openGauss每日一练第16天 | openGauss逻辑结构：表管理4

1.创建表，为表添加字段

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

2.删除表中的已有字段

--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
 sex    | boolean               | 
Indexes:
    "test_pkey" PRIMARY KEY, btree (id) TABLESPACE pg_default

--执行下面的SQL语句，删除列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 | confupdtype | confdeltype | co
nfmatchtype | conislocal | coninhcount | connoinherit | consoft | conopt | con
key | confkey | conpfeqop | conppeqop | conffeqop | conexclop | conbin | consr
c | conincluding 
---------+--------------+---------+---------------+-------------+-------------
-+----------+----------+----------+-----------+-------------+-------------+---
------------+------------+-------------+--------------+---------+--------+----
----+---------+-----------+-----------+-----------+-----------+--------+------
--+--------------
(0 rows)

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=# \d test
             Table "public.test"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 age    | integer               | default 25

5.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

6.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

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=# \d mytest
            Table "public.mytest"
 Column |         Type          | Modifiers  
--------+-----------------------+------------
 id     | bigint                | not null
 name   | character varying(50) | not null
 stuage | bigint                | default 25

8.删除表

omm=# DROP TABLE mytest;
DROP TABLE

omm=# \dt
No relations found.