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                [TOC] * * * * * #### 外鍵約束 外鍵約束使用最多的兩種情況: 1)父表更新時子表也更新,父表刪除時如果子表有匹配的項,刪除失敗; 2)父表更新時子表也更新,父表刪除時子表匹配的項也刪除。 前一種情況,在外鍵定義中,使用 ~~~ ON UPDATE CASCADE ON DELETE RESTRICT; ~~~ 后一種情況,可以使用 ~~~ ON UPDATE CASCADE ON DELETE CASCADE; ~~~ InnoDB允許你使用ALTER TABLE在一個已經存在的表上增加一個新的外鍵: #### 增加外鍵 ~~~ ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name, ...) REFERENCES tbl_name (index_col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option] ~~~ #### 刪除外鍵 InnoDB支持使用ALTER TABLE來刪除外鍵: ~~~ ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol; ~~~
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