i have this sqlite table:
CREATE TABLE frames (videomd5 TEXT, framemd5 TEXT, type TEXT, PRIMARY KEY (videomd5, framemd5))
As you can see, the table has a combined PRIMARY KEY because it is allowed that one of the fields has the same values but never both at once.
Currently I'm performing a check like this
SELECT framemd5 FROM frames WHERE framemd5='$digest' AND videomd5='$videomd5'
before adding something to the table to avoid a PRIMARY KEY CONTRAINTs but i feel there is a better way to handle it. Should i fire the INSERT without checking first and handel the CONSTRAINT afterwards? If so, how is this best done in perl?
Thank you
Yes you can use the "OR IGNORE" in your insert, eg:
sqlite> create table bla (id INT PRIMARY KEY);
sqlite> insert into bla values (1);
sqlite> insert into bla values (1);
Error: column id is not unique
sqlite> insert or ignore into bla values (1);
sqlite>
Refer to the official doc for the details:
http://www.sqlite.org/syntaxdiagrams.html#insert-stmt
As you said yourself - there are two ways, do some benchmarks or select the one which matches your coding style.
I prefer first approach, though, than handling primary key violation exceptions
Related
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.
This question already has answers here:
PostgreSQL foreign key not existing, issue of inheritance?
(2 answers)
Closed 8 years ago.
create table abstract_addresses (
address_id int primary key
);
create table phone_numbers (
phone_number text not null unique
) inherits (abstract_addresses) ;
create table contacts (
name text primary key,
address_id int not null references abstract_addresses(address_id)
);
insert into phone_numbers values (1, '18005551212'); --works
select * from abstract_addresses;
address_id
1
select * from phone_numbers;
address_id phone_number
1 18005551212
insert into contacts values ('Neil', 1); --error
I get this error message:
ERROR: insert or update on table "contacts" violates foreign key constraint "contacts_address_id_fkey"
SQL state: 23503
Detail: Key (address_id)=(1) is not present in table "abstract_addresses".
Just a bad use-case for postgresql table inheritance?
Per the caveats in the docs:
A serious limitation of the inheritance feature is that indexes (including unique constraints) and foreign key constraints only apply to single tables, not to their inheritance children. This is true on both the referencing and referenced sides of a foreign key constraint.
http://www.postgresql.org/docs/current/static/ddl-inherit.html
To do what you want:
Create a table with only an id — like you did.
Don't use inheritance. Really don't. It's useful to partition log tables; not for what you're doing.
Make phone number ids default to nextval('abstract_addresses_address_id_seq'), or whatever the sequence name is.
Add a foreign key in phone_numbers referencing abstract_addresses (address_id). Make it deferrable, initially deferred.
Add an after insert trigger on phone_numbers that inserts a new row in abstract_addresses when needed.
If appropriate, add an after delete trigger on phone_numbers that cascade deletes abstract_addresses — make sure it occurs after the delete, else affected rows will report incorrect values when you delete from phone_numbers.
That way, you'll have an abstract_address for use in occasional tables that need such a thing, while still being able to have a hard reference to phone_numbers where the latter is what you actually want.
One caveat to be aware of: it doesn't play well with ORMs.
I would like to know how to use trim function when declaring primary key in db2.
For example, in table Employee(name,id) how to restrict the usage of these two insert statements:
insert into Employee(name,id) values (jay,1);
insert into Employee(name,id) values (jay,1);
In the above statements both name and id are primary keys.
maybe you mean unique? if so, well you can easily solve this problem, all you have to do, is to create a primary key on bouth fields ;-)
something like this:
alter table Employee add constraint Employee_pk primary key (name, id)
I have a table like:
CREATE TABLE test(
id integer not null default nextval('test_id_seq'::regclass),
client_name_id integer not null
);
Foreign-key constraints:
"test_client_name_id_fkey" FOREIGN KEY (client_name_id) REFERENCES company(id) DEFERRABLE INITIALLY DEFERRED
and company table:
CREATE TABLE company(
id integer not null default nextval('company_id_seq'::regclass),
company_name character varying(64) not null
)
Now I have trigger on test table which fetch id from company table using provided value client_name_id which is string by matching it with company_name. but when I insert record PostgreSQL return error that client_name_id is string and int required which is true.
