Does MySQL support foreign keys

mysql Do you support foreign keys

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mysql Support foreign keys . stay MySQL in , Foreign keys are mainly used to establish the relationship between master tables and slave tables , You can establish a connection for the data of two tables , Constrains the consistency and integrity of data in two tables ; When a record is deleted from the main table , The corresponding records from the table must be changed accordingly . A table can have one or more foreign keys , The foreign key can be null , If not null , Then the value of each foreign key must be equal to a value of the primary key in the primary table ; The number of columns and corresponding data types in the foreign key must be the same as those in the primary key of the primary table .

The operating environment of this tutorial ：windows7 System 、mysql8 edition 、Dell G3 The computer .

mysql Support foreign keys .

MySQL Foreign keys （FOREIGN KEY）

A foreign key is a field in a specified table that matches another field in another table . Foreign keys set constraints on data in related tables , This makes MySQL Ability to maintain referential integrity .

Foreign keys are used to establish the relationship between the master table and the slave table , Establish a connection between the data of two tables , Constrains the consistency and integrity of data in two tables .

For two tables that have an association , The table where the primary key in the associated field is located is the primary table （ Parent table ）, The table where the foreign key is located is the slave table （ Sub table ）.

When a record is deleted from the main table , The corresponding records from the table must be changed accordingly . A table can have one or more foreign keys , The foreign key can be null , If not null , Then the value of each foreign key must be equal to a value of the primary key in the primary table .

Let's take a look at the following database diagram in the sample database .

We have two watches ：customers and orders, Every customer has zero or more orders , Each order can only belong to one customer .customers Table and orders The relationship between tables is one to many , it orders from customerNumber Field to create a foreign key in the table specified by the .customers In the table customerNumber Fields and orders In the table customerNumber The primary key field is related .

customers Tables are called parent tables or reference tables ,orders Tables are called child tables or reference tables .

A foreign key can be a column or a group of columns . The columns in the child table usually refer to the primary key columns in the parent table .

A table can have multiple foreign keys , Each foreign key in a child table can reference a different parent table .

Rows in the child table must contain values that exist in the parent table , for example ,orders Each order record in the table must have customers surface customerNumber Exists in . therefore , Multiple orders can reference the same customer , This relationship is called a （ Customer ） To more than one （ Order ） Or one to many .

Sometimes , The child table and the parent table are the same . The foreign key refers to the primary key of the table , for example , Next employees surface ：

reportTo Column is a foreign key , It refers to employeeNumber As employees The primary key column of the table , To reflect the reporting structure between employees , That is, each employee reports to another employee , Employees can have zero or more direct reports . We have a tutorial on self join to help you query data based on this table .

reportTo Foreign keys are also called recursive or self referencing foreign keys .

Foreign keys enforce referential integrity , Helps you automatically maintain data consistency and integrity . for example , You cannot create an order for a customer that does not exist .

Besides , You can customerNumber Set cascading on the deletion of foreign keys , In order to delete customers Customers in the table , All orders associated with the customer will also be deleted . This saves the use of multiple DELETE sentence or DELETE JOIN The time and energy of the statement .

Same as delete , You can also update for customerNumber Foreign key definitions cascade , So that multiple UPDATE Sentence or UPDATE JOIN Statement to execute cross table updates .

Be careful ： stay MySQL in ,InnoDB The storage engine supports foreign keys , So you have to create InnoDB Tables can use foreign key constraints .

mysql When defining a foreign key , The following rules need to be followed ：

• The primary table must already exist in the database , Or the table that is currently being created . In the latter case , Then the master table and the slave table are the same table , Such a table is called a self reference table , This structure is called self referential integrity .

• The primary key must be defined for the primary table .

• Primary key cannot contain null value , But null values are allowed in foreign keys . in other words , As long as each non null value of the foreign key appears in the specified primary key , The content of this foreign key is correct .

• Specify the column name or combination of column names after the table name of the main table . This column or combination of columns must be the primary key or candidate key of the main table .

• The number of columns in the foreign key must be the same as the number of columns in the primary key of the primary table .

