openGauss每日一练第14天 | 学习心得体会

学习目标

学习表的约束、表的默认值、自增类型等技术

课程作业

1.创建表的时候定义列级约束

创建表的时候为表定义列级约束：在列级定义了primary key约束（id列）和not null约束（name列）。

omm=# create table test( id bigint primary key,name varchar(50) not null,age int);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "test_pkey" for table "test"
CREATE TABLE
omm=# insert into test values(1,'user1',50);
INSERT 0 1
omm=# select * from test;
 id | name  | age 
----+-------+-----
  1 | user1 |  50
(1 row)

omm=# \d test 
            Table "public.test"
 Column |         Type          | Modifiers 
--------+-----------------------+-----------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | 
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

omm=# 

2.创建表的时候定义表级约束

创建表时定义表级约束:在表列级定义了primary key约束（id列），在列级定义了not null约束（name列）。

create table test001(
id bigint,
name varchar(50) not null, – 创建列级not null约束
age int,
primary key(id) – 创建表级约束
);

omm=# create table test001( id bigint, name varchar(50) not null, age  int, primary key(id) );
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "test001_pkey" for table "test001"
CREATE TABLE
omm=# insert into test001 values(1,'user1',50);
INSERT 0 1
omm=# select * from test001;
 id | name  | age 
----+-------+-----
  1 | user1 |  50
(1 row)

omm=# \d test001
           Table "public.test001"
 Column |         Type          | Modifiers 
--------+-----------------------+-----------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | 
Indexes:
    "test001_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

omm=# 

3.为表的属性定义默认值

create table test002(
id bigint,
name varchar(28) not null,
age int default 20, – 为该列定义默认值为20
primary key(id)
);

omm=# create table test002(
     id bigint,
      name varchar(28) not null,
    age  int default 20, 
      primary key(id)
   );
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "test002_pkey" for table "test002"
CREATE TABLE
omm=# insert into test002(id,name) values(1,'user1');
INSERT 0 1
omm=# insert into test002(id,name) values(2,'user2');
INSERT 0 1
omm=# select * from test002;
 id | name  | age 
----+-------+-----
  1 | user1 |  20
  2 | user2 |  20
(2 rows)

omm=#

4.如果在创建表的时候，没有为某列定义默认值，缺省的默认值是空值null

omm=# drop table if exists test;
DROP TABLE
omm=# create table test(
   id bigint,
      name varchar(50) not null,
    age  int, primary key(id)                            
     );       

NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "test_pkey" for table "test"
CREATE TABLE
omm=# insert into test(id,name) values(1,'user1');
INSERT 0 1
omm=# select * from test;
 id | name  | age 
----+-------+-----
  1 | user1 |    
(1 row)

omm=# 

5.创建表时使用自增数据类型

使用serial数据类型

omm=# create table invoice(invoicenum serial NOT NULL,name varchar(20));
NOTICE:  CREATE TABLE will create implicit sequence "invoice_invoicenum_seq" for serial column "invoice.invoicenum"
CREATE TABLE
omm=# insert into invoice(name) values('user1');
INSERT 0 1
omm=# insert into invoice(name) values('user2');
INSERT 0 1
omm=# insert into invoice(name) values('user3');
INSERT 0 1
omm=# select * from invoice;
 invoicenum | name  
------------+-------
          1 | user1
          2 | user2
          3 | user3
(3 rows)

omm=# 

6.使用现有的表创建新表

将创建新表，并且会将旧表的数据拷贝给新表：

omm=# CREATE TABLE newtestwithdata AS SELECT * FROM invoice;
INSERT 0 3
omm=#  SELECT * FROM newtestwithdata;
 invoicenum | name  
------------+-------
          1 | user1
          2 | user2
          3 | user3
(3 rows)

omm=# 

创建和旧表的表结构相同的新表，但是不会将旧表的数据拷贝给新表：

omm=# CREATE TABLE testnewwithoutdata AS SELECT * FROM invoice WHERE 1=2;
INSERT 0 0
omm=# SELECT * FROM testnewwithoutdata;
 invoicenum | name 
------------+------
(0 rows)

omm=#