So I'm learning some SQL as a bit of a side project since my SQL sucks. I have the following table I created:
CREATE TABLE deliveries (
pid INTEGER,
FOREIGN KEY (pid) REFERENCES person_lives_at,
);
I want to alter it to be a table like this one:
CREATE TABLE deliveries (
pid INTEGER,
FOREIGN KEY (pid) REFERENCES employee,
);
How can I achieve that? I'm using Oracle SQL Developer
Since you created an FK constraint without a name, oracle assigned a system-generated name to the constraint like SYS_xxxxxx. To find the constraint name:
select constraint_name from all_Constraints
where table_name = 'DELIVERIES'
In my test case, it returned "SYS_C0016779". I then drop the constraint:
alter table deliveries drop constraint SYS_C0016779
Then add the new constraint:
ALTER TABLE deliveries
ADD CONSTRAINT PID_EMP_FK -- or whatever you want to call it.
FOREIGN KEY (pid)
REFERENCES employee(pid); -- or whatever the name of the column is
Related
I am trying to change the primary key of one of my tables from a simple to a composite key, where the composite should be composed of the old pk column and a newly created one.
I followed along this answer from a very similar question: https://stackoverflow.com/a/27832197/1948454
It almost works, except that there is no value set in the dependant table for the new column.
Here's the situation:
Suppose I have a table for a catalog, and a table for catalog entries. Before:
-- DDL Catalog
CREATE TABLE public.Catalog (
name_ VARCHAR(255) NOT NULL,
foo_ VARCHAR(255) NULL,
CONSTRAINT Catalog_pkey PRIMARY KEY (name_)
);
-- DDL CatalogEntry
CREATE TABLE public.CatalogEntry (
pricekey_ VARCHAR(255) NOT NULL,
pricekeyroot_ VARCHAR(255) NOT NULL,
catalog_name_ VARCHAR(255) NULL,
bar_ VARCHAR(255) NULL,
CONSTRAINT CatalogEntry_pkey PRIMARY KEY (pricekey_, pricekeyroot_)
);
-- public.CatalogEntry FOREIGN KEYs
ALTER TABLE public.CatalogEntry ADD CONSTRAINT CatalogEntry_catalog_name__fkey FOREIGN KEY (catalog_name_) REFERENCES Catalog(name_) ON DELETE CASCADE;
So CatalogEntry.catalog_name references to Catalog.name_.
Now I need to add another column version_ to the Catalog table, indicating a version of some catalog. This means I will have to create a new composite pk composed of name_ and version_. Here's my script:
-- UPDATE script
-- add the new version column and set all values to default of 1
ALTER TABLE Catalog ADD version_ INTEGER;
UPDATE Catalog SET version_ = 1;
ALTER TABLE Catalog ALTER column version_ SET not null;
-- update primary key and foreign key
BEGIN;
-- first, drop fkey constraint on CatalogEntry
ALTER TABLE CatalogEntry DROP CONSTRAINT CatalogEntry_catalog_name__fkey;
-- then, update Catalog primary key
ALTER TABLE Catalog DROP CONSTRAINT Catalog_pkey,
ADD CONSTRAINT Catalog_uni_name UNIQUE (name_),
ADD PRIMARY KEY (name_, version_);
-- now add new foreign key again to CatalogEntry
ALTER TABLE CatalogEntry ADD catalog_version_ INTEGER;
ALTER TABLE CatalogEntry
ADD CONSTRAINT CatalogEntry_catalog_name__catalog__fkey FOREIGN KEY (catalog_name_, catalog_version_)
references Catalog(name_, version_ ) ON DELETE CASCADE;
COMMIT;
-- finally, remove unique constraint on name since it is not needed anymore
ALTER TABLE Catalog DROP CONSTRAINT Catalog_uni_name;
After performing these steps, the primary and foreign key appear to be set correctly - but the value of CatalogEntry.catalog_version_ is null. The corresponding value of Catalog.version_ is set correctly to 1.
Where is my mistake? Do I also have to set CatalogEntry.catalog_version_ manually to 1? I would have assumed that it would be set automatically.
The value of CatalogEntry.catalog_version_ doesn't magically get set just because you define a foreign key constraint.
What effectively happened is that no row in CatalogEntry references a row in Catalog. The reason is that the default for foreign key constraints is MATCH SIMPLE, see the documentation:
MATCH SIMPLE allows any of the foreign key columns to be null; if any of them are null, the row is not required to have a match in the referenced table.
You should create the foreign key constraint as MATCH FULL so that either all or none of the columns must be NULL. Then you would have received an error creating the foreign key.
Solution: update CatalogEntry and set the column to 1 there as well, then define the foreign key with MATCH FULL.
My problem is that i have two tables with each table having a foreign key to the other table.
Each time , i execute the SQL file containing the creation of the two tables, it gives me an error that he doesn't find the other table. I'm working with sqlplus to execute the sql file.
Here's an example of SQL file i tried with :
create table A(
Age number(3),
name number(3) constraint A_FK references B(name))
/
create table B(
Age number(3) constraint B_FK references A(Age),
name number(3))
And even if i reverse the order, it gives the same error.
Thanks for help.
This is a problem of cycles in foreign keys. One method is to add all foreign keys after table creation (as I think the other answers propose).
