MySQL定义外键：以实现表之间数据约束（mysql定义外键）
MySQL is the world’s most popular open source database platform. It is widely used by web and mobile developers to manage and store both small-scale and large-scale data. The concept of foreign keys is a defining feature of relational databases, such as MySQL, that allows developers to define a relation between two tables. By making use of foreign keys in MySQL, developers are able to ensure that data stored in one table corresponds to the data stored in another table.
Foreign keys are made up of two components – the primary key and the foreign key. The primary key is usually a unique identifier for the table, such as an auto-incrementing ID, while the foreign key defines the relation between the two tables. This is generally done by simply matching the primary key of one table to the foreign key of the other table. The relation between two tables can also be defined by adding an additional column to the table with the foreign key.
MySQL allows developers to define foreign keys in several different ways. The simplest and most popular way of creating a foreign key is to use the “ALTER TABLE” command. This command is used to modify the structure of an existing table. We can add a new foreign key to an existing table with the following command:
ALTER TABLE table_name ADD FOREIGN KEY (column_name) REFERENCES other_table (other_column_name);
This command will ensure that the values stored in the “column_name” column of the “table_name” table will always correspond to the values stored in the “other_column_name” column of the “other_table” table. If a value is inserted in either one of these columns, it must first be checked against the values present in the other table, and if the value does not exist, and an error will be thrown.
It is also possible to set up cascading foreign keys using the “ON DELETE CASCADE” and “ON UPDATE CASCADE” options. These are used to ensure that if a record is either updated or removed from a table, the related records in the other table will automatically be updated or removed as well. For example, if a record is removed from the “other_table”, the related record in the “table_name” will also be removed.
Foreign keys can be used to enforce a wide range of data integrity rules and constraints, such as ensuring that data is consistent across different tables, and preventing data from being entered unless it meets certain criteria. Through proper use of foreign keys, developers can create robust and reliable applications that make use of multiple tables.
