1. 背景

? ?*?MySQL有兩種常用的引擎類型MyISAM和InnoDB。目前只有InnoDB引擎類型支持外鍵約束。

? ?*?本表的列必須與外鍵類型相同, 外鍵必須是外表的唯一鍵(或主鍵)。

? ?* 設置外建的列不能設置 NO NULL 字段屬性。

2. 外建作用

? ?* 使兩張表形成關聯,外鍵只能引用外表中的列的值

? ?*?保持數據一致性，完整性，控制存儲在外鍵表中的數據

3. 外鍵實驗 [ 員工 --> 部門 ]

? ?* 創建外鍵依賴的外表 departments

mysql>?CREATE?TABLE?departments(->?id?BIGINT?PRIMARY?KEY?NOT?NULL?AUTO_INCREMENT,->?name?VARCHAR(64)?NOT?NULL->?)ENGINE=INNODB?CHARSET=utf8mb4; Query?OK,?0?rows?affected?(0.05?sec)

? ?* 創建員工表 empoyees, 并建立外鍵

? ? ?指定外鍵關鍵字：?FOREIGN KEY(列名)?

? ? ?引用外鍵關鍵字：?REFERENCES <外鍵表名>(外鍵列名)　

? ? ?事件觸發限制: ?[默認] no action

? ? ? ? ? ? ?ON DELETE ?SET NULL ? 當外表字段刪除時，本表設置為NULL(空值) [ 不推薦設置此薦 ]

? ? ? ? ON UPDATE?CASCADE ? 當外表字段更新時，本表外鍵級聯更新

? ? ? ? ? ? ?ON DELETE/UPDATE SET DEFAULT 當有事件觸發時，設置為默認值

　　　　ON DELETE/UPDATE?RESTRICT ? ? ?當有事件觸發時，限制外表中的外鍵改動

mysql>?CREATE?TABLE?empoyees(?->?id?BIGINT?PRIMARY?KEY?NOT?NULL?AUTO_INCREMENT,->?sex?ENUM('M',?'F')?NOT?NULL,->?age?INT?NOT?NULL,->?department?BIGINT,->?FOREIGN?KEY(department)?->?REFERENCES?departments(id)?->?ON?DELETE?SET?NULL->?ON?UPDATE?CASCADE->?)ENGINE=INNODB?CHARSET=utf8mb4; Query?OK,?0?rows?affected?(0.12?sec)

? ?* 對部門表 departments 插入數據

mysql>?INSERT?INTO?departments?SELECT?NULL,?'dev'; Query?OK,?1?row?affected?(0.01?sec) Records:?1??Duplicates:?0??Warnings:?0mysql>?INSERT?INTO?departments?SELECT?NULL,?'test'; Query?OK,?1?row?affected?(0.01?sec) Records:?1??Duplicates:?0??Warnings:?0mysql>?INSERT?INTO?departments?SELECT?NULL,?'ops'; Query?OK,?1?row?affected?(0.01?sec) Records:?1??Duplicates:?0??Warnings:?0

? ?* 查看部門表數據

mysql>?SELECT?*?FROM?departments; +----+------+ |?id?|?name?| +----+------+ |??1?|?dev??| |??2?|?test?| |??3?|?ops??| +----+------+ 3?rows?in?set?(0.00?sec)

? ?* 插入正常數據 （部門編號存在于部門表中）

mysql>?INSERT?INTO?empoyees?SELECT?NULL,?'M',?22,?2; Query?OK,?1?row?affected?(0.01?sec) Records:?1??Duplicates:?0??Warnings:?0mysql>?SELECT?*?FROM?empyees; ERROR?1146?(42S02):?Table?'mytest.empyees'?doesn't?exist mysql>?SELECT?*?FROM?empoyees; +----+-----+-----+------------+ |?id?|?sex?|?age?|?department?| +----+-----+-----+------------+ |??1?|?M???|??22?|??????????2?| +----+-----+-----+------------+ 1?row?in?set?(0.01?sec)

? ?* 插入非正常數據 (部門編號不存在于部門表中)　[ 部門表中不存在id為4的列 ]

mysql>?INSERT?INTO?empoyees?SELECT?NULL,?'M',?22,?4; ERROR?1452?(23000):?Cannot?add?or?update?a?child?row:?a?foreign?key?constraint?fails?(`mytest`.`empoyees`,?CONSTRAINT?`empoyees_ibfk_1`?FOREIGN?KEY?(`department`)?REFERENCES?`departments`?(`id`)?ON?DELETE?SET?NULL?ON?UPDATE?CASCADE)

? ?*?修改部門表數據 [ 員工表中已有數據關聯部門表中id為2 ]

mysql>?SELECT?*?FROM?empoyees; +----+-----+-----+------------+ |?id?|?sex?|?age?|?department?| +----+-----+-----+------------+ |??1?|?M???|??22?|??????????2?| +----+-----+-----+------------+ 1?row?in?set?(0.01?sec)mysql>?UPDATE?departments?SET?id?=?4?WHERE?id=2; Query?OK,?1?row?affected?(0.04?sec) Rows?matched:?1??Changed:?1??Warnings:?0mysql>?SELECT?*?FROM?empoyees; +----+-----+-----+------------+ |?id?|?sex?|?age?|?department?| +----+-----+-----+------------+ |??1?|?M???|??22?|??????????4?| +----+-----+-----+------------+ 1?row?in?set?(0.01?sec)

? ?* 刪除部門表數據 [ 員工表中已有數據關聯部門表中id為4 ]

mysql>?SELECT?*?FROM?empoyees; +----+-----+-----+------------+ |?id?|?sex?|?age?|?department?| +----+-----+-----+------------+ |??1?|?M???|??22?|??????????2?| +----+-----+-----+------------+ 1?row?in?set?(0.01?sec)mysql>?DELETE?FROM?departments?WHERE?id?=?4; Query?OK,?1?row?affected?(0.01?sec)mysql>?SELECT?*?FROM?empoyees; +----+-----+-----+------------+ |?id?|?sex?|?age?|?department?| +----+-----+-----+------------+ |??1?|?M???|??22?|???????NULL?| +----+-----+-----+------------+ 1?row?in?set?(0.00?sec)

4. 總結

以需求驅動技術，技術本身沒有優略之分，只有業務之分。

轉載于:https://blog.51cto.com/lisea/1943689

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