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8 kwi 2021 · 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);
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Learn how to use the FOREIGN KEY constraint to prevent actions that would destroy links between tables in MySQL. See examples of creating, naming, and dropping FOREIGN KEY constraints on CREATE TABLE and ALTER TABLE statements.
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.
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:
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.
Here is the basic syntax of defining a foreign key constraint in the CREATE TABLE or ALTER TABLE statement: [CONSTRAINT constraint_name] FOREIGN KEY [foreign_key_name] (column_name, ...) REFERENCES parent_table(colunm_name,...) [ON DELETE reference_option] [ON UPDATE reference_option] Code language: SQL (Structured Query Language) (sql)
17 sie 2017 · To achieve this, we employ SET NULL as a constraint when creating the foreign key: ALTER TABLE books ADD CONSTRAINT fk_books_genres_genre_id FOREIGN KEY (genre_id) REFERENCES genres (id) ON DELETE SET NULL. Important! Before you specify the SET NULL action, make sure that you have declared the “genre_id” column in the “books” table as ...
