学习目标

学习openGuass数据库中如何对表进行修改

1.创建表，为表添加字段

[omm@ogdb1 ~]$ gsql -p 40000 -r
gsql ((openGauss 3.1.0 build 4e931f9a) compiled at 2022-09-29 14:19:24 commit 0 last mr  )
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.

--首先创建一张测试表。
omm=# drop table if exists test;
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
omm=#

--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
omm=#

--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

2.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

3.删除表的已有约束、添加约束

--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
omm=# select * from pg_constraint where conname like 'test_pkey';
 conname | connamespace | contype | condeferrable | condeferred | convalidated | conrelid | contypid | conindid | confrelid | confup
dtype | confdeltype | confmatchtype | conislocal | coninhcount | connoinherit | consoft | conopt | conkey | confkey | conpfeqop | co
nppeqop | conffeqop | conexclop | conbin | consrc | conincluding
---------+--------------+---------+---------------+-------------+--------------+----------+----------+----------+-----------+-------
------+-------------+---------------+------------+-------------+--------------+---------+--------+--------+---------+-----------+---
--------+-----------+-----------+--------+--------+--------------
(0 rows)


--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=# select * from pg_constraint where conname like 'test_pkey';
  conname  | connamespace | contype | condeferrable | condeferred | convalidated | conrelid | contypid | conindid | confrelid | conf
updtype | confdeltype | confmatchtype | conislocal | coninhcount | connoinherit | consoft | conopt | conkey | confkey | conpfeqop |
conppeqop | conffeqop | conexclop | conbin | consrc | conincluding
-----------+--------------+---------+---------------+-------------+--------------+----------+----------+----------+-----------+-----
--------+-------------+---------------+------------+-------------+--------------+---------+--------+--------+---------+-----------+-
----------+-----------+-----------+--------+--------+--------------
 test_pkey |         2200 | p       | f             | f           | t            |    16595 |        0 |    16601 |         0 |
        |             |               | t          |           0 | t            | f       | f      | {1}    |         |           |
          |           |           |        |        |
(1 row)

omm=#
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

4.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=#
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

5.修改表字段的数据类型

omm=# alter table test alter column age type bigint;
ALTER TABLE
omm=#
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

6.修改表字段的名字

--修改表字段的名字,并再次查询确认
omm=# alter table test rename column age to stuage;
ALTER TABLE
omm=#
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

7.修改表的名字

--修改表的名字。执行下面的SQL语句，将表test的名字变更为mytest：
omm=# alter table test rename to mytest;
ALTER TABLE
omm=#
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

8.删除表

omm=# drop table mytest;
DROP TABLE
omm=#