Database related concepts Review

Database related concepts Review

What are the three-level modes of the database system architecture?

A: The three modes are the mode, the external mode, and the internal mode.

A schema is also called a logical or conceptual schema. It describes the logical structure and features of all data in a database. A Schema must contain the logical structure of data and the relationship between data.

The external mode is also called the user mode or sub-mode. It is the description of the local data structure and features seen by the database users. It is the local logical structure of the data, and the external mode is part of the mode, it is derived from a pattern. A conceptual pattern can have several external patterns.

The internal mode is also called the storage mode, which describes the physical structure of data, storage methods and access policies on the storage media.

Briefly describe the second-level ing composition of the database system architecture?

A: In the three-level mode, the transition from the external mode to the internal mode abstraction layer is implemented by the two-level ing functions provided by the database management system, that is, the external mode/mode ing and the mode/Internal Mode ing.

The external mode/mode ing is used to define the ing between the external mode and the conceptual mode. This ing allows you to modify the external mode/mode image when modifying the conceptual mode, the external mode remains unchanged as much as possible, that is, the change of the conceptual mode does not affect the external mode and applications, thus achieving the logical independence of data.

Mode/Internal Mode ing defines the correspondence between the conceptual mode and the internal mode, that is, the correspondence between the conceptual record and the internal record. When the storage structure of the database changes, that is, when the internal mode is changed, as long as the corresponding mode/internal mode image is changed, the mode is saved as much as possible, while the external mode and application have less influence, thus realizing the physical independence of data.

Through two-level ing, the user's operations on the database are simplified, so that the user can perform physical operations on the database only by performing logical operations on the database, while also facilitating data security and confidentiality.

Can I explain the candidate keys and primary keys in the link model?

A: The candidate key can be used to uniquely identify or distinguish an attribute or attribute group of a tuples in a link. It is called a candidate key. A primary key is used to specify a primary key from a candidate key to identify a tuples.

Interpreting external keywords in a relational model?

A: If a field in a table is not the primary keyword or candidate keyword of the table, but the primary keyword or candidate keyword of another table, this field (attribute) is called an external keyword, or a foreign key for short.

What are the main tables and slave tables in the relational model?

A: The master table and the slave table are two tables associated with foreign keys. The table with foreign keys as primary keys is called the master table, and the table with foreign keys is called the slave table.

What are the basic operations of relational databases?

A: the basic operations of relational databases include selection, projection, and connection.

Select an operation that selects the tuples that meet the given conditions from the specified link to form a new link.

Projection is to select several attributes from the property set of the specified link to form a new relationship.

Join is to combine the tuples in the two links according to the specified conditions to generate a new link.