六、FOREIGN KEY ---- 外鍵約束
外鍵可以是單個字段，也可以是多個字段。所謂的外鍵約束就是引用字段必須在被引用字段中存在，除非引用字段部分為NULL或全部為NULL（由MATCH TYPE決定），否則INSERT或UPDATE時將返回失敗，且被引用字段必須有唯一約束或是主鍵。
外鍵約束語法相對較復雜一點，創建外鍵的語法如下：
ALTER TABLE tbl_foreign CONSTRAINT fk_constraint FOREIGN KEY（col1,col2） REFERENCES tbl_foreign_refd(refd_col1,refd_col2) MATCH [SIMPLE|FULL] ON DELETE [CASCADE|NO ACTION] ON UPDATE [CASCADE|NO ACTION];
其中：
tbl_foreign : 引用表
fk_constraint : 外鍵約束名稱
(col1,col2) : 引用表中引用字段
tbl_foreign_refd : 被引用表
(refd_col1,refd_col2) : 被引用表中被引用字段，和(col1,col2)對應
MATCH [SIMPLE|FULL] : 外鍵匹配模式,如果引用字段全部不是NULL,則強匹配,否則根據匹配模式進行弱匹配。
--SIMPLE,默認值，只要引用字段中任一字段為NULL，則不要求與被引用字段強匹配；
--FULL,只有引用字段全部為NULL，才不要求與被引用字段強匹配。
ON DELETE [CASCADE | NO ACTION] : 默認NO ACTION
--CASCADE,刪除被引用表數據級聯刪除引用表數據
--NO ACTION,刪除被引用表數據必須先刪除引用表數據，否則，如果引用表如果存在數據，直接刪除被引用表數據返回失敗。
ON UPDATE [CASCADE | NO ACTION] : 默認NO ACTION
--CASCADE,更新被引用表時級聯更新引用表數據
--NO ACTION,更新被引用表時必須先刪除引用表數據，否則，如果引用表存在數據，直接更新被引用表數據返回失敗。
1.創建測試表(引用表tbl_foreign和被引用表tbl_foreign_refd)
create table tbl_foreign_refd( a int not null, b int not null, c varchar ); alter table tbl_foreign_refd add constraint pk_tbl_foreign_refd_a_b primary key(a,b); create table tbl_foreign( a int, b int, c varchar ); alter table tbl_foreign add constraint fk_tbl_foreign_a_b foreign key(a,b) references tbl_foreign_refd(a,b);
上表中完整外鍵其實如下，因為match,on delete,on update會自動使用默認值。
test=# alter table tbl_foreign add constraint fk_tbl_foreign_a_b foreign key(a,b) references tbl_foreign_refd(a,b) match simple on delete no action on update no action;
測試例1.match simple on delete no action on update no action
test=# insert into tbl_foreign_refd (a,b) values (1,1),(1,2),(1,3); INSERT 0 3 test=# insert into tbl_foreign(a,b) values (1,1),(1,2); INSERT 0 2 test=# insert into tbl_foreign(a,b) values (2,1); ERROR: insert or update on table "tbl_foreign" violates foreign key constraint "fk_tbl_foreign_a_b" DETAIL: Key (a, b)=(2, 1) is not present in table "tbl_foreign_refd". test=# insert into tbl_foreign(a) values (2); INSERT 0 1 test=# insert into tbl_foreign(a) values (1); INSERT 0 1 test=# select * from tbl_foreign; a | b | c ---+------+------ 1 | 1 | NULL 1 | 2 | NULL 2 | NULL | NULL 1 | NULL | NULL (4 rows) test=# delete from tbl_foreign_refd where a=1 and b=1; ERROR: update or delete on table "tbl_foreign_refd" violates foreign key constraint "fk_tbl_foreign_a_b" on table "tbl_foreign" DETAIL: Key (a, b)=(1, 1) is still referenced from table "tbl_foreign". test=# update tbl_foreign_refd set a=3 where a=1 and b=1; ERROR: update or delete on table "tbl_foreign_refd" violates foreign key constraint "fk_tbl_foreign_a_b" on table "tbl_foreign" DETAIL: Key (a, b)=(1, 1) is still referenced from table "tbl_foreign".
測試例2.match full on delete cascade on update cascade
刪除外鍵約束，清空數據，重新增加外鍵
test=# alter table tbl_foreign drop constraint fk_tbl_foreign_a_b ; ALTER TABLE test=# delete from tbl_foreign; DELETE 4 test=# alter table tbl_foreign add constraint fk_tbl_foreign_a_b foreign key(a,b) references tbl_foreign_refd(a,b) match full on delete cascade on update cascade; ALTER TABLE
test=# insert into tbl_foreign(a,b) values (1,1),(1,2); INSERT 0 2 test=# insert into tbl_foreign(a) values (2); ERROR: insert or update on table "tbl_foreign" violates foreign key constraint "fk_tbl_foreign_a_b" DETAIL: MATCH FULL does not allow mixing of null and nonnull key values. test=# insert into tbl_foreign(c) values (2); INSERT 0 1 test=# select * from tbl_foreign; a | b | c ------+------+------ 1 | 1 | NULL 1 | 2 | NULL NULL | NULL | 2 (3 rows) test=# delete from tbl_foreign_refd where a=1 and b=1; DELETE 1 test=# update tbl_foreign_refd set a=2 where a=1 and b=2; UPDATE 1 test=# select * from tbl_foreign; a | b | c ------+------+------ NULL | NULL | 2 2 | 2 | NULL (2 rows) test=# update tbl_foreign set a=3 where a=2; ERROR: insert or update on table "tbl_foreign" violates foreign key constraint "fk_tbl_foreign_a_b" DETAIL: Key (a, b)=(3, 2) is not present in table "tbl_foreign_refd".