You can also just do that for the first table:
create table A (
Age number(3) primary key,
name number(3)
);
create table B (
name number(3) primary key,
Age number(3),
constraint B_FK foreign key (age) references A(Age)
);
alter table B add constraint A_FK foreign key (name) references B(name);
Here is a db<>fiddle.
Notes:
Foreign keys should reference primary keys, so I added that declaration as well.
I recommend making the primary key the first column in the table.
You can also define the constraint inline for one of the tables (i.e. age number(3) constraint b_fk references a(age)).
The table column(s) that is referred by a foreign key must exist at the time when the constraint is created. Since you have some kind of cyclic reference between the tables, you need to do this in three steps:
first create one table without the foreign key
create the second table (with its foreign key)
finally add the foreign key to the first table with an alter table statement
You also need the referred column to have a unique or primary key constraint set up, otherwise you would get error ORA-02270: no matching unique or primary key for this column-list.
create table A(
age number(3) primary key,
name number(3)
);
create table B(
age number(3) constraint B_FK references A(Age),
name number(3) primary key
);
alter table A add constraint A_FK foreign key (name) references B(name);
Demo on DB Fiddle
Side note: I am quite suspicious about your sample structure, but this could be because your oversimplified it in the question.
It fails because the reference table doesn't exist yet.
Create the tables without the key first. Then drop one and recreated it with the reference. Then drop the 2nd and recreate it with the reference.
Create table first and then ADD the CONSTRAINT
ALTER TABLE A
ADD FOREIGN KEY (name) REFERENCES B(name);
ALTER TABLE B
ADD FOREIGN KEY (age) REFERENCES A(age);
i want to add to a primary key in one table a references to the primary key of another table.
my code:
CREATE TABLE[payment]
(ID int Primary key)
CREATE TABLE [tab]
(ID int Primary key references tab2(ID))
Alter Table payment
alter column ID
ADD constraint fk_payment
references tab(ID)
i get the error that the syntax near constraint is wrong, but i don't know what to change
because of the not changeable order of the table Alter table is the only option. to reference from one table to the other doesn't work cause I've references from that table to another one already.
i need two one-to-one-relations from one table to another
If you want to add a FK constraint, just use this code:
ALTER TABLE dbo.payment
ADD CONSTRAINT fk_payment
FOREIGN KEY(ID) REFERENCES dbo.tab(ID)
You don't need to alter the column or table - just add the constraint
I am trying to create two tables using the following SQL:
create table student(sid char(20) primary key,name char(20),age int,hours char(10) references courses(cid));
create table courses(cid char(10),cname char(10),grader char(20) references student(sid));
However I get the following error:
1: ERROR: relation "courses" does not exist
3: ERROR: relation "student" does not exist
Is there any way or syntax which can solve this problem?
You would need to create the tables first (without REFERENCES clause). After that create your foreign keys manually by statement ALTER TABLE mytable ADD CONSTRAINT mytablefk FOREIGN KEY... But first I'd consider if there really is a relationship from table courses to table student!
Rather than creating the Foreign Key constraints at the same time as the tables with the References short-hand, you can add one or both of them afterwards with an Alter Table Add Constraint command. See the Alter Table page in the PostgrSQL manual here.
As mu pointed out, the target of a foreign key has to have a Unique or Primary Key constraint defined, so I've added that on the cid column in the example below.
In your case, it could look something like this:
create table student(sid char(20) primary key,name char(20),age int,hours char(10));
create table courses(cid char(10) primary key,cname char(10),grader char(20));
Alter Table student Add Constraint fk_student_hours_cid Foreign Key (hours) References courses(cid);
Alter Table courses Add Constraint fk_courses_grader_sid Foreign Key (grader) References student(sid);
I have a table in Oracle which has following schema:
City_ID Name State Country BuildTime Time
When I declared the table my primary key was both City_ID and the BuildTime, but now I want to change the primary key to three columns:
City_ID BuildTime Time
How can I change the primary key?
Assuming that your table name is city and your existing Primary Key is pk_city, you should be able to do the following:
ALTER TABLE city
DROP CONSTRAINT pk_city;
ALTER TABLE city
ADD CONSTRAINT pk_city PRIMARY KEY (city_id, buildtime, time);
Make sure that there are no records where time is NULL, otherwise you won't be able to re-create the constraint.
You will need to drop and re-create the primary key like this:
alter table my_table drop constraint my_pk;
alter table my_table add constraint my_pk primary key (city_id, buildtime, time);
However, if there are other tables with foreign keys that reference this primary key, then you will need to drop those first, do the above, and then re-create the foreign keys with the new column list.
An alternative syntax to drop the existing primary key (e.g. if you don't know the constraint name):
alter table my_table drop primary key;
Sometimes when we do these steps:
alter table my_table drop constraint my_pk;
alter table my_table add constraint my_pk primary key (city_id, buildtime, time);
The last statement fails with
ORA-00955 "name is already used by an existing object"
Oracle usually creates an unique index with the same name my_pk. In such a case you can drop the unique index or rename it based on whether the constraint is still relevant.
You can combine the dropping of primary key constraint and unique index into a single sql statement:
alter table my_table drop constraint my_pk drop index;
check this:
ORA-00955 "name is already used by an existing object"