The role of foreign keys:
Maintain data consistency and integrity, primarily to control the storage of data in foreign key tables. Causes the two tables to be associated, the foreign key can only reference the values of the columns in the outside!
A B two tables
A table contains the customer number, customer name
Table B contains orders for each customer
After the foreign key is available
You can only delete customer x in table A if you are sure that there are no customer x orders in table B
The prerequisite for establishing a foreign key is that the column in this table must be the same as the foreign key type (the foreign key must be the outer primary key).
Specify PRIMARY KEY Keywords: foreign key (column name)
Referencing foreign key keywords: references < foreign key table name > (foreign key column name)
Event Trigger restrictions: On delete and on update, to set parameter cascade (following foreign key changes), restrict (Restrict foreign key changes in the surface), set NULL (set NULL value), set default (set default), [Default]no action
Outtable table primary key ID type int
To create a table containing foreign keys:
CREATE TABLE Temp (
Name Char (20),
FOREIGN key (ID) references outtable (ID) on DELETE cascade on UPDATE cascade);
Description: Set the ID column as a foreign key reference to the outer outtable ID column when the value of the foreign key deletes the corresponding column in this table screen except when the value of the foreign key changes the corresponding column value in this table.