Appendix B: Glossary of SQL Terms

This appendix defines common SQL terms.

A

ACID

ACID stands for Atomicity, Consistency, Isolation, and Durability, the four properties that guarantee database transactions are reliable.

C

Column

A column, also called a field or attribute, is a vertical entity in a database table that contains all the information associated with a specific data type.

Constraint

A constraint is a rule or condition enforced on a database to maintain data integrity. Uniqueness, primary key, foreign key, and check constraints are examples.

D

Database

A database is an organized collection of structured data stored and managed in a computer system, usually by a database management system (DBMS).

DDL (Data Definition Language)

DDL is the subset of SQL used to define, modify, and remove the structure of database objects such as tables, indexes, and views.

DML (Data Manipulation Language)

DML is the subset of SQL used to retrieve, insert, update, and delete data in a database, usually through SELECT, INSERT, UPDATE, and DELETE statements.

I

Index

An index is a data structure that speeds up data retrieval from a database table by giving quick access to rows based on the values of specific columns.

J

Join

A join is an SQL operation that combines rows from two or more tables based on a related column between them, so you can retrieve related data from several tables in a single query.

K

Key

A key is a column or set of columns used to identify rows or to link tables. A primary key or unique key identifies each row in its table. A foreign key holds values of a key in another table, so it links the two tables and doesn’t have to be unique in its own table.

Q

Query

A query is a request for data from a database, usually a SELECT statement that retrieves the rows matching specified criteria. The word is also used loosely for any SQL statement, including INSERT, UPDATE, and DELETE, which change data instead of retrieving it.

R

Relational database

A relational database is a database that organizes data into tables with rows and columns and uses keys to establish relationships between the tables. The software that manages one is a relational database management system (RDBMS).

S

Schema

A schema is a collection of database objects, including tables, views, indexes, and constraints, that defines the structure and organization of a database.

T

Transaction

A transaction is a sequence of one or more SQL statements executed as a single unit of work. Transactions keep database operations consistent and preserve their integrity.

V

View

A view is a virtual table generated by an SQL query. It presents data from one or more tables in a predefined format, and you can query it like a table without repeating the SQL behind it. A view doesn’t store its own copy of the data, so changing data through a view changes the underlying tables.