• The data type of the column in the foreign key must be the same as that of the corresponding column in the primary key of the primary table .

Create foreign keys for tables

MySQL Create foreign key Syntax

The following syntax describes how to use the CREATE TABLE The foreign key is defined in the sub table of the statement .

CONSTRAINT constraint_nameFOREIGN KEY foreign_key_name (columns)REFERENCES parent_table(columns)ON DELETE actionON UPDATE action

Let's study grammar in more detail ：

• CONSTRAINT Clause allows you to define constraint names for foreign key constraints . If you omit it ,MySQL A name will be automatically generated .

• FOREIGN KEY Clause specifies the column in the child table that references the primary key column in the parent table . You can put a foreign key name in FOREIGN KEY After Clause , Or let MySQL Create a name for you . Please note that ,MySQL Will be used automatically foreign_key_name Name create index .

• REFERENCES Clause specifies the parent table and column referenced by the column in the child table . The number of columns in the specified child table and parent table FOREIGN KEY and REFERENCES It has to be the same .

• ON DELETE Clause allows you to define the contents of records in the child table when deleting records in the parent table . If omitted ON DELETE Clause and delete records in the parent table that contain records in the child table ,MySQL Will refuse to delete . Besides ,MySQL It also provides you with operations , So that you can use other options , for example ON DELETE CASCADE  , requirement MySQL Delete the records in the sub table , When the records in the parent table are deleted , Records will reference records in the parent table . If you do not want to delete the related records in the sub table , Please switch to ON DELETE SET NULL operation .MySQL The foreign key column value in the child table will be set to NULL When deleting records in the parent table , The condition is that the foreign key column in the child table must accept NULL value . Please note that , If you use ON DELETE NO ACTION or ON DELETE RESTRICT operation ,MySQL Will refuse to delete .

• ON UPDATE Clause allows you to specify what happens to the rows in the child table when the rows in the parent table are updated . You can omit ON UPDATE Clause , So that when updating rows in the parent table MySQL Reject any updates to the rows in the child table .ON UPDATE CASCADE Operations allow you to perform cross table updates , And when the parent table is updated ON UPDATE SET NULL When the line in , The operation resets the values in the rows in the child table to the values NULL.ON UPDATE NO ACTION or UPDATE RESTRICT Action rejects any updates .

MySQL Create a 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 .products In the table cat_id The field is defined with UPDATE ON CASCADE and DELETE ON RESTRICT Foreign keys for actions .

CREATE DATABASE IF NOT EXISTS dbdemo; USE dbdemo; CREATE TABLE categories(   cat_id int not null auto_increment primary key,   cat_name varchar(255) not null,   cat_description text) ENGINE=InnoDB; CREATE TABLE products(   prd_id int not null auto_increment primary key,   prd_name varchar(355) not null,   prd_price decimal,   cat_id int not null,   FOREIGN KEY fk_cat(cat_id)   REFERENCES categories(cat_id)   ON UPDATE CASCADE   ON DELETE RESTRICT)ENGINE=InnoDB;

Add foreign keys to the table

MySQL Add foreign key syntax

To add a foreign key to an existing table , Please use... With the above foreign key definition syntax ALTER TABLE sentence ：

ALTER TABLE table_nameADD CONSTRAINT constraint_nameFOREIGN KEY foreign_key_name(columns)REFERENCES parent_table(columns)ON DELETE actionON UPDATE action;

MySQL Add a foreign key example

Now? , Let's add a new one called vendors New table of , And change products Table to include suppliers ID Field ：

USE dbdemo; CREATE TABLE vendors(    vdr_id int not null auto_increment primary key,    vdr_name varchar(255))ENGINE=InnoDB; ALTER TABLE products ADD COLUMN vdr_id int not null AFTER cat_id;

To add a foreign key to a table products, Please use the following sentence ：

ALTER TABLE productsADD FOREIGN KEY fk_vendor(vdr_id)REFERENCES vendors(vdr_id)ON DELETE NO ACTIONON UPDATE CASCADE;

Now? ,products The table has two foreign keys , A quote categories surface , Another reference vendors surface .

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