drop foreign key
ALTER TABLE table_name
DROP CONSTRAINT fk_name;drop foreign key
ALTER TABLE table_name
DROP CONSTRAINT fk_name;sql foreign key
CREATE TABLE orders (
id int NOT NULL,
user_id int,
product_id int,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES users(id),
FOREIGN KEY (product_id) REFERENCES products(id)
);sql foreign key
create table Jobs(
job_id number not null,
job_title varchar(30),
min_salary number,
max_salary number
);
create table job_history(
employee_id number not null,
start_date date,
end_date date,
job_id number not null,
department_id number
);
alter table jobs add constraint pk_jobs primary key(job_id);
alter table job_history add constraint fk_job foreign key(job_id) references jobs(job_id);foreign key constraint
CREATE TABLE Students ( /* Create table with foreign key - Way 1 */
ID INT NOT NULL
Name VARCHAR(255)
LibraryID INT
PRIMARY KEY (ID)
FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID)
);
CREATE TABLE Students ( /* Create table with foreign key - Way 2 */
ID INT NOT NULL PRIMARY KEY
Name VARCHAR(255)
LibraryID INT FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID)
);
ALTER TABLE Students /* Add a new foreign key */
ADD FOREIGN KEY (LibraryID)
REFERENCES Library (LibraryID);what is foreign key
Foreign Key:
It is a column that comes from a different table and
using Foreign key tables are related each other
It is the primary key of another table
It can be duplicate or null for another tableCopyright © 2021 Codeinu
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