[Java Advanced] Detailed explanation of data definition language (DDL)

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Data Definition Language (DDL) is a part of SQL (Structured Query Language), which is used to define, manage and control the structure and elements of a database. DDL allows database administrators, developers, and other users to create, modify, and delete database objects such as tables, indexes, views, etc. In this article, we will delve into the basic concepts of DDL, including table creation, modification, and deletion, as well as other important DDL-related topics.

What is data definition language (DDL)?

Data Definition Language (DDL) is a subset of SQL, mainly used to define and manage database structures, including the following aspects:

Creation of tables : DDL allows you to create new tables and specify table columns, data types, constraints, etc.
Table modification : You can use DDL to modify the structure of an existing table, such as adding new columns, deleting columns, modifying column data types, etc.
Deletion of tables : DDL allows you to delete tables that are no longer needed, thus freeing up database resources.
Index creation and deletion : Indexes are data structures used to speed up data retrieval, and DDL can be used to create and delete indexes.
Creation and deletion of views : DDL allows you to create virtual tables that are derived from one or more real tables.
Definition of constraints : You can use DDL to define constraints in the table, such as primary keys, foreign keys, unique constraints, etc., to maintain data integrity and consistency.
Management of schemas : DDL also allows you to manage schemas in your database, which are logical containers of database objects.

Let us now delve into some key concepts and operations in DDL.

Table creation

Creating tables is an important aspect of DDL, which defines the structure of the data stored in the database. Here is a simple DDL example to create a table named "users":

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100),
    birthdate DATE
);

Let’s break down the above DDL statement:

CREATE TABLE: This is the keyword to create the table.
users: This is the name of the table.
(user_id INT PRIMARY KEY, username VARCHAR(50) NOT NULL, email VARCHAR(100), birthdate DATE): This is the column definition for the table. Each column has a name, data type, and optional constraints. In this example, we define four columns: user_id, username, emailand birthdate. user_idColumn is defined as primary key.

The above DDL statement creates a table named "users" that contains four columns. user_idA column is a primary key, which means its values must be unique and not null. usernameColumn is a string with a maximum length of 50, and is not allowed to be empty. emailColumn is a string with a maximum length of 100 and can be empty. birthdateColumn is a date type column.

Table modification

In addition to creating tables, DDL also allows you to modify existing table structures. The following are some common table modification operations:

Add Column : You can use ALTER TABLEstatements to add new columns to an existing table. For example, to add a new phonecolumn to the table named "users", you can execute the following DDL statement:
```
ALTER TABLE users
ADD COLUMN phone VARCHAR(20);
```
Modify a column : You can modify a column's data type, length, or other properties. For example, to emailincrease the maximum length of a column from 100 to 150, you can execute the following DDL statement:
```
ALTER TABLE users
MODIFY COLUMN email VARCHAR(150);
```
Drop Column : If a column is no longer needed, you can ALTER TABLEdrop it using the statement. For example, to delete phonea column, you can execute the following DDL statement:
```
ALTER TABLE users
DROP COLUMN phone;
```

table deletion

DDL also allows you to delete tables that are no longer needed. Deleting a table is a cautious operation because it permanently deletes the table and its data. The following is an example DDL statement to delete a table:

DROP TABLE users;

The above DDL statement will delete the table named "users".

Index creation and deletion

Indexes are key data structures used to speed up data retrieval. DDL allows you to create and delete indexes. The following is an example of DDL to create and delete an index:

Create an index : To create an index on a column of a table, you can use CREATE INDEXthe statement. For example, to create an index named "idx_username" on the table named "users", you can execute the following DDL statement:
```
CREATE INDEX idx_username
ON users (username);
```
The above DDL statement will create an index on the "username" column.
Dropping an index : If an index is no longer needed, you can DROP INDEXdrop it using the statement. For example, to delete the index named "idx_username", you can execute the following DDL statement:
```
DROP INDEX idx_username
ON users;
```

View creation and deletion

Views are virtual tables that are derived from one or more real tables. DDL allows you to create and delete views. Here is an example DDL for creating and deleting views:

