Foreign Keys
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MySQL will set the foreign key column values in the child table to NULL when the record in the parent table is deleted, with a condition that the foreign key column in the child table must accept NULL values. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT. If you re-create a table that was dropped, it must have a definition that conforms to the foreign key constraints referencing it.
The ON UPDATE CASCADE action allows you to perform a cross-table update, and the ON UPDATE SET NULL action resets the values in the rows in the child table to NULL values when the rows in the parent table are updated. One foreign key references a two-column index in the product table. Additionally, MySQL requires that the referenced columns be indexed for performance reasons.
SQL - Foreign Key - What is foreign key in database? You have the options of NO ACTION, CASCADE, SET NULL, or SET DEFAULT.
Summary: in this tutorial, you will learn about MySQL foreign key and how to create, add, and drop foreign key constraints in MySQL. Introduction to MySQL foreign key A foreign key is a field in a table that matches another field of another table. A foreign key places constraints on data in the related tables, which enables MySQL to maintain referential integrity. We have two tables: customers and orders. Each customer has zero or more orders and each order belongs to only one customer. The relationship between customers table and orders table is one-to-many, and it is established by a foreign key in the orders table specified by the customerNumber field. The customerNumber field in the orders table relates to the customerNumber primary key field in the customers table. The customers table is called parent table or referenced table, and the orders table is known as child table or referencing table. A foreign key can be a column or a set of columns. The columns in the child table often refer to the columns in the parent table. A table may have more than one foreign key, and each foreign key in the child table may refer to a different parent table. A row in the child table must contain values that exist in the parent table e. Multiple orders can refer to the same customer therefore, this relationship is called one customer to many orders , or one-to-many. Sometimes, the child and parent tables are the same. The foreign key refers back to the primary key of the table e. We have a specific to help you query data against this kind of table. The reportTo foreign key is also known as recursive or self-referencing foreign key. Foreign keys enforce referential integrity that helps you maintain the consistency and integrity of the data automatically. For example, you cannot create an order for a non-existent customer. In addition, you can set up a cascade on delete action for the customerNumber foreign key so that when you delete a customer in the customers table, all the orders associated with the customer are also deleted. This saves you time and efforts of using multiple or a. The same as deletion, you can also define a cascade on update action for the customerNumber foreign key to perform the cross-table update without using multiple statements or an. In MySQL, the InnoDB supports foreign keys so that you must create InnoDB tables in order to use foreign key constraints. Creating foreign keys for tables MySQL creating foreign key syntax The following syntax illustrates how to define a foreign key in a child table in statement. If you omit it, MySQL will generate a name automatically. You can put a foreign key name after FOREIGN KEY clause or leave it to let MySQL create a name for you. The number of columns in the child table and parent table specified in the FOREIGN KEY and REFERENCES must be the same. If you omit the ON DELETE clause and delete a record in the parent table that has records in the child table refer to, MySQL will reject the deletion. In addition, MySQL also provides you with actions so that you can have other options such as that ask MySQL to delete records in the child table that refers to a record in the parent table when the record in the parent table is deleted. MySQL will set the foreign key column values in the child table to NULL when the record in the parent table is deleted, with a condition that the foreign key column in the child table must accept NULL values. Notice that if you use ON DELETE NO ACTION or ON DELETE RESTRICT action, MySQL will reject the deletion. You can omit the ON UPDATE clause to let MySQL reject any updates to the rows in the child table when the rows in the parent table are updated. The ON UPDATE CASCADE action allows you to perform a cross-table update, and the ON UPDATE SET NULL action resets the values in the rows in the child table to NULL values when the rows in the parent table are updated. The ON UPDATE NO ACTION or UPDATE RESTRICT actions reject any updates. MySQL creating table foreign key example The following example creates a dbdemo database and two tables: categories and products. Each category has one or more products and each product belongs to only one category. If you omit it, MySQL generates a constraint name for you. However, if you disable the foreign key checks, you can load data in any orders. Another example is that, unless you disable the foreign key checks, you cannot that is referenced by a foreign key constraint. When you drop a table, any constraints that you defined for the table are also removed. To disable foreign key checks, you use the following statement:.
Then, open brackets and add in the name of the column in this table that will be the foreign key. For storage engines that do not support foreign keys such asMySQL Server foreign key syntax and ignores foreign key specifications. For each row in the referencing table the table contains the FOREIGN KEYthe foreign key must match an existing primary key in the referenced table the table contains the PRIMARY KEY. Even if that happens, we will make sure foreign key syntax useful comments are taken out of spam and approved. Syntax Note: MariaDB accepts the REFERENCES clause in ALTER TABLE and CREATE TABLE statements, but that syntax does nothing. Hope to see you again and again. Foreign Keys and Other MySQL Statements Table and column identifiers in a FOREIGN KEY. This can be up to 30 characters. In our examples, we will reference the parent and child tables below. Want to improve the above article. In that case, the child table row containing such a foreign key is permitted to be inserted, and does not match any row in the referenced parent table. The constraint here is that all customers must be associated with an agent that is already in the 'agents' table.
