_at ...
...
primary key ( member_id , project_id ) ,
)
}
The id column is superfluous.
For debugging purposes do
SELECT GROUP_CONCAT(pbm.member_id) AS member_ids,
GROUP_CONCAT(u.name) as names,
GROUP_CONCAT(u.surname) as surnames,
GROUP_CONCAT(u.country) as countries,
GROUP_CONCAT(pt.tasks_completed) as tasks
FROM project_board_members pbm
JOIN users u
ON (u.id = pbm.member_id)
JOIN project_team pt
ON (pt.user_id = pbm.member_id)
WHERE pbm.project_id = '5'
GROUP BY pbm.member_id
All the fields that list multiple entries in the result are messing up the rowcount in your resultset.
To Fix that you can do:
SELECT pbm.member_id
u.name,
u.surname,
u.country,
pt.tasks_completed
FROM (SELECT
p.project_id, p.member_id
FROM project_board_members p
WHERE p.project_id = '5'
LIMIT 1
) AS pbm
JOIN users u
ON (u.id = pbm.member_id)
JOIN project_team pt
ON (pt.user_id = pbm.member_id)