I tried to create a new table (tableB) with a foreign key constraint to another table (tableA) and just wondering if I can create along with this all constraints and indexes needed. My goal would be to have a single create table statement with no need of an alter table… statement afterwards and no other create index… statement. Is this possible? Thanks for any hint in advance :)
create table tableA
(
id number
, constraint tableApk primary key (id)
);
create table tableB
(
id number
, constraint tableBfk foreign key (id) references tableA (id)
on delete cascade
using index (
create index tableBfkidx on tableB (id)
)
);
That isn't allowed. Per the documentation a using_index_clause can only be specified for unique or primary constraints.
Best of luck.
Related
Is there a way i can declare a disabled foreign key in the table creation in SQL
i don't want to do it by altering the table if possible.
From the CONSTRAINT documentation, use the DISABLE keyword:
CREATE TABLE table1 (
ID NUMBER PRIMARY KEY
)
CREATE TABLE table2 (
OTHER_ID CONSTRAINT table2__other_id__fk REFERENCES table1 (id) DISABLE
);
fiddle
I am new to sql and have not have much experience in it
Trying to create a relationship between two tables using a constraint.
adding a unique constraint and relationship for force the userid and consumerid between table1 and table2
any sample will help me
Thanks
You can use this FOREIGN KEY (blabla) REFERENCES secondTable(blabla) but only if the table is not created yet. Write it inside your CREATE TABLE query.
But if it already exists you can add it using ALTER TABLE tablename ADD CONSTRAINT `FK_myKey` FOREIGN KEY (`blabla`) REFERENCES `secondTable` (`blabla`) ON DELETE CASCADE ON UPDATE CASCADE;
How to enforce a constraint of foreign key on columns of same table in SQL while entering values in the following table:
employee:
empid number,
manager number (must be an existing employee)
Oracle call this a self-referential integrity constraint. The documentation is here for a description,
You create a self-referential constraint in the same manner you would a normal one:
alter table employees
add constraint employees_emp_man_fk
foreign key ( manager_no )
references employees ( emp_id )
on delete set null
;
I'm assuming that your manager_no is nullable. I've added set null here as a delete cascade would probably wipe out a significant amount of your table.
I can't think of a better way of doing this. Deleting a manager should not result in the deletion of all their employees so you have to set null and have a trigger on the table to alert you to anyone with no manager.
I always like this site, which is good for simple references. and don't forget to have an index on the FK as well or Tom will yell at you :-).
One can also utilise standard Oracle syntax to create a self-referential FK in the create table statement, which would look like the following.
create table employees
( emp_id number
, other_columns ...
, manager_no number
, constraint employees_pk
primary key (emp_id)
, constraint employees_man_emp_fk
foreign key ( manager_no )
references employees ( emp_id )
on delete set null
);
EDIT:
In answer to #popstack's comment below:
Whilst you can do this in one statement not being able to alter a table is a fairly ridiculous state of affairs. You should definitely analyze a table that you're going to be selecting from and you will still want an index on the foreign key ( and possibly more columns and / or more indexes ) otherwise whenever you use the foreign key you're going to do a full table scan. See my link to asktom above.
If you're unable to alter a table then you should, in descending order of importance.
Find out how you can.
Change your DB design as a FK should have an index and if you can't have one then FKs are probably not the way to go. Maybe have a table of managers and a table of employees?
SELF REFERENCES QUERY...
Alter table table_name ADD constraints constraints_name foreign key(column_name1,column_name2..) references table_name(column_name1,column_name2...) ON DELETE CASCADE;
EX- ALTER TABLE Employee ADD CONSTRAINTS Fr_key( mgr_no) references employee(Emp_no) ON DELETE CASCADE;
CREATE TABLE TABLE_NAME (
`empid_number` int ( 11) NOT NULL auto_increment,
`employee` varchar ( 100) NOT NULL ,
`manager_number` int ( 11) NOT NULL ,
PRIMARY KEY (`empid_number`),
CONSTRAINT `manager_references_employee`
FOREIGN KEY (`manager_number`) REFERENCES (`empid_number`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
Hope it helps!
I've a sql table with a primary key(Auto Incremented) and a foreign key.Now I need to modify the table by modifying the foreign key to second primary key so that its values are not allowed to duplicate.
How do i alter my table without affecting the data? Need the sql code.
Regards,
Vix
If I understand your request, you want to force the foreign key to be unique within the given table so your schema looks like:
Create Table Table1
(
Id int not null primary key clustered
, ForeignId not null
, ...
, Constraint FK_Table1_Table2
Foreign Key ( ForeignId )
References Table2( Id )
)
And you now want to force ForeignId to be unique in this table, correct? You would do the following:
Alter Table Table1
Add Constraint UC_Table1_ForeignId Unique Nonclustered ( ForeignId )
create table d(id numeric(1), code varchar(2))
After I create the above table how can I add a composite primary key on both fields and also a foreign key?
In Oracle, you could do this:
create table D (
ID numeric(1),
CODE varchar(2),
constraint PK_D primary key (ID, CODE)
);
alter table d add constraint pkc_Name primary key (id, code)
should do it. There's lots of options to a basic primary key/index depending on what DB your working with.
The ALTER TABLE statement presented by Chris should work, but first you need to declare the columns NOT NULL. All parts of a primary key need to be NOT NULL.
You don't need to create the table first and then add the keys in subsequent steps. You can add both primary key and foreign key while creating the table:
This example assumes the existence of a table (Codes) that we would want to reference with our foreign key.
CREATE TABLE d (
id [numeric](1),
code [varchar](2),
PRIMARY KEY (id, code),
CONSTRAINT fk_d_codes FOREIGN KEY (code) REFERENCES Codes (code)
)
If you don't have a table that we can reference, add one like this so that the example will work:
CREATE TABLE Codes (
Code [varchar](2) PRIMARY KEY
)
NOTE: you must have a table to reference before creating the foreign key.
If using Sql Server Management Studio Designer just select both rows (Shift+Click) and Set Primary Key.