MySQL数据库应用 2023-03-18

MySQL practice

1.create a database with name example, and set the character set to utf8
2.alter the example database, and set the character set to gbk
3.drop the example database

 

here are the SQL commands to perform the tasks you mentioned:

1. To create a database with name example, and set the character set to utf8, use the following SQL command:

CREATE DATABASE example CHARACTER SET utf8;

2. To alter the example database, and set the character set to gbk, use the following SQL command:

ALTER DATABASE example CHARACTER SET gbk;

3. To drop the example database, use the following SQL command:

DROP DATABASE example;

 

1.通过UI来操作，

2.通过UI的新建query来执行命令

3.直接通过命令行mysql -u root -p

 

 

Here are the SQL statements to complete the tasks:

1. Create a database with name example, and set the character set to utf8:

sql

CREATE DATABASE example CHARACTER SET utf8;

2. Alter the example database, and set the character set to gbk:

sql

ALTER DATABASE example CHARACTER SET gbk;

3. Drop the example database:

sql

DROP DATABASE example;

Note: Be careful when using the DROP DATABASE statement as it permanently deletes the database and all its contents. Make sure to double-check the database name before executing the statement.

 

Do you forgot to add default keyword before Character in above command

You are correct, my apologies for the mistake. When specifying the character set for a database, you should use the "DEFAULT CHARACTER SET" clause to set the default character set for the database.

Here are the corrected SQL statements:

1. Create a database with name example, and set the character set to utf8:

sql

CREATE DATABASE example DEFAULT CHARACTER SET utf8;

2. Alter the example database, and set the character set to gbk:

sql

ALTER DATABASE example DEFAULT CHARACTER SET gbk;

3. Drop the example database:

sql

DROP DATABASE example;

Thank you for bringing this to my attention.

 

CREATE DATABASE db_name

[[DEFAULT] CHARACTER SET charset_name]

[[DEFAULT] COLLATE collation_name]

Does [DEFAULT] mean that this part can be omitted?

Yes, the [DEFAULT] keyword in the CREATE DATABASE statement means that it is optional. If you omit [DEFAULT], the statement will still work the same way.

For example, the following two statements are equivalent:

sql

CREATE DATABASE db_name DEFAULT CHARACTER SET charset_name DEFAULT COLLATE collation_name; CREATE DATABASE db_name CHARACTER SET charset_name COLLATE collation_name;

In both cases, the DEFAULT keyword is optional and can be omitted.

 

show databases with character set and collation in MySQL

To display a list of databases in MySQL with their respective character set and collation, you can use the following SQL query:

SELECT 
  schema_name AS `Database`,
  default_character_set_name AS `Character Set`,
  default_collation_name AS `Collation`
FROM 
  information_schema.SCHEMATA;

 

This query retrieves the database names from the information_schema.SCHEMATA table and displays their corresponding default character set and collation in separate columns.

Note that this query only shows the default character set and collation for each database. If you want to see the character set and collation for individual tables or columns within a database, you will need to query the information_schema.COLUMNS or information_schema.TABLES table.

 

Is MySQL support nchar and nvarchar like SQL Server?

MySQL does not have a specific NCHAR or NVARCHAR data type like SQL Server. However, you can achieve similar functionality in MySQL by using the CHARACTER SET and COLLATE clauses.

MySQL supports the CHAR data type, which allows you to specify a character set and collation for the column. For example, you can define a column to use the utf8mb4 character set and the utf8mb4_unicode_ci collation like this:

CREATE TABLE my_table (
  id INT,
  name CHAR(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci
);

In this example, the name column is defined as a CHAR column with a length of 50, and it uses the utf8mb4 character set and the utf8mb4_unicode_ci collation. This allows you to store Unicode data in the column, similar to how NCHAR and NVARCHAR work in SQL Server.

Note that the maximum length of a CHAR column depends on the character set being used. For example, if you are using the utf8mb4 character set, the maximum length of a CHAR column is 255 characters. If you need to store longer strings, you can use the VARCHAR data type instead, which also supports the CHARACTER SET and COLLATE clauses.

CREATE TABLE my_table (
  id INT,
  name VARCHAR(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci
);

In this example, the name column is defined as a VARCHAR column with a length of 50, and it uses the utf8mb4 character set and the utf8mb4_unicode_ci collation.

 

create a student table in mysql with id, sNo, sName, sex, birthday, deptName, remark columns, make id the primary key usigned int, not null, auto increase. sNo and sName are not null .

Here's an example SQL query to create a student table in MySQL with the specified columns and constraints:

CREATE TABLE student (
  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  sNo VARCHAR(20) NOT NULL,
  sName VARCHAR(50) NOT NULL,
  sex ENUM('M', 'F'),
  birthday DATE,
  deptName VARCHAR(50),
  remark TEXT
);

In this example:

• The id column is defined as an unsigned integer, not null, and auto-incremented to serve as the primary key for the table.
• The sNo and sName columns are defined as VARCHAR data types and marked as not null.
• The sex column is defined as an ENUM data type with the values 'M' and 'F', to represent male and female, respectively.
• The birthday column is defined as a DATE data type.
• The deptName column is defined as a VARCHAR data type.
• The remark column is defined as a TEXT data type, which can store larger amounts of text.

You can customize the column data types and constraints as needed for your specific use case.

 

Do you forgot to specify the engine and charset?

You're right, specifying the engine and character set for the table is also a good practice. Here's an updated version of the query that includes the ENGINE and CHARSET options:

CREATE TABLE student (
  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  sNo VARCHAR(20) NOT NULL,
  sName VARCHAR(50) NOT NULL,
  sex ENUM('M', 'F'),
  birthday DATE,
  deptName VARCHAR(50),
  remark TEXT
) ENGINE=InnoDB CHARSET=utf8mb4;

In this example, the table is created with the InnoDB storage engine and the utf8mb4 character set, which supports storing characters from the full Unicode character set. You can change these options to match your specific needs.

How many engine types are there in MySQL? and what's the differences between them?

There are several storage engines available in MySQL, each with its own strengths and weaknesses. Here's a brief overview of some of the most common storage engines:

1. InnoDB: This is the default and most widely used storage engine in MySQL. It supports transactions, row-level locking, and foreign key constraints, making it a good choice for applications that require data integrity and high concurrency.

2. MyISAM: This storage engine is simple and efficient, but does not support transactions or foreign keys. It's a good choice for read-heavy workloads, such as data warehouses or web servers that serve mostly static content.

3. Memory: This storage engine stores data in memory instead of on disk, making it very fast but not suitable for large amounts of data or data that needs to persist between server restarts.

4. CSV: This storage engine stores data in comma-separated values format, making it easy to import and export data in a human-readable format. However, it does not support indexes or efficient querying.

5. NDB: This storage engine is designed for high availability and scalability, using a distributed architecture to store data across multiple nodes. It's a good choice for large, mission-critical applications that need to handle a high volume of transactions.

There are several other storage engines available as well, including Merge, Archive, and Blackhole, each with their own unique features and use cases. When choosing a storage engine, it's important to consider the specific needs of your application and choose an engine that provides the necessary features and performance characteristics.

 

 

 

作者:Chuck Lu    GitHub