2.刪除外鍵約束
alter table tbl_foreign drop constraint fk_tbl_foreign_a_b ;
3.增加外鍵約束
和唯一鍵，主鍵一樣，增加外鍵約束前首先要刪除臟數據，對外鍵來說臟數據針對不同的match type來說是不一樣的。
match simple : 引用字段全部是NOT NULL，且在被引用表中不存在的。
match full : 引用字段部分是NULL和全部是NOT NULL且在被引用表中不存在的。
情況一：增加match simple類型外鍵
第一步：刪除外鍵，清空表，寫入測試數據
test=# alter table tbl_foreign drop constraint fk_tbl_foreign_a_b ; ALTER TABLE test=# delete from tbl_foreign; DELETE 2 test=# insert into tbl_foreign(a,b) values (1,2),(2,2),(1,1); INSERT 0 3 test=# insert into tbl_foreign(a) values (3),(4); INSERT 0 2 test=# insert into tbl_foreign(c) values (5); INSERT 0 1 test=# select * from tbl_foreign; a | b | c ------+------+------ 1 | 2 | NULL 2 | 2 | NULL 1 | 1 | NULL 3 | NULL | NULL 4 | NULL | NULL NULL | NULL | 5 (6 rows) test=# select * from tbl_foreign_refd ; a | b | c ---+---+------ 1 | 3 | NULL 2 | 2 | NULL (2 rows)
對於要增加的外鍵來說，(1,1,NULL),(1,2,NULL)是臟數據。
第二步：查詢臟數據
test=# select * from tbl_foreign where not exists (select null from tbl_foreign_refd where tbl_foreign_refd.a=tbl_foreign.a and tbl_foreign_refd.b=tbl_foreign.b) and a is not null and b is not null; a | b | c ---+---+------ 1 | 1 | NULL 1 | 2 | NULL (2 rows)
第三步：刪除臟數據
將上面SQL語句中的select替換成delete即可。
test=# delete from tbl_foreign where not exists (select null from tbl_foreign_refd where tbl_foreign_refd.a=tbl_foreign.a and tbl_foreign_refd.b=tbl_foreign.b) and a is not null and b is not null; DELETE 2
第四步：增加外鍵
test=# alter table tbl_foreign add constraint fk_tbl_foreign_a_b foreign key(a,b) references tbl_foreign_refd(a,b) match simple; ALTER TABLE
情況二：增加match full類型外鍵
第一步：刪除外鍵，清空表，寫入測試數據
test=# alter table tbl_foreign drop constraint fk_tbl_foreign_a_b ; ALTER TABLE test=# delete from tbl_foreign; DELETE 4 test=# insert into tbl_foreign(a,b) values (1,2),(2,2),(1,1); INSERT 0 3 test=# insert into tbl_foreign(a) values (3),(4); INSERT 0 2 test=# insert into tbl_foreign(c) values (5); INSERT 0 1 test=# select * from tbl_foreign; a | b | c ------+------+------ 1 | 2 | NULL 2 | 2 | NULL 1 | 1 | NULL 3 | NULL | NULL 4 | NULL | NULL NULL | NULL | 5 (6 rows) test=# select * from tbl_foreign_refd ; a | b | c ---+---+------ 1 | 3 | NULL 2 | 2 | NULL (2 rows)
對於要增加的外鍵來說，(1,1,NULL),(1,2,NULL),(3,NULL,NULL),(4,NULL,NULL)是臟數據。
第二步：查詢臟數據
test=# select * from tbl_foreign where not exists (select null from tbl_foreign_refd where tbl_foreign_refd.a=tbl_foreign.a and tbl_foreign_refd.b=tbl_foreign.b) and ((a is not null and b is not null) or (a is not null and b is null) or(a is null and b is not null)); a | b | c ---+------+------ 1 | 1 | NULL 1 | 2 | NULL 3 | NULL | NULL 4 | NULL | NULL (4 rows)
第三步：刪除臟數據
將上面SQL語句的SELECT替換成DELETE即可。
test=# delete from tbl_foreign where not exists (select null from tbl_foreign_refd where tbl_foreign_refd.a=tbl_foreign.a and tbl_foreign_refd.b=tbl_foreign.b) and ((a is not null and b is not null) or (a is not null and b is null) or(a is null and b is not null)); DELETE 4
第四步：增加外鍵約束
test=# alter table tbl_foreign add constraint fk_tbl_foreign_a_b foreign key(a,b) references tbl_foreign_refd(a,b) match full; ALTER TABLE