Create a view : To create a view, you can use CREATE VIEWstatements. For example, the following DDL statement creates a view named "active_users" that displays all users with the status "active" in the table named "users":
```
CREATE VIEW active_users AS
SELECT * FROM users
WHERE status = 'active';
```
Deleting a view : To delete a view, you can use DROP VIEWthe statement. For example, to delete the view named "active_users", you can execute the following DDL statement:
```
DROP VIEW active_users;
```

Definition of constraints

Constraints are rules used to maintain the integrity and consistency of data. DDL allows you to define various constraints. Here are some common constraints:

Primary key constraints : Primary key constraints ensure that the values in a column or a set of columns are unique and not null. For example, the following DDL statement defines a primary key constraint named "pk_user_id", which user_idsets the column as the primary key:
```
ALTER TABLE users
ADD CONSTRAINT pk_user_id PRIMARY KEY (user_id);
```
Foreign key constraints : Foreign key constraints are used to establish an association between two tables. It ensures that values in one table exist in another table. For example, the following DDL statement defines a foreign key constraint that sets user_ida column as a reference to another table:
```
ALTER TABLE orders
ADD CONSTRAINT fk_user_id
FOREIGN KEY (user_id)
REFERENCES users (user_id);
```
Unique Constraint : A unique constraint ensures that the values in a column or set of columns are unique, but can be null. For example, the following DDL statement defines a unique constraint, setting emaila column to be unique:
```
ALTER TABLE users
ADD CONSTRAINT uk_email UNIQUE (email);
```
Check constraints : Check constraints are used to enforce specific conditions. For example, the following DDL statement defines a check constraint that ensures that agethe value of a column is greater than or equal to 18:
```
ALTER TABLE users
ADD CONSTRAINT chk_age CHECK (age >= 18);
```

Mode management

A database schema is a logical container used to organize and manage database objects such as tables, views, and indexes. DDL allows you to create, modify and delete schemas. Here are some schema-related DDL operations:

Creating a schema : To create a new schema, you can use CREATE SCHEMAstatements. For example, the following DDL statement creates a new schema named "my_schema":
```
CREATE SCHEMA my_schema;
```
Modify schema : DDL allows you to modify the properties of an existing schema. For example, to modify the owner of the schema named "my_schema", you can execute the following DDL statement:
```
ALTER SCHEMA my_schema OWNER TO new_owner;
```
Deleting a schema : To delete a schema and all the objects it contains, you can use DROP SCHEMAthe statement. For example, the following DDL statement deletes the schema named "my_schema":
```
DROP SCHEMA my_schema CASCADE;
```
CASCADEoption deletes all objects in the schema.

Precautions

There are some important considerations to consider when using DDL:

Data loss : Deleting and modifying tables can result in data loss, so you should be careful to back up your data before performing these operations.
Transaction management : DDL statements usually implicitly commit transactions, so pay attention to transaction consistency before and after DDL operations.
Permissions : Specific permissions are usually required to perform DDL operations. Ensure that the user has sufficient permissions to perform the required DDL operations.
Performance impact : Creating, modifying, and deleting large tables, indexes, or views can have an impact on database performance and needs to be performed within a maintenance window.
Constraints : When defining constraints, make sure they are correct, otherwise data may be inconsistent.
Use Patterns with Care : Be careful when creating patterns and don't create too many unnecessary patterns that can lead to clutter.

in conclusion

Data Definition Language (DDL) is a powerful tool in SQL for defining, managing, and controlling database structures. Through DDL, you can create, modify, and delete database objects such as tables, indexes, and views, as well as define constraints and management modes. However, DDL should be used with caution to ensure the consistency and integrity of the database. Before performing DDL operations, it is recommended to back up important data to prevent unexpected situations. I hope this article helped you better understand the basic concepts and operations of DDL, so you can manage your database structure more effectively.

Author information

Author: Fanyi
CSDN: https://techfanyi.blog.csdn.net
Nuggets: https://juejin.cn/user/4154386571867191