I have read everything I could find on the web regarding JOIN and foreign key relationships, but can not seem to get my head around a solution to solve my problem.
As you can see in the relationship diagram image my CUSTOMER table have a foreign key relation with TASK_IN table.
TASK_IN table have a foreign key relation with TASK_ID table.
I want to retrieve all data from the TASK_ID table. Can this be done via the CUSTOMER and TASK_IN table foreign key relation - if yes, how would the SQL query look (PHP)?
SELECT * FROM task_id AS taskid INNER JOIN task_in AS taskin ON taskid.id = taskin.id INNER JOIN customer AS cust ON taskin.id = cust.id WHERE id = YOUR_VALUE_HERE
EDIT:
this is just basic sql syntax, you can read more here.
Related
What the title says. I've tried connecting two tables with two foreign keys referencing the same primary key and cannot get it to work.
Use table aliases. For example:
select *
from money_transfer t
join account s on t.sender_id = s.id
join account r on t.receiver_id = r.id
Here you can see two references to the same PK (account.id). The first time the table is given the alias s and the second time the same table is given the alias r. This way you can differentiate which one you want to use on each join predicate.
I have these tables:
tutors:
email firstname, lastname...
courses:
url tutorid description
reviews:
review courseid author
author in reviews and tutorid in courses is foreign key = tutors.email.
I need to delete all tutors that have 2 or more courses without a description.
I first tried to just select such tutors:
select tutors.email, COUNT(courses.url)
from courses
left join tutors on courses.tutorid = tutors.email
where
description is null group by (tutors.email);
this works fine. However I'm not sure how to delete the tutors with the given emails, considering the tutors.email is a foreign key in other tables.
If you don't delete the records in related tables that contains the foreign key, you will eventually create something called orphaned records, and that is something that goes against the referential integrity rule in general in RDBMS.
It is also possible that the RDBMS will enforce referential integrity, and you wont be able to do that before you delete all the associate records in the other tables first.
table 1 is called (athlete) and table2 is called (training_session.id) the primary key to table 1 is ID, and the table 2 has the primary key Athelete_id
I want to delete a person from my database by using his name, which I've called "Pet". However, he is also connected to another table which stores his training session. So (ID 1) on table 1 is connected to table 2 (athlete id1)
I struggle a lot, I try using INNER JOIN.
DELETE athlete,training_session FROM athlete
INNER JOIN
training_session ON training_session.id = athlete.name
WHERE
athlete.name = "Pet;
I have something wrong with my syntax, is it correct to use Inner Join or have I misunderstood
You should have set up foreign key constraints with Cascade deletions to simplify the logic and all you would have needed than was to delete from athlete. So I would suggest you add it.
For more info you can take a look at:
http://www.mysqltutorial.org/mysql-on-delete-cascade/
I do not understand MySQL delete when I need to delete data in a table with data from another table that depend on it.
For example if I want to delete a data in table 'factory', all data at table 'room' that depends on data at table 'factory is also deleted'.
Fac_ID is the primary key in 'factory' and foreign key in 'room'
below is my SQL code.
DELETE * FROM factory
LEFT JOIN room ON room.Fac_ID = factory.Fac_ID
WHERE factory.Fac_ID = :Fac_ID
Can any one help me?
I think you need a separate delete for this.
First is to delete foreign data
delete from room where Fac_ID = :Fac_ID
Then delete primary data
delete from factory where Fac_ID = :Fac_ID
Unless your table design is ON DELETE CASCADE (supported only in INNODB), you only need to delete the primary data
There is a small mistake in your query because of which I think you are facing problem.
As per my understanding you have some records in Main table and some records in refrenced table. There are some case for which main table has some id but there is not entry in refrence table for that id. And for handling that case you applied left join.
But in your query you wrote reference table on left so basically it is taking all of the record from reference table which is kind of inner join in this case.
So for correcting this you need to interchange the key id pass in your query or you may use right join to select all records from main table.
