An index is a data structure that improves query performance by providing quick access to specific data
Types of indexes:
- Clustered index: reorders the physical records of a table according to the index
- Non-clustered index: creates a separate data structure that contains the index keys and pointers to the table records
Index characteristics:
- Uniqueness: ensures that each value in the indexed column is unique
- Composite index: combines multiple columns into a single index
Index benefits:
- Improves query performance
- Reduces disk I/O
- Supports efficient sorting and grouping

Normalization is the process of organizing data in a database to minimize data redundancy and dependency
Normalization rules:
1. First Normal Form (1NF): each table cell must contain a single value
2. Second Normal Form (2NF): each non-key attribute must depend on the entire primary key
3. Third Normal Form (3NF): if a table is in 2NF, and a non-key attribute depends on another non-key attribute, then it should be moved to a separate table
Benefits of normalization:
- Reduces data redundancy
- Improves data integrity
- Simplifies database design

A join is a method of combining data from two or more tables based on a related column
Types of joins:
- Inner join: returns only the rows that have matching values in both tables
- Left join: returns all the rows from the left table and the matching rows from the right table
- Right join: returns all the rows from the right table and the matching rows from the left table
- Full outer join: returns all rows from both tables
Join types:
- Equi-join: joins tables based on equal values
- Non-equi join: joins tables based on non-equal values
- Self-join: joins a table with itself

Database design is the process of creating a conceptual representation of a database
Design principles:
- Data independence: separates data from the physical storage
- Data abstraction: provides a simplified view of the data
- Data normalization: minimizes data redundancy and dependency
Design steps:
1. Requirements gathering: identifies the data requirements
2. Conceptual design: creates a high-level design of the database
3. Logical design: creates a detailed design of the database
4. Physical design: implements the database on a specific platform
Design considerations:
- Scalability: ability to handle increasing data volumes
- Security: protects the database from unauthorized access
- Performance: optimizes query execution and data retrieval

规范化是将数据库中的数据组织成最小化数据冗余和依赖的过程
规范化规则： 1.第一范式 (1NF)：每个表单元格必须包含单个值 2.第二范式 (2NF)：每个非键属性必须依赖于整个主键 3.第三范式 (3NF)：如果表满足 2NF，并且非键属性依赖于另一个非键属性，则应将其移到单独的表中
规范化优点：
- 降低数据冗余
- 提高数据完整性
- 简化数据库设计

连接是根据相关列将两个或多个表的数据结合的方法
连接类型：
- 内连接：返回两个表中具有匹配值的行
- 左连接：返回左表中的所有行和右表中的匹配行
- 右连接：返回右表中的所有行和左表中的匹配行
- 完整外连接：返回两个表中的所有行
连接类型：
- 等值连接：根据等值连接表
- 非等值连接：根据非等值连接表
- 自连接：将表与自身连接