hai guys,
i am planning to create a table view and the detail view for the corresponding row, all the datas are fetch from the sqlite database. For this i have created two tables in the database table one for the datas in the table view list and table two for the detail view of the table list. i set the product id as the primary key for the table one. i want the product id as the foreign key for table two. i don't know how to set the foreign key for it and don't know how to retrieve from the table two and display it in the detail view. please help me to do this.
Thanks in advance...
1.Change your DB Settings enable to Foreign Keys.
2.Create the child table like this
table1 is the parent table having id1 as primary key.
CREATE TABLE "table1" ("id1" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL)
table2 is the child table having id2 as a foreign key with reference to id1 of table1.
CREATE TABLE table2 (
id2 INTEGER,
parent_id INTEGER,
description TEXT,
FOREIGN KEY (id2) REFERENCES table1(id1)
)
Use equijoin to retrieve the data from the tables.
select * from table1,table2 where table1.id1=table2.id2;
or
select table2.* from table2,table1 where table1.id1=table2.id2;
to retrieve the data from single table alone.
Hope this helps to you!
Related
Currently, I have the following 3 tables.
CREATE TABLE customer (
id SERIAL PRIMARY KEY
);
CREATE TABLE google_subscription (
fk_customer_id INTEGER NOT NULL UNIQUE,
CONSTRAINT fk_customer_id_constraint
FOREIGN KEY(fk_customer_id)
REFERENCES customer(id)
ON DELETE RESTRICT
);
CREATE TABLE apple_subscription (
fk_customer_id INTEGER NOT NULL UNIQUE,
CONSTRAINT fk_customer_id_constraint
FOREIGN KEY(fk_customer_id)
REFERENCES customer(id)
ON DELETE RESTRICT
);
google_subscription is having fk_customer_id referencing to customer table id.
apple_subscription is having fk_customer_id referencing to customer table id.
I was wondering, is it ever possible to create a constraint, such that customer table id, will only be found in either google_subscription or apple_subscription, but NOT both?
No that is not possible. A constraint cannot span across multiple tables. Instead you can have another table subscriptions where you can have a unique index on customer(id). Or you can have another column in customer table which will hold only 1 subscription at a time.
I created a table in PostgreSQL with a foreign key constraint.
I dropped the table to which the foreign key belongs. Now how to alter the table or how to defer the foreign key present in the table?
To clarify:
I have a table named test. It has a column called subjectName, which is a foreign key of subject Table. Now I dropped subject table. How to remove the FK constaint on table test
Assuming the following tables:
create table subject
(
name varchar(10) primary key
);
create table test
(
some_column integer,
subject_name varchar(10) not null references subject
);
there are two scenarios what could have happened when you dropped the table subject:
1. you didn't actually drop it:
drop table subject;
ERROR: cannot drop table subject because other objects depend on it
Detail: constraint test_subject_name_fkey on table test depends on table subject
Hint: Use DROP ... CASCADE to drop the dependent objects too.
2. you did drop it, then the foreign key is gone as well.
drop table subject cascade;
NOTICE: drop cascades to constraint test_subject_name_fkey on table test
which tells you that the foreign key constraint was automatically dropped.
Perhaps your question in not exactly what you mean. Are you wanting to remove the which was a foreign key from the table. As amply indicated if you dropped the parent table then the FK is also dropped. However the column itself is not dropped from the child table. To remove that you need to alter the table.
alter table test drop column subject_name;
See demo here
I have a postgres table (lets call this table Events) with a composite foreign key to another table (lets call this table Logs). The Events table looks like this:
CREATE TABLE Events (
ColPrimary UUID,
ColA VARCHAR(50),
ColB VARCHAR(50),
ColC VARCHAR(50),
PRIMARY KEY (ColPrimary),
FOREIGN KEY (ColA, ColB, ColC) REFERENCES Logs(ColA, ColB, ColC)
);
In this case, I know that I can efficiently search for Events by the primary key, and join to Logs.
What I am interested in is if this foreign key creates an index on the Events table which can be useful even without joining. For example, would the following query benefit from the FK?
SELECT * FROM Events
WHERE ColA='foo' AND ColB='bar'
Note: I have run the POSTGRES EXPLAIN for a very similar case to this, and see that the query will result in a full table scan. I am not sure if this is because the FK is not helpful for this query, or if my data size is small and a scan is more efficient at my current scale.
PostgreSQL does not automatically create an index on the columns on which a foreign key is defined. If you need such an index, you will have to create it yourself.
It is usually a good idea to have such an index, so that modifications on the parent table that affect the referenced columns are efficient.
I'm trying to add a foreign key constraint to an existing table:
ALTER TABLE address
ADD FOREIGN KEY(company_id)
REFERENCES company(company_id) ON DELETE CASCADE DEFERRABLE;
That fails with:
ERROR: insert or update on table "address" violates \
foreign key constraint "address_company_id_fkey"
DETAIL: Key (company_id)=(83376) is not present in table "company".
Yet the company table does have that key:
DB=> SELECT company_id FROM company WHERE company_id = 83376;
company_id
------------
83376
(1 row)
I suspect that this is due to table inheritance (old database, very historic reasons), company is a base table and one derived table is the person table. Which is the one containing the actual key:
DB=> SELECT company_id FROM person WHERE company_id = 83376;
company_id
------------
83376
(1 row)
I'm specifically targeting the base table (assuming it contains the data of all derived tables) because the address rows refer to different derived tables.
Is there a way to make that work?
Or as an alternative, kinda even better, is there a way to have foreign keys targeting specific derived tables?
Is it possible to allow two optional foreign key?
For example:
table a (id, nameA)
table b(id nameB)
table c(id ,name)
that the name in table c is a foreign key
and it can contracts or the id from table a or the id from table b.
If its possible, what the way to create it?