我经常使用ON DELETE CASCADE，但我从不使用ON UPDATE CASCADE，因为我不确定在什么情况下它会有用。
为了便于讨论，让我们看一些代码。
CREATE TABLE parent (
id INT NOT NULL AUTO_INCREMENT,
PRIMARY KEY (id)
);
CREATE TABLE child (
id INT NOT NULL AUTO_INCREMENT, parent_id INT,
INDEX par_ind (parent_id),
FOREIGN KEY (parent_id)
REFERENCES parent(id)
ON DELETE CASCADE
);
对于ON DELETE CASCADE，如果删除带有id的父级，则child中的记录parent_id = parent。Id将被自动删除。这应该没有问题。
This means that ON UPDATE CASCADE will do the same thing when id of the parent is updated?
If (1) is true, it means that there is no need to use ON UPDATE CASCADE if parent.id is not updatable (or will never be updated) like when it is AUTO_INCREMENT or always set to be TIMESTAMP. Is that right?
If (2) is not true, in what other kind of situation should we use ON UPDATE CASCADE?
What if I (for some reason) update the child.parent_id to be something not existing, will it then be automatically deleted?
好吧，我知道，上面的一些问题可以通过编程测试来理解，但我也想知道这其中是否有依赖于数据库供应商的。
请照点光。
ON UPDATE和ON DELETE指定当父表中的一行被更新和删除时将执行哪个操作。允许的操作包括:NO ACTION、CASCADE、SET NULL和SET DEFAULT。
删除父表中的行操作
如果您删除父表中的一行或多行，您可以设置以下操作之一:
ON DELETE NO ACTION: SQL Server raises an error and rolls back the delete action on the row in the parent table.
ON DELETE CASCADE: SQL Server deletes the rows in the child table that is corresponding to the row deleted from the parent table.
ON DELETE SET NULL: SQL Server sets the rows in the child table to NULL if the corresponding rows in the parent table are deleted. To execute this action, the foreign key columns must be nullable.
ON DELETE SET DEFAULT: SQL Server sets the rows in the child table to their default values if the corresponding rows in the parent table are deleted. To execute this action, the foreign key columns must have default definitions. Note that a nullable column has a default value of NULL if no default value specified.
By default, SQL Server appliesON DELETE NO ACTION if you don’t explicitly specify any action.
父表中行更新操作
如果您更新父表中的一行或多行，您可以设置以下操作之一:
ON UPDATE NO ACTION: SQL Server raises an error and rolls back the update action on the row in the parent table.
ON UPDATE CASCADE: SQL Server updates the corresponding rows in the child table when the rows in the parent table are updated.
ON UPDATE SET NULL: SQL Server sets the rows in the child table to NULL when the corresponding row in the parent table is updated. Note that the foreign key columns must be nullable for this action to execute.
ON UPDATE SET DEFAULT: SQL Server sets the default values for the rows in the child table that have the corresponding rows in the parent table updated.
FOREIGN KEY (foreign_key_columns)
REFERENCES parent_table(parent_key_columns)
ON UPDATE <action>
ON DELETE <action>;
请参阅参考教程。
