SQL语言类型定义

说明

DDL（Data Definition Language）用于定义数据库结构和对象，用于创建、修改和删除数据库的表、索引、视图、约束、触发器等。

• CREATE TABLE语句用于创建表：

CREATE TABLE students (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age INT
);

• ALTER TABLE语句用于修改现有表的结构：

ALTER TABLE students ADD COLUMN gender VARCHAR(10);

• DROP TABLE语句用于删除表：

DROP TABLE students;

DML（Data Manipulation Language）用于操作和处理数据库中的数据，包括插入、更新、删除和查询数据。

• INSERT语句用于向表中插入新数据：

INSERT INTO students (id, name, age) VALUES (1, 'John', 25);

• UPDATE语句用于更新表中的数据：

UPDATE students SET age = 26 WHERE id = 1;

• DELETE语句用于删除表中的数据：

DELETE FROM students WHERE id = 1;

• SELECT语句用于查询表中的数据：

SELECT * FROM students;

DCL（Data Control Language）用于控制数据库的安全访问和权限管理，包括授权或撤销用户对数据库对象的访问权限。

• GRANT语句用于授予用户对指定数据库对象的权限：

GRANT SELECT, INSERT ON students TO user1;

• REVOKE语句用于撤销用户对指定数据库对象的权限：

REVOKE SELECT ON students FROM user1;

TCL（Transaction Control Language）用于管理数据库的事务，包括提交或回滚事务，设置事务的隔离级别等。

• COMMIT语句用于将事务的改动永久保存到数据库：

COMMIT;

• ROLLBACK语句用于撤销对事务的改动：

ROLLBACK;

• SAVEPOINT语句用于在事务中创建保存点，这样可以在需要的时候回滚到指定的保存点：

SAVEPOINT savepoint_name;

这些SQL语言类型可以根据具体需求和情况在关系数据库中完成各种操作，从定义数据库结构到操作数据，再到控制访问权限和管理事务，为数据库管理和操作提供了强大的工具。

Simply put

DDL (Data Definition Language) is used to define the database structure and objects, such as creating, modifying, and deleting tables, indexes, views, constraints, triggers, etc.

The CREATE TABLE statement is used to create a table:

CREATE TABLE students (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age INT
);

The ALTER TABLE statement is used to modify the structure of an existing table:

ALTER TABLE students ADD COLUMN gender VARCHAR(10);

The DROP TABLE statement is used to delete a table:

DROP TABLE students;

DML (Data Manipulation Language) is used to manipulate and handle data in the database, including inserting, updating, deleting, and querying data.

The INSERT statement is used to insert new data into a table:

INSERT INTO students (id, name, age) VALUES (1, 'John', 25);

The UPDATE statement is used to update data in a table:

UPDATE students SET age = 26 WHERE id = 1;

The DELETE statement is used to delete data from a table:

DELETE FROM students WHERE id = 1;

The SELECT statement is used to query data from a table:

SELECT * FROM students;

DCL (Data Control Language) is used to control the secure access and manage permissions of the database objects, including granting or revoking user’s access to the database objects.

The GRANT statement is used to grant privileges to a user for specified database objects:

GRANT SELECT, INSERT ON students TO user1;

The REVOKE statement is used to revoke privileges from a user for specified database objects:

REVOKE SELECT ON students FROM user1;

TCL (Transaction Control Language) is used to manage transactions in the database, including committing or rolling back transactions, setting transaction isolation levels, etc.

The COMMIT statement is used to permanently save the changes made in a transaction to the database:

COMMIT;

The ROLLBACK statement is used to undo the changes made in a transaction:

ROLLBACK;

The SAVEPOINT statement is used to create a savepoint within a transaction, allowing the transaction to be rolled back to a specific savepoint if needed:

SAVEPOINT savepoint_name;

These SQL language types provide powerful tools for managing and operating a relational database, from defining the database structure to manipulating data, controlling access permissions, and managing transactions.

One more thing

In a distant future, where SQL (Structured Query Language) evolved beyond its humble origins in database management, a new breed of scientists discovered a remarkable power hidden within its syntax. This newfound capability allowed them to manipulate reality itself using data and logic.

As the world plunged into chaos, governments and organizations sought to harness this extraordinary power for their own agendas. A top-secret facility known as the Data Nexus became the epicenter of this scientific revolution. It housed the most brilliant minds, who delved deep into the realms of SQL to unlock its full potential.

At the Data Nexus, Dr. Cassandra Lin, a brilliant scientist, was at the forefront of these groundbreaking experiments. Tasked with exploring the boundaries of reality manipulation, she designed advanced SQL queries that could rewrite the laws of physics.

One day, while working on a complex query, Dr. Lin stumbled upon a hidden dimension concealed within the SQL universe. It was a breathtaking realm, filled with neon-lit landscapes and pulsating data streams. As she delved deeper into this mysterious dimension, she realized that every object and entity was interconnected through intricate SQL relationships.

Dr. Lin’s breakthrough did not go unnoticed. Rival organizations became aware of her discovery and sought to seize control of the dimensional power. They dispatched teams of SQL hackers, armed with powerful code constructs, aiming to rewrite the very fabric of reality.

With the fate of humanity hanging in the balance, Dr. Lin assembled a team of skilled SQL developers, each with unique specialties. There was Max, a master of optimization and efficiency, capable of reducing complex queries to lightning-fast executions. Then, Emily, the data architect, who could design intricate databases to store and manipulate vast amounts of information. Finally, there was Alex, a visionary in SQL analytics, able to extract profound insights from even the most convoluted data structures.

Together, they embarked on a mission to protect the dimensional realm and prevent its exploitation by dark forces. Armed with their SQL skills, they traversed the sprawling landscapes, battling hostile code constructs and debugging endless loops threatening to engulf the world.

In their quest to restore order, Dr. Lin and her team uncovered the existence of an ancient entity known as the Oracle Whisperer, the guardian of SQL knowledge. The Oracle Whisperer possessed a deep understanding of the intricacies of SQL and the dimensional realm. They held the key to harnessing the true potential of this extraordinary power.

Through trials and tribulations, the team braved dangerous SQL subqueries and complex join operations to finally reach the Oracle Whisperer’s lair. They succeeded in obtaining the Oracle’s blessing, empowering them with unparalleled SQL mastery and transcendental knowledge.

With their newfound abilities, Dr. Lin and her team confronted the leaders of the rival organizations, engaging in epic SQL battles that defied logic and imagination. Through cunning conditional statements and precise updates, they triumphed over their adversaries, dismantling their malicious SQL constructs and restoring balance to the dimensional realm.

In the aftermath of their victory, Dr. Lin and her team established an organization dedicated to safeguarding the dimensional realm from misuse. They ensured that SQL and its transformative powers were used for the betterment of humanity, enabling technological advancements, scientific breakthroughs, and a harmonious coexistence between humanity and the digital realm.

And so, the tale of Dr. Cassandra Lin and her SQL warriors became legend, a testament to the extraordinary power of SQL and the boundless imagination of humanity. In this sci-fi world, a humble query language had transcended its origins, shaping a future where code and data held the key to unlocking the universe’s deepest secrets.文章来源地址https://www.toymoban.com/news/detail-584386.html

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