How can I tell PostgreSQL not to verify inserted row as I have taken care of it in my triggers.
What you are trying to do is very unorthodox. Are you sure, this is what you want? Of course, you cant enter a string (with non-digits) into an integer column. No surprise there, right? If you want to enter the text instead, you'd have to add a text column instead - with a fk-constraint to company(company_name) if you want to match your current layout.
ALTER TABLE test ALTER DROP COLUMN client_name_id; -- drops fk constraint, too
ALTER TABLE test ADD COLUMN client_name REFERENCES company(company_name);
You would need a UNIQUE constraint on company.company_name to allow this.
However, I would advise to rethink your approach. Your table layout seems proper as it is. The trigger is the unconventional element. Normally, you would reference the primary key, just like you have it now. No trigger needed. To get the company name, you would join the table in a SELECT:
SELECT *
FROM test t
JOIN company c ON t.client_name_id = c.id;
Also, these non-standard modifiers should only be there if you need them: DEFERRABLE INITIALLY DEFERRED. Like, when you have to enter values in table test before you enter the referenced values in table company (in the same transaction).
I've got two tables - one is Product and one is ProductSearchResult.
Whenever someone tries to Insert a SearchResult with a product that is not listed in the Product table the foreign key constrain is violattet, hence i get an error.
I would like to know how i could get my database to automatically create that missing Product in the Product Table (Just the ProductID, all other attributes can be left blank)
Is there such thing as CASCADE ON INSERT? If there is, i was not able not get it working.
Rules are getting executed after the Insert, so because we get an Error beforehand there are useless if you USE an "DO ALSO". If you use "DO INSTEAD" and add the INSERT Command at the End you end up with endless recursion.
I reckon a Trigger is the way to go - but all my attempts to write one failed.
Any recommendations?
The Table Structure:
CREATE TABLE Product (
ID char(10) PRIMARY KEY,
Title varchar(150),
Manufacturer varchar(80),
Category smallint,
FOREIGN KEY(Category) REFERENCES Category(ID) ON DELETE CASCADE);
CREATE TABLE ProductSearchResult (
SearchTermID smallint NOT NULL,
ProductID char(10) NOT NULL,
DateFirstListed date NOT NULL DEFAULT current_date,
DateLastListed date NOT NULL DEFAULT current_date,
PRIMARY KEY (SearchTermID,ProductID),
FOREIGN KEY (SearchTermID) REFERENCES SearchTerm(ID) ON DELETE CASCADE,
FOREIGN KEY (ProductID) REFERENCES Product ON DELETE CASCADE);
Yes, triggers are the way to go. But before you can start to use triggers in plpgsql, you
have to enable the language. As user postgres, run the command createlang with the proper parameters.
Once you've done that, you have to
Write function in plpgsql
create a trigger to invoke that function
See example 39-3 for a basic example.
Note that a function body in Postgres is a string, with a special quoting mechanism: 2 dollar signs with an optional word in between them, as the quotes. (The word allows you to quote other similar quotes.)
Also note that you can reuse a trigger procedure for multiple tables, as long as they have the columns your procedure uses.
So the function has to
check if the value of NEW.ProductID exists in the ProductSearchResult table, with a select statement (you ought to be able to use SELECT count(*) ... INTO someint, or SELECT EXISTS(...) INTO somebool)
if not, insert a new row in that table
If you still get stuck, come back here.
In any case (rules OR triggers) the insert needs to create a new key (and new values for the attributes) in the products table. In most cases, this implies that a (serial,sequence) surrogate primary key should be used in the products table, and that the "real world" product_id ("product number") should default to NULL, and be degraded to a candidate key.
BTW: a rule can be used, rules just are tricky to implement correctly for N:1 relations (they need the same kind of EXISTS-logic as in Bart's answer above).
Maybe cascading on INSERT is not such a good idea after all. What do you want to happen if someone inserts a ProductSearchResult record for a not-existing product? [IMO a FK is always a domain; you cannot just extend a domain just by referring to a not-existant value for it; that would make the FK constraint meaningless]