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/
Related
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 a address table which is referenced from 6 other tables (sometimes multiple tables). Some of those tables have around half a million records (and the address table around 750000 records). I want to have a periodical query running which deletes all records that are not referenced from any of the tables.
The following sub-queries is not a option, because the query never finishes - the scope is too big.
delete from address where address_id not in (select ...)
and not in (select ...) and not in (select ...) ...
What I was hoping was that I could use the foreign key constraint and I could simply delete all records for which the foreign key constraint does not stop me (because there is no reference to the table). I could not find a way to do this (or is there?). Anybody another good idea to tackle this problem?
You can try this ways
DELETE
address
FROM
address
LEFT JOIN other_table ON (address.id = other_table.ref_field)
LEFT JOIN other_table ON (address.id = other_table2.ref_field)
WHERE
other_table.id IS NULL AND other_table2.id IS NULL
OR
DELETE
FROM address A
WHERE NOT EXISTS (
SELECT 1
FROM other_table B
WHERE B.a_key = A.id
)
I always use this:
DELETE FROM table WHERE id NOT IN (SELECT id FROM OTHER table)
I'd do this by first creating a TEMPORARY TABLE (t) that is a UNION of the IDs in the 6 referencing tables, then run:
DELETE x FROM x LEFT JOIN t USING (ID) WHERE x.ID IS NULL;
Where x is the address table.
See 'Multiple-table syntax' here:
http://dev.mysql.com/doc/refman/5.0/en/delete.html
Obviously, your temporary table should have its PRIMARY KEY on ID. It may take some time to query and join, but I can't see a way round it. It should be optimized, unlike the multiple sub-query version.
It is not so simple as it is written in the title.
I have a table, for binding user accounts together.
For example: a person signs up via standard site functionality. On the next day he signs up via the Facebook account.
I have user_user table with a_user_id, b_user_id fields. These fields are foreign keys to the standard user table.
I make unique key for 2 fields.
But. How to avoid the situation like this one:
a_user_id: 1
b_user_id : 2
and the next row:
a_user_id: 2
b_user_id : 1
Both rows mean the same. Can MySQL handle this?
Are there any better approaches for this case?
Introduce the constraint a_user_id < b_user_id. Honestly said, I till now always held this constraint on the programming side, This prevents cycles: self references and a-b-a.
First one would need to normalize the table, probably using a temporary table in MySQL.
Normalisation of the old table:
Assuming a table u2(a_user_id NOT NULL, b_user_id NOT NULL):
DELETE FROM u2
WHERE a_user_id = b_user_id;
ALTER TABLE u2 ADD COLUMN other ...;
UPDATE u2
SET
other = a_user_id,
a_user_id = b_user_id,
b_user_id = other
WHERE
a_user_id > b_user_id;
ALTER TABLE u2 REMOVE COLUMN other;
(Unchecked syntax)
I ve got a database modelize like this,
One mother table let's call it table_mother, and severals child tables.
Relation beetween table_mother and childs is like this:
All tables childs have a foreign key kind of name as the id of the mother table (id_table_mother) (relationship is 1->n as id_table_mother is uniq and tbale child can get several entries for id_table_mother)
I would like to delete all records in childs table wich are related no more with the mother table, for now i try something like this
DELETE FROM tb_child_1,tb_child_2,tb_child_3
WHERE
tb_child_1.id_table_mother
AND tb_child_2.id_table_mother
AND tb_child_3.id_table_mother
NOT IN (SELECT id_table_mother FROM tb_table_mother);
Thx
edit : this is how I ended for now
delete from tb_child_1 where id_mother not in (select id_mother from tb_mother_table);
delete from tb_child_2 where id_mother not in (select id_mother from tb_mother_table);
any "global" solution ?
also my database is not innodb so I can 't go with foreigh keys and stuff
You have to build FOREIGN KEY Constraints to be performed on delete or update to know more about FOREIGN KEY Constraints visit http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
Write 3 queries like this one -
DELETE
tb_child_1
FROM
tb_table_mother
LEFT JOIN
tb_child_1
ON tb_table_mother.id_table_mother = tb_child_1.id_table_mother
WHERE
tb_child_1.id_table_mother IS NULL;
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)