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SQL FOREIGN KEY on ALTER TABLE. To create a FOREIGN KEY constraint on the "PersonID" column when the "Orders" table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Orders. ADD FOREIGN KEY (PersonID) REFERENCES Persons (PersonID);
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.
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax: ALTER TABLE Orders ADD CONSTRAINT FK_PersonOrder
8 kwi 2021 · Syntax –. ALTER TABLE table_name ADD FOREIGN KEY (column_name) REFERENCE table_name (Referencing column_name in table_name); Query –. ALTER TABLE exam ADD FOREIGN KEY (student_id) REFERENCES student (student_id);
Add Foreign Key using ALTER TABLE. The following SQL command adds a Foreign Key to an existing table. ALTER TABLE Subject ADD FOREIGN KEY (StudentID) REFERENCES Student(StudentID); Drop foreign key MySQL. For dropping a Foreign key from a MySQL table, you need to know the name of the foreign key first.
The essential syntax for a defining a foreign key constraint in a CREATE TABLE or ALTER TABLE statement includes the following: [CONSTRAINT [symbol]] FOREIGN KEY. [index_name] (col_name, ...) REFERENCES tbl_name (col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option] reference_option:
First, specify the name of the table from which you want to drop the foreign key after the ALTER TABLE keywords. Second, specify the constraint name after the DROP FOREIGN KEY keywords. Notice that constraint_name is the name of the foreign key constraint specified when you created or added the foreign key constraint to the table.
