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外键可以是单个字段，也可以是多个字段。所谓的外键约束就是引用字段必须在被引用字段中存在，除非引用字段部分为NULL或全部为NULL（由MATCH TYPE决定），否则INSERT或UPDATE时将返回失败，且被引用字段必须有唯一约束或是主键。
外键约束语法相对较复杂一点，创建外键的语法如下：
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 : 外键约束名称
(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)对应
MATCH [SIMPLE|FULL] : 外键匹配模式,如果引用字段全部不是NULL,则强匹配,否则根据匹配模式进行弱匹配。
--SIMPLE,默认值，只要引用字段中任一字段为NULL，则不要求与被引用字段强匹配；
--FULL,只有引用字段全部为NULL，才不要求与被引用字段强匹配。
ON DELETE [CASCADE | NO ACTION] : 默认NO ACTION
--CASCADE,删除被引用表数据级联删除引用表数据
--NO ACTION,删除被引用表数据必须先删除引用表数据，否则，如果引用表如果存在数据，直接删除被引用表数据返回失败。
ON UPDATE [CASCADE | NO ACTION] : 默认NO ACTION
--CASCADE,更新被引用表时级联更新引用表数据
--NO ACTION,更新被引用表时必须先删除引用表数据，否则，如果引用表存在数据，直接更新被引用表数据返回失败。
1.创建测试表(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);
上表中完整外键其实如下，因为match,on delete,on update会自动使用默认值。
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;
测试例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".
测试例2.match full on delete cascade on update cascade
删除外键约束，清空数据，重新增加外键
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.删除外键约束
alter table tbl_foreign drop constraint fk_tbl_foreign_a_b ;
3.增加外键约束
和唯一键，主键一样，增加外键约束前首先要删除脏数据，对外键来说脏数据针对不同的match type来说是不一样的。
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。
情况一：增加match simple类型外键
第一步：删除外键，清空表，写入测试数据
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)
对于要增加的外键来说，(1,1,NULL),(1,2,NULL)是脏数据。
第二步：查询脏数据
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：删除脏数据
将上面SQL语句中的select替换成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：增加外键
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
情况二：增加match full类型外键
第一步：删除外键，清空表，写入测试数据
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)
对于要增加的外键来说，(1,1,NULL),(1,2,NULL),(3,NULL,NULL),(4,NULL,NULL)是脏数据。
第二步：查询脏数据
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：删除脏数据
将上面SQL语句的SELECT替换成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：增加外键约束
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
postgresql----数据库表约束----FOREIGN KEY
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原文地址：http://www.cnblogs.com/alianbog/p/5598393.html
