2019独角兽企业重金招聘Python工程师标准>>> 
概述
一般来说,每一个都需要一个主键,主键的值一般是通过数据库的auto_increment或者使用插入sequence来保证每条记录分配唯一的值.
Oracle在12c中实现了表中序列的auto_increment,12c之前只能通过trigger来自己实现.
PG中有个数据类型serial.
创建带serial的表
创建和使用
db01=# create table t_t3(id serial,name varchar(30));
CREATE TABLE
db01=# insert into t_t3(name) values('aaa');
INSERT 0 1
db01=# insert into t_t3(name) values('aaa');
INSERT 0 1
db01=# insert into t_t3(name) select generate_series(1,10) || 'a';
INSERT 0 10
db01=# select * from t_t3;
id | name
----+------
1 | aaa
2 | aaa
......
11 | 9a
12 | 10a
(12 rows)
只需要插入id的默认值,自动会帮你生成序列号所以下面的方式也可以
也可以通过关键字default
db01=# insert into t_t3 values(default,'bbb');
INSERT 0 1
db01=# select * from t_t3 where name = 'bbb';
id | name
----+------
13 | bbb
(1 row)
使用了serial类型的自动,不要主动插入,否则就会主键冲突
db01=# alter table t_t3 add constraint pk_t_t3 primary key(id);
ERROR: could not create unique index "pk_t_t3"
DETAIL: Key (id)=(1) is duplicated.
db01=# delete from t_t3 where name = 'a';
DELETE 1
db01=# alter table t_t3 add constraint pk_t_t3 primary key(id);
ALTER TABLE
##serial的内部##
db01=# \ds
List of relations
Schema | Name | Type | Owner
--------+-------------+----------+-------
hippo | t_t3_id_seq | sequence | hippo
(1 row)
db01=# select currval('t_t3_id_seq');
currval
---------
13
(1 row)
serial内部其实帮助我们实现了:
- 创建了一个sequence,名字是
表名_字段名_seq - 设置id字段为NOT NULL DEFAULT nextval('此sequence')
- 关联此sequence到相关字段.
我们自己来实现一下:
db01=# create table t4(id bigint,name varchar(30));
CREATE TABLE
db01=# create sequence seq_t4 increment by 2 start 1 cache 20 owned by t4.id;
CREATE SEQUENCE
db01=# select nextval('seq_t4');
1
db01=# select nextval('seq_t4');
3
db01=# alter table t4 alter id set not null,alter id set default nextval('seq_t4');
ALTER TABLE
db01=# insert into t4(name) values('a');
INSERT 0 1
db01=# insert into t4(name) values('a');
INSERT 0 1
db01=# select * from t4;
5 | a
7 | a
删除
db01=# \ds
List of relations
Schema | Name | Type | Owner
--------+-------------+----------+-------
hippo | seq_t4 | sequence | hippo
hippo | t_t3_id_seq | sequence | hippo
(2 rows)
db01=# drop table t4;
DROP TABLE
db01=# \ds
List of relations
Schema | Name | Type | Owner
--------+-------------+----------+-------
hippo | t_t3_id_seq | sequence | hippo
(1 row)
因为这个sequence关联在表的字段上,所以删除表字段会一起删除
除非owner没有指定
补充
serial类型有3中,smallserial,serial,bigserial,底层分别对应smallint,integer,bigint,分别占2bit,4bit,8bit.
查看sequence的属性
db01=# select * from t_t3_id_seq;
sequence_name | last_value | start_value | increment_by | max_value | min_value | cache_value | log_cnt | is_cycled | is_called
---------------+------------+-------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
t_t3_id_seq | 13 | 1 | 1 | 9223372036854775807 | 1 | 1 | 22 | f | t
(1 row)
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