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The FOREIGN KEY constraint is a key used to link two tables together. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. The following SQL creates a FOREIGN KEY on the "PersonID" column when the "Orders" table is created:
ALTER TABLE child_table_name ADD FOREIGN KEY (child_table_column) REFERENCES parent_table_name(parent_table_column); child_table_name is that table in which we want to add constraint. child_table_column is that table column in which we want to add foreign key.
22 lip 2024 · Creating a new table with a foreign key requires CREATE TABLE permission in the database, and ALTER SCHEMA permission on the schema in which the table is being created. Creating a foreign key in an existing table requires ALTER TABLE permission on the table.
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.
Learn how to create a foreign key for a table in SQL using different methods and syntax. See examples of single-column and multi-column foreign keys, and how to modify an existing table with a foreign key.
Adding FOREIGN KEY contraints to existing tables. To add a FOREIGN KEY constraint to existing table, you use the ALTER TABLE statement. ALTER TABLE table_1 ADD CONSTRAINT fk_name FOREIGN KEY (fk_key_column) REFERENCES table_2(pk_key_column) Code language: SQL (Structured Query Language) (sql)
You can add a foreign key constraint to an existing table using the following ALTER TABLE syntax: ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (col_name, ...) REFERENCES tbl_name (col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option]
