sql常用命令之增删改查

Posted 2020-11-23 落日峡谷

1. 显示数据库 show databases;

show databases;

2. 显示当前数据库、用户和版本

select current_database();   # hive

select database();   #mysql

select user();   #显示当前用户
select version();   #版本

3. 创建/删除数据库

create database db_name;
drop database db_name；#删除数据库

4. 选择某个数据库

use db_name;

5. 显示当前数据库下的表

show tables;

6. 创建表 create table

create table if not exists my_test(
   id INT UNSIGNED AUTO_INCREMENT,
   name_people VARCHAR(40) NOT NULL,
     submission_time DATETIME,
   PRIMARY KEY(id)
)ENGINE=InnoDB DEFAULT CHARSET=utf8;

7. 描述表结构DESC或者DESCRIBE、show columns from

DESC my_test;
SHOW COLUMNS FROM my_test; #显示表结构

8. 插入数据 insert into

INSERT my_test(
    id,
    name_people,
    submission_time
)
VALUES
    (1, \'Mary\', NOW());

9 .查询 select

select * from my_test;
select name_people from my_test;
select name_peole from my_test where id=1;

10. 删除一条记录 delete

delete from my_test where id=1;

11. 删除表中的所有记录，但是保存原来的表结构

delete from my_test;

12. 更新表的某一列 update

UPDATE my_test SET name_people = \'陛下\' WHERE id=1

13. 更新多列数据，用 \',\' 逗号隔开

UPDATE my_test SET name_people = \'将军\', submission_time = NOW() WHERE id=1

 14. 增加新列 alter，然后update

ALTER table my_test add name_adress VARCHAR(50) not null;
update my_test set name_adress=\'北京\';

 15. 复制另一张表的数据和表结构，来新建表 create，使用as关键词

CREATE TABLE my_test_copy AS SELECT * FROM  my_test;
SELECT * from my_test_copy;

 16. 删除表

drop table table_name;

 17. 增加主键，需要先删除原来的主键，否则会报错：[Err] 1068 - Multiple primary key defined

#增加主键
alter table my_test add phone_number VARCHAR(30) not null;
update my_test set phone_number=123456789;
alter table my_test add primary key (phone_number); #[Err] 1068 - Multiple primary key defined，需要先删除原来的主键

18. 删除主键，删除主键出现：[Err] 1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key

alter table my_test drop primary key; 
#一般的error：[Err] 1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key
# 表示自增列只能有1列，且这列必须为key，也就是建表的时候，如果是自增列，必须用primary key标识
# 例如：
create table if not exists my_test(
   id INT UNSIGNED AUTO_INCREMENT,
   name_people VARCHAR(40) NOT NULL,
     submission_time DATETIME,
   PRIMARY KEY(id)
)ENGINE=InnoDB DEFAULT CHARSET=utf8;
#出现错误的原因是：主键是自增属性，不能直接删除

19. 建立一个新表，但是主键不设置为自增属性，然后就可以删除主键

create table if not exists my_test_key(
   id INT UNSIGNED,
   name_people VARCHAR(40) NOT NULL,
   submission_time DATETIME,
   PRIMARY KEY(id)
)ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT my_test_key(
    id,
    name_people,
    submission_time
)
VALUES
    (1, \'Tom\', NOW());

show keys from my_test_key; #显示主键

alter table my_test_key drop primary key; 

show keys from my_test_key; #显示主键

20. 建立索引 create index

create index index_one on my_test_key(submission_time);

 

 21. 建立唯一索引 create unique index

create unique index index_two on my_test_key(name_people);

 

 22. 显示索引 show index

show index from my_test_key; #显示索引，my_test_key属于表名

23. 显示主键 show keys

show keys from my_test_key; #显示主键，my_test_key属于表名

24. 删除索引 drop index

drop index index_one on my_test_key;
alter table my_test_key drop index index_two;

 

#欢迎交流