数据库操作
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PS D:\WorkSpace\MachineLearning\DL_learning> mysql -u root -p
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mysql> 显示所有数据库
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| atguigudb |
| dbtest14 |
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
6 rows in set (0.00 sec)创建数据库
mysql> create database itcast;
Query OK, 1 row affected (0.11 sec)
mysql> create database if not exists test1;
Query OK, 1 row affected (0.10 sec)
mysql> create database test2 default charset utf8mb4;
Query OK, 1 row affected (0.08 sec)删除数据库
mysql> drop database itcast;
Query OK, 0 rows affected (0.15 sec)
mysql> drop database if exists test2;
Query OK, 0 rows affected (0.08 sec)进入某个数据库进行查询 操作
mysql> use test1;
Database changed查看当前处于哪个数据库
mysql> select database();
+------------+
| database() |
+------------+
| test1 |
+------------+
1 row in set (0.00 sec)表结构
查询所有表
mysql> use atguigudb;
Database changed
mysql> show tables;
+---------------------+
| Tables_in_atguigudb |
+---------------------+
| countries |
| departments |
| emp_details_view |
| employees |
| job_grades |
| job_history |
| jobs |
| locations |
| order |
| regions |
+---------------------+
10 rows in set (0.00 sec)查询表的结构
创建一张表，；分号表示语句结构，enter才可以执行
mysql> create table tb_user(
-> id int,
-> name varchar(50),
-> age int,
-> gender varchar(1)
-> );
Query OK, 0 rows affected (0.95 sec)mysql> show tables;
+-----------------+
| Tables_in_test1 |
+-----------------+
| tb_user |
+-----------------+
1 row in set (0.00 sec)查询表结构
mysql> desc tb_user;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(50) | YES | | NULL | |
| age | int | YES | | NULL | |
| gender | varchar(1) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
查询表的结构（SQL语言）
mysql> show create table tb_user;
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table
|
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_user | CREATE TABLE `tb_user` (
`id` int DEFAULT NULL,
`name` varchar(50) DEFAULT NULL,
`age` int DEFAULT NULL,
`gender` varchar(1) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)修改表结构
mysql> create table emp(
-> id int comment '编号',
-> workno varchar(10) comment '工号',
-> name varchar(10) comment '姓名',
-> gender char(1) comment '性别',
-> age tinyint unsigned comment '年龄',
-> idcard char(18) comment '身份证号',
-> entrydate date comment '入职时间'
-> ) comment '员工表';
Query OK, 0 rows affected (0.39 sec)添加字段
mysql> alter table emp add nickname varchar(20) comment '昵称';
Query OK, 0 rows affected (0.26 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> describe emp;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| workno | varchar(10) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| idcard | char(18) | YES | | NULL | |
| entrydate | date | YES | | NULL | |
| nickname | varchar(20) | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)修改某列的列名和列的数据类型
把nickname改为username
mysql> alter table emp change nickname username varchar(30) comment '用户名';
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> describe emp;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| workno | varchar(10) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| idcard | char(18) | YES | | NULL | |
| entrydate | date | YES | | NULL | |
| username | varchar(30) | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)修改表名
把表名：emp改为employee
mysql> alter table emp rename to employee;
Query OK, 0 rows affected (0.50 sec)
mysql> describe emp;
ERROR 1146 (42S02): Table 'test1.emp' doesn't exist
mysql> describe employee;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| workno | varchar(10) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| idcard | char(18) | YES | | NULL | |
| entrydate | date | YES | | NULL | |
| username | varchar(30) | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)
mysql> show tables;
+-----------------+
| Tables_in_test1 |
+-----------------+
| employee |
| tb_user |
+-----------------+
2 rows in set (0.00 sec)修改表的字段类型
把username 的类型从varchar(30) 改为 char(28)
mysql> alter table employee modify column username char(28);
Query OK, 0 rows affected (2.45 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> describe employee;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| workno | varchar(10) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| idcard | char(18) | YES | | NULL | |
| entrydate | date | YES | | NULL | |
| username | char(28) | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)删除某一列
删除username那一列
mysql> alter table employee drop username;
Query OK, 0 rows affected (0.27 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> describe employee;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| workno | varchar(10) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| idcard | char(18) | YES | | NULL | |
| entrydate | date | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
7 rows in set (0.00 sec)删除表
删除表
mysql> drop tables if exists tb_user;
Query OK, 0 rows affected (0.22 sec)
mysql> show tables;
+-----------------+
| Tables_in_test1 |
+-----------------+
| employee |
+-----------------+
1 row in set (0.00 sec)删除表，重新创建表
表的数据没了。新表和旧的表的结构不变。（数据没了，结构还在）
mysql> truncate table employee;
Query OK, 0 rows affected (0.93 sec)
mysql> show tables;
+-----------------+
| Tables_in_test1 |
+-----------------+
| employee |
+-----------------+
1 row in set (0.00 sec)
mysql> describe employee;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| workno | varchar(10) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| idcard | char(18) | YES | | NULL | |
| entrydate | date | YES | | NULL | |
| username | varchar(30) | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)