The database uses "foreign keys" to establish relationships between tables. Foreign keys can be used to establish an association between the master table and the slave table, establish a connection for the data in the two tables, and constrain the consistency and integrity of the data in the two tables.
The operating environment of this tutorial: Windows 7 system, mysql version 5.8, Dell G3 computer.
The database uses "foreign keys" to establish relationships between tables.
The foreign key constraint (FOREIGN KEY) of the database is a special field of the table and is often used together with the primary key constraint. For two tables with an associated relationship, the table where the primary key in the associated field is located is the primary table (parent table), and the table where the foreign key is located is the secondary table (child table).
Foreign keys are used to establish the association between the master table and the slave table, establish a connection for the data in the two tables, and constrain the consistency and integrity of the data in the two tables. For example, a fruit stall only has four kinds of fruits: apples, peaches, plums, and watermelons. Then, when you come to the fruit stall to buy fruits, you can only choose apples, peaches, plums, and watermelons. Other fruits are not available for purchase.
When a record is deleted from the main table, the corresponding record from the table must also be changed accordingly. A table can have one or more foreign keys, and the foreign key can be null. If it is not null, the value of each foreign key must be equal to a certain value of the primary key in the main table.
When defining foreign keys, you need to comply with the following rules:
The main table must already exist in the database, or be the table currently being created. If it is the latter case, the master table and the slave table are the same table. Such a table is called a self-referential table, and this structure is called self-referential integrity.
A primary key must be defined for the main table.
The primary key cannot contain null values, but null values are allowed in foreign keys. That is, as long as every non-null value of the foreign key appears in the specified primary key, the contents of the foreign key are correct.
Specify the column name or a 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 main table.
The data type of the column in the foreign key must be the same as the data type of the corresponding column in the primary key of the main table.
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