Summary of simple operations of libraries and tables in mysql (with examples)

This article brings you a summary of the simple operations of libraries and tables in MySQL (with examples). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. Library operations

1. Create database

Create database:

Create database library name charset utf8; charset uft8 Optional

1.2 Database naming convention:

Can be distinguished by letters, numbers, underscores, @, #, $

Write

Uniqueness

Cannot use keywords such as create select

Cannot use numbers alone

Maximum 128 digits

2. Basic database operations:

查看数据库
show databases;
show create database db1;
select database();
选择数据库
USE 数据库名
删除数据库
DROP DATABASE 数据库名;
修改数据库
alter database db1 charset utf8;

2. Table operations:

A table is equivalent to a file. A record in the table is equivalent to a row of content in the file. A record has a corresponding title, which is called

The cid cname teacher_id in the first line is a field, and each other line is a record.

Create table:

#语法：
create table 表名(
字段名1 类型[(宽度) 约束条件],
字段名2 类型[(宽度) 约束条件],
字段名3 类型[(宽度) 约束条件]
);
#注意：
1. 在同一张表中，字段名是不能相同
2. 宽度和约束条件可选、非必须，宽度指的就是字段长度约束，例如：char(10)里面的10
3. 字段名和类型是必须的

# 事例

mysql> create database db1 charset utf8;

mysql> use db1;

mysql> create table t1(  
    -> id int, 
    -> name varchar(50),
    -> sex enum('male','female'),
    -> age int(3)
    -> );

mysql> show tables; #查看db1库下所有表名

mysql> desc t1;
+-------+-----------------------+------+-----+---------+-------+
| Field | Type                  | Null | Key | Default | Extra |
+-------+-----------------------+------+-----+---------+-------+
| id    | int(11)               | YES  |     | NULL    |       |
| name  | varchar(50)           | YES  |     | NULL    |       |
| sex   | enum('male','female') | YES  |     | NULL    |       |
| age   | int(3)                | YES  |     | NULL    |       |
+-------+-----------------------+------+-----+---------+-------+

mysql> select id,name,sex,age from t1;
Empty set (0.00 sec)

mysql> select * from t1;
Empty set (0.00 sec)

mysql> select id,name from t1;
Empty set (0.00 sec)

View table structure:

mysql> describe t1; #查看表结构，可简写为：desc 表名
+-------+-----------------------+------+-----+---------+-------+
| Field | Type                  | Null | Key | Default | Extra |
+-------+-----------------------+------+-----+---------+-------+
| id    | int(11)               | YES  |     | NULL    |       |
| name  | varchar(50)           | YES  |     | NULL    |       |
| sex   | enum('male','female') | YES  |     | NULL    |       |
| age   | int(3)                | YES  |     | NULL    |       |
+-------+-----------------------+------+-----+---------+-------+


mysql> show create table t1\G; #查看表详细结构，可加\G

Insert data

mysql> insert into t1 values
    -> (1,'chao',18,'male'),
    -> (2,'sb',81,'female')
    -> ;
mysql> select * from t1;
+------+------+------+--------+
| id   | name | age  | sex    |
+------+------+------+--------+
|    1 | chao |   18 | male   |
|    2 | sb |   81 | female |
+------+------+------+--------+



mysql> insert into t1(id) values 
    -> (3),
    -> (4);
mysql> select * from t1;
+------+------+------+--------+
| id   | name | age  | sex    |
+------+------+------+--------+
|    1 | chao |   18 | male   |
|    2 | sb |   81 | female |
|    3 | NULL | NULL | NULL   |
|    4 | NULL | NULL | NULL   |
+------+------+------+--------+

Modify table: alter table has fixed syntax format

语法：
1. 修改表名
      ALTER TABLE 表名 
                          RENAME 新表名;

2. 增加字段
      ALTER TABLE 表名
                          ADD 字段名  数据类型 [完整性约束条件…], #注意这里可以通过逗号来分割，一下添加多个约束条件
                          ADD 字段名  数据类型 [完整性约束条件…];
      ALTER TABLE 表名
                          ADD 字段名  数据类型 [完整性约束条件…]  FIRST; #添加这个字段的时候，把它放到第一个字段位置去。
      ALTER TABLE 表名
                          ADD 字段名  数据类型 [完整性约束条件…]  AFTER 字段名;#after是放到后的这个字段的后面去了，我们通过一个first和一个after就可以将新添加的字段放到表的任意字段位置了。
                            
3. 删除字段
      ALTER TABLE 表名 
                          DROP 字段名;

4. 修改字段
      ALTER TABLE 表名 
                          MODIFY  字段名 数据类型 [完整性约束条件…];
      ALTER TABLE 表名 
                          CHANGE 旧字段名 新字段名 旧数据类型 [完整性约束条件…];  #change比modify还多了个改名字的功能，这一句是只改了一个字段名
      ALTER TABLE 表名 
                          CHANGE 旧字段名 新字段名 新数据类型 [完整性约束条件…];#这一句除了改了字段名，还改了数据类型、完整性约束等等的内容

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