DELETE * FROM factory
LEFT JOIN room ON factory.Fac_ID = room.Fac_ID --- here you applied left join
WHERE factory.Fac_ID = :Fac_ID
MySQL allows you to delete rows from multiple tables at the same time. The syntax is:
DELETE f, r
FROM factory f LEFT JOIN
room r
ON r.Fac_ID = f.Fac_ID
WHERE f.Fac_ID = :Fac_ID;
However, this is better set up as a cascading delete foreign key relationship between the two tables.
I have four Database Tables like these:
Book
ID_Book |ID_Company|Description
BookExtension
ID_BookExtension | ID_Book| ID_Discount
Discount
ID_Discount | Description | ID_Company
Company
ID_Company | Description
Any BookExtension record via foreign keys points indirectly to two different ID_Company fields:
BookExtension.ID_Book references a Book record that contains a Book.ID_Company
BookExtension.ID_Discount references a Discount record that contains a Discount.ID_Company
Is it possible to enforce in Sql Server that any new record in BookExtension must have Book.ID_Company = Discount.ID_Company ?
In a nutshell I want that the following Query must return 0 record!
SELECT count(*) from BookExtension
INNER JOIN Book ON BookExstension.ID_Book = Book.ID_Book
INNER JOIN Discount ON BookExstension.ID_Discount = Discount.ID_Discount
WHERE Book.ID_Company <> Discount.ID_Company
or, in plain English:
I don't want that a BookExtension record references a Book record of a Company and a Discount record of another different Company!
Unless I've misunderstood your intent, the general form of the SQL statement you'd use is
ALTER TABLE FooExtension
ADD CONSTRAINT your-constraint-name
CHECK (ID_Foo = ID_Bar);
That assumes existing data already conforms to the new constraint. If existing data doesn't conform, you can either fix the data (assuming it needs fixing), or you can limit the scope (probably) of the new constraint by also checking the value of ID_FooExtension. (Assuming you can identify "new" rows by the value of ID_FooExtension.)
Later . . .
Thanks, I did indeed misunderstand your situation.
As far as I know, you can't enforce that constraint the way you want to in SQL Server, because it doesn't allow SELECT queries within a CHECK constraint. (I might be wrong about that in SQL Server 2008.) A common workaround is to wrap a SELECT query in a function, and call the function, but that's not reliable according to what I've learned.
You can do this, though.
Create a UNIQUE constraint on Book
(ID_Book, ID_Company). Part of it will look like UNIQUE (ID_Book, ID_Company).
Create a UNIQUE constraint on Discount (ID_Discount, ID_Company).
Add two columns to
BookExtension--Book_ID_Company and
Discount_ID_Company.
Populate those new columns.
Change the foreign key constraints
in BookExtension. You want
BookExtension (ID_Book,
Book_ID_Company) to reference
Book (ID_Book, ID_Company). Similar change for the foreign key
referencing Discount.
Now you can add a check constraint to guarantee that BookExtension.Book_ID_Company is the same as BookExtension.Discount_ID_Company.
I'm not sure how [in]efficient this would be but you could also use an indexed view to achieve this. It needs a helper table with 2 rows as CTEs and UNION are not allowed in indexed views.
CREATE TABLE dbo.TwoNums
(
Num int primary key
)
INSERT INTO TwoNums SELECT 1 UNION ALL SELECT 2
Then the view definition
CREATE VIEW dbo.ConstraintView
WITH SCHEMABINDING
AS
SELECT 1 AS Col FROM dbo.BookExtension
INNER JOIN dbo.Book ON dbo.BookExtension.ID_Book = Book.ID_Book
INNER JOIN dbo.Discount ON dbo.BookExtension.ID_Discount = Discount.ID_Discount
INNER JOIN dbo.TwoNums ON Num = Num
WHERE dbo.Book.ID_Company <> dbo.Discount.ID_Company
And a unique index on the View
CREATE UNIQUE CLUSTERED INDEX [uix] ON [dbo].[ConstraintView]([Col] ASC)