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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.
Following is the basic syntax of defining a foreign key constraint in CREATE TABLE and ALTER TABLE constraint in MySQL. [CONSTRAINT constraint_name] FOREIGN KEY [foreign_key_name] (col_name, ...) REFERENCES parent_tbl_name (col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option]
10 lip 2024 · Using ALTER TABLE Statement to Add FOREIGN KEY. We can define a foreign key using the ALTER TABLE statement in MySQL. This is the case where you already have a table and need to get a foreign key in it. The foreign key constraint helps maintain referential integrity between the two tables.
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)
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Orders
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:
In this article, we introduced what foreign keys are, the rules for foreign keys, and how to use them in MySQL. In relational databases, foreign keys are used to define a constrained relationship between two entities. Foreign keys are useful for ensuring data integrity.
