我对这个基本但重要的问题有些怀疑,因此我决定以身作则。
让我们来创建测试表主有两列,CON_ID具有独特的约束和ind_id通过唯一索引收录。
create table master (
con_id integer unique,
ind_id integer
);
create unique index master_unique_idx on master (ind_id);
Table "public.master"
Column | Type | Modifiers
--------+---------+-----------
con_id | integer |
ind_id | integer |
Indexes:
"master_con_id_key" UNIQUE CONSTRAINT, btree (con_id)
"master_unique_idx" UNIQUE, btree (ind_id)
在表描述(psql中的\ d)中,您可以根据唯一索引来区分唯一约束。
独特性
为了以防万一,让我们检查唯一性。
test=# insert into master values (0, 0);
INSERT 0 1
test=# insert into master values (0, 1);
ERROR: duplicate key value violates unique constraint "master_con_id_key"
DETAIL: Key (con_id)=(0) already exists.
test=# insert into master values (1, 0);
ERROR: duplicate key value violates unique constraint "master_unique_idx"
DETAIL: Key (ind_id)=(0) already exists.
test=#
它按预期工作!
外键
现在,我们将使用两个外键定义明细表,这两个外键引用master中的两列。
create table detail (
con_id integer,
ind_id integer,
constraint detail_fk1 foreign key (con_id) references master(con_id),
constraint detail_fk2 foreign key (ind_id) references master(ind_id)
);
Table "public.detail"
Column | Type | Modifiers
--------+---------+-----------
con_id | integer |
ind_id | integer |
Foreign-key constraints:
"detail_fk1" FOREIGN KEY (con_id) REFERENCES master(con_id)
"detail_fk2" FOREIGN KEY (ind_id) REFERENCES master(ind_id)
好吧,没有错误。让我们确保它起作用。
test=# insert into detail values (0, 0);
INSERT 0 1
test=# insert into detail values (1, 0);
ERROR: insert or update on table "detail" violates foreign key constraint "detail_fk1"
DETAIL: Key (con_id)=(1) is not present in table "master".
test=# insert into detail values (0, 1);
ERROR: insert or update on table "detail" violates foreign key constraint "detail_fk2"
DETAIL: Key (ind_id)=(1) is not present in table "master".
test=#
这两列都可以在外键中引用。
约束使用索引
您可以使用现有的唯一索引添加表约束。
alter table master add constraint master_ind_id_key unique using index master_unique_idx;
Table "public.master"
Column | Type | Modifiers
--------+---------+-----------
con_id | integer |
ind_id | integer |
Indexes:
"master_con_id_key" UNIQUE CONSTRAINT, btree (con_id)
"master_ind_id_key" UNIQUE CONSTRAINT, btree (ind_id)
Referenced by:
TABLE "detail" CONSTRAINT "detail_fk1" FOREIGN KEY (con_id) REFERENCES master(con_id)
TABLE "detail" CONSTRAINT "detail_fk2" FOREIGN KEY (ind_id) REFERENCES master(ind_id)
现在,列约束描述之间没有区别。
部分索引
在表约束声明中,您无法创建部分索引。它直接由来自定义的create table ...
。在唯一索引声明中,可以设置WHERE clause
创建部分索引。您还可以在表达式上创建索引(不仅在列上),还可以定义其他一些参数(排序规则,排序顺序,NULL放置)。
您不能使用部分索引添加表约束。
alter table master add column part_id integer;
create unique index master_partial_idx on master (part_id) where part_id is not null;
alter table master add constraint master_part_id_key unique using index master_partial_idx;
ERROR: "master_partial_idx" is a partial index
LINE 1: alter table master add constraint master_part_id_key unique ...
^
DETAIL: Cannot create a primary key or unique constraint using such an index.