Data Dictionary Entries

A data dictionary is a centralized repository of information about the data in a database, including the structure, relationships, and constraints. It serves as a reference guide for database administrators, developers, and end-users, helping them understand the database schema and ensure proper data usage. A completed data dictionary typically contains the following entries:

1. Table Name: This entry provides the name of each table in the database. It serves as a starting point for understanding the organization of data and the purpose of each table.
2. Column Name: This entry lists the name of each column within a table. Column names should be descriptive and clearly indicate the type of data stored in that column.
3. Data Type: This entry specifies the data type of each column, such as integer, float, date, or text. Data types define the nature of the data stored in the column and the operations that can be performed on it.
4. Column Description: This entry provides a brief description of the purpose and content of each column. A well-written column description helps users and developers understand the significance of the data stored in the column and ensures proper data usage.
5. Default Value: This entry indicates the default value for a column, if any. Default values are automatically assigned to a column when a new row is inserted without specifying a value for that column. Specifying default values can help maintain data consistency and prevent errors due to missing data.
6. Constraints: This entry lists any constraints applied to the columns, such as primary key, foreign key, unique, check, or not null. Constraints enforce data integrity rules and ensure that the data stored in the database adheres to the defined business rules and requirements.
7. Relationships: This entry describes the relationships between tables in the database, including the type of relationship (one-to-one, one-to-many, or many-to-many) and the columns involved in the relationship. Relationships define how tables are connected and provide context for understanding the database structure and data dependencies.
8. Indexes: This entry lists any indexes created on the columns to optimize query performance. Indexes are important for ensuring efficient data retrieval and can significantly impact the performance of database operations.

By documenting these entries in a completed data dictionary, you provide a comprehensive reference for understanding the database schema, data organization, and relationships. This information is invaluable for maintaining data integrity, ensuring proper data usage, and facilitating communication among team members who interact with the database.

Once you have used SQL to create a schema, the RDBMS stores information about the database and its tables in a series of files called the data dictionary^[1] (also called a catalog).

1. The data dictionary is the foundation of the database, it is where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns.
2. catalog: Also called the data dictionary. The catalog is the foundation of the database, it is where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns.

The data dictionary is the foundation of the database--it's where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns. If you modify a table, such as by adding a column or constraint, the RDBMS automatically updates the data dictionary to reflect the change. Formats for data dictionaries differ among RDBMSs, but they usually contain:

1. Definitions of table columns
2. Referential-integrity constraints among tables
3. Permissions
4. Definitions of views and custom data domains

When a user asks the RDBMS to work with database data, the system looks in the data dictionary to find the table and columns the user requested. If they exist, the RDBMS makes sure the user has permission to work with the data and then writes it to the screen via a virtual table. Since the data dictionary contains referential-integrity constraints, the RDBMS can also ensure that the proposed change or update follows those constraints. The next lesson describes the tables that are part of the data dictionary.

[1]:

[1]data dictionary: The cross-reference of definitions and specifications for data categories and their relationships.