openGauss每日一练第16天-随堂笔记

1. 学习目标
   openGauss每日一练第15天-随堂笔记

2. 课程学习

• 环境准备

--首先创建一张测试表。
omm=# drop table if exists test;
NOTICE:  table "test" does not exist, skipping
DROP TABLE
omm=# create table test(
      id bigint,
      name varchar(50) not null,
      age  int default 20,
      primary key(id)
     );
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "test_pkey" for table "test"
CREATE TABLE

• 为表添加字段

--查看表test的信息
omm=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 20
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

--为表test新增一列，列名为sex，数据类型为Boolean：
omm=# alter table test add column sex Boolean;
ALTER TABLE

--执行下面gsql命令，查看表test的信息
omm=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 20
 sex    | boolean               | 
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 删除表中的已有字段

--执行下面的SQL语句，删除刚刚添加的列sex：
omm=# alter table test drop column sex;
ALTER TABLE

--执行下面gsql命令，再次查看表test的信息
omm=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 20
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 删除表的已有约束

--表test上有一个名叫test_pkey的PRIMARY KEY约束。执行下面的SQL语句，删除这个约束：
omm=# alter table test drop constraint test_pkey;
ALTER TABLE

--执行下面的gsql命令，再次查看表test的信息：
omm=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 20

--或直接查看约束是否被删除
select * from pg_constraint  where conname like 'test_pkey';

• 为表添加约束

--执行下面的SQL命令，为表test添加刚刚删除的主键约束：
omm=# alter table test add constraint test_pkey primary key(id);
NOTICE:  ALTER TABLE / ADD PRIMARY KEY will create implicit index "test_pkey" for table "test"
ALTER TABLE

--再次查看表test的信息：
omm=# \x
Expanded display is on.
omm=# select * from pg_constraint  where conname like 'test_pkey';
-[ RECORD 1 ]-+----------
conname       | test_pkey
connamespace  | 2200
contype       | p
condeferrable | f
condeferred   | f
convalidated  | t
conrelid      | 16389
contypid      | 0
conindid      | 16395
confrelid     | 0
confupdtype   |  
confdeltype   |  
confmatchtype |  
conislocal    | t
coninhcount   | 0
connoinherit  | tconkey        | {1}
confkey       | 
conpfeqop     | 
conppeqop     | 
--More--
consoft       | f
conopt        | f
conffeqop     | 
conexclop     | 
conbin        | 
consrc        | 
conincluding  | 

omm=# \d test
omm=#              Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 20
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 修改表字段的默认值

--执行下面的SQL语句，将age的默认值变更为25
omm=# alter table test alter column age set default 25;
ALTER TABLE
omm=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 25
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 修改表字段的数据类型

omm=# alter table test ALTER COLUMN age TYPE bigint;
ALTER TABLE

omm=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | bigint                | default 25
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 修改表字段的名字

omm=# ALTER TABLE test RENAME COLUMN age TO stuage;
ALTER TABLE
omm=# \d test
              Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 stuage | bigint                | default 25
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 修改表的名字

omm=# ALTER TABLE test RENAME TO mytest;
ALTER TABLE
omm=# \d mytest
            Table "public.mytest"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 stuage | bigint                | default 25
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

• 删除表

omm=# DROP TABLE mytest;
DROP TABLE

3. 总结
   今天学习内容主要是表相关字段类型、字段属性的修改，以及表、表字段的的参数变更。