A-Z Database Glossary

A comprehensive reference of database terms from A to Z written for beginners and anyone learning how data is stored, managed, and retrieved. Each term includes a plain-English definition and a real world example.

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A

Term	Definition	Example
ACID Properties	A set of four guarantees — Atomicity, Consistency, Isolation, Durability — that ensure database transactions are processed reliably even when something goes wrong	When you transfer money between bank accounts, ACID ensures the amount is deducted from one account and added to the other — never half-done
Aggregate Function	A function that performs a calculation on a group of rows and returns a single result	COUNT(), SUM(), AVG(), MIN(), and MAX() — e.g. SUM(sales) to get total revenue across all orders
ALTER TABLE	An SQL command used to add, remove, or modify columns and constraints in an existing table	Adding a "phone_number" column to a "customers" table that was created without it
Anomaly (Data Anomaly)	An error or inconsistency in a database caused by poor design — specifically by data redundancy	Storing a customer's city in multiple rows, then updating only some of them so different rows show different cities for the same customer
Attribute	A characteristic or property of an entity in a database — equivalent to a column in a table	In a "Student" table, "name", "age", and "enrollment_date" are attributes
Auto-Increment	A feature that automatically generates a unique, incrementing number each time a new row is inserted	A new customer gets ID 1001, the next gets 1002, and so on — without the user specifying the number

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B

Term	Definition	Example
Backup	A copy of the database saved at a point in time so it can be restored if data is lost or corrupted	Scheduling an automatic nightly backup of a hospital's patient records database
Base Table	A real, physically stored table in a database, as opposed to a view which is virtual	The "orders" table where actual order data lives — as distinct from a "recent_orders" view built on top of it
Batch Processing	Processing a large volume of data records together at scheduled intervals rather than one at a time in real time	Running payroll calculations for all 5,000 employees every Friday night in a single batch job
BCNF (Boyce-Codd Normal Form)	A stricter version of Third Normal Form (3NF) that eliminates certain remaining redundancies in table design	Resolving a table where a non-key column can determine part of the primary key — a subtle design flaw BCNF catches
Binary Large Object (BLOB)	A data type used to store large binary files — such as images, audio, or documents — directly in the database	Storing a user's profile photo as a BLOB in the database instead of just saving a file path
Buffer Pool	A region of memory where the database caches recently accessed data pages to reduce slow disk reads	The database serving the same popular product page from memory instead of hitting the disk every time

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C

Term	Definition	Example
Candidate Key	Any column or combination of columns that could uniquely identify each row in a table — before one is chosen as the primary key	A "students" table where both "student_id" and "national_id" uniquely identify each student — both are candidate keys
Cardinality	The number of unique values in a column, or the nature of the relationship between two tables (one-to-one, one-to-many, many-to-many)	A "gender" column with 3 values has low cardinality; a "customer_id" column with millions of unique values has high cardinality
CASCADE	A rule applied to a foreign key that automatically propagates changes — when a parent row is deleted or updated, child rows follow	Deleting a customer automatically deletes all their orders, because the foreign key is set to CASCADE
Check Constraint	A rule enforced at the database level that limits what values can be entered in a column	Ensuring the "age" column never accepts values below 0 or above 150
Clustered Index	An index that physically sorts and stores table rows in the order of the index key — only one can exist per table	A "customers" table sorted on disk by customer_id, making lookups by ID very fast
Column	A vertical field in a table that holds one specific type of data across all rows	The "email" column in a "users" table stores an email address for every user
Composite Key	A primary key made up of two or more columns combined, because no single column is unique enough on its own	In an "order_items" table, the combination of "order_id" and "product_id" together uniquely identifies each line item
Concurrency	Multiple users or processes accessing and modifying the database at the same time	Hundreds of shoppers adding items to their carts simultaneously on an e-commerce site
Constraint	A rule applied to a column or table that restricts what data can be entered, enforcing data integrity	NOT NULL, UNIQUE, CHECK, PRIMARY KEY, and FOREIGN KEY are common constraints
CRUD	The four basic operations of any database — Create, Read, Update, Delete	Adding a new user (Create), viewing their profile (Read), changing their email (Update), and removing their account (Delete)
Cursor	A database object that allows you to retrieve query results one row at a time, rather than all at once	Iterating through a list of 10,000 customers one by one to process each individually inside a stored procedure

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D

Term	Definition	Example
Data Definition Language (DDL)	SQL commands that define or modify the structure of a database — its tables, columns, and constraints	CREATE TABLE, ALTER TABLE, DROP TABLE — used by developers to build or change the database schema
Data Integrity	The accuracy, consistency, and reliability of data throughout its lifecycle in the database	Ensuring a "date_of_birth" field never contains a future date, and a "country_code" field only accepts valid country codes
Data Lake	A large storage repository that holds raw, unstructured, or semi-structured data in its native format until it is needed	A company storing all clickstream logs, emails, and sensor data in Amazon S3 before deciding how to analyse them
Data Manipulation Language (DML)	SQL commands that retrieve or change the actual data within tables	SELECT, INSERT, UPDATE, DELETE — used every time you query or modify records
Data Mart	A smaller, subject-specific version of a data warehouse focused on one department or business function	A sales data mart containing only revenue, pipeline, and quota data for the sales team
Data Migration	The process of moving data from one system, format, or location to another	Moving customer records from an old Oracle database to a new PostgreSQL system
Data Redundancy	Storing the same piece of data in multiple places, which wastes storage and risks inconsistency	Storing a customer's city name in both the "customers" table and every row of the "orders" table
Data Type	The classification that defines what kind of value a column can hold	INT for whole numbers, VARCHAR for text, DATE for dates, BOOLEAN for true/false values
Data Warehouse	A large, centralised database designed specifically for reporting and business intelligence rather than day-to-day operations	Amazon Redshift or Snowflake storing years of sales history so analysts can run trend reports
Database	An organised collection of structured data stored electronically, managed by a Database Management System (DBMS)	A library's catalogue system storing every book's title, author, availability, and borrower history
Database Administrator (DBA)	The person responsible for installing, configuring, securing, monitoring, and maintaining a database system	A DBA who schedules backups, tunes slow queries, and manages user access permissions
Database Management System (DBMS)	The software layer that sits between users and the physical database, handling storage, retrieval, and security	MySQL, PostgreSQL, Oracle, Microsoft SQL Server — the engine that makes databases work
Deadlock	A situation where two transactions are each waiting for the other to release a resource, causing both to freeze indefinitely	Transaction A holds a lock on Table 1 and wants Table 2; Transaction B holds Table 2 and wants Table 1 — neither can proceed
Denormalisation	The deliberate introduction of redundancy into a database to improve read performance at the cost of write complexity	Storing a pre-calculated "total_order_value" in the orders table to avoid joining three tables every time a report runs
Dirty Read	Reading data that another transaction has changed but not yet committed — which may be rolled back	Reading a bank balance that is being updated mid-transfer and seeing an amount that will never actually be the final value
DROP	An SQL command that permanently deletes a table, database, or other object and all the data inside it	DROP TABLE customers — immediately and irreversibly removes the entire customers table
Dump	A complete export of a database's structure and data into a file, typically used for backup or migration	Running pg_dump in PostgreSQL to export the entire database to a .sql file

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E

Term	Definition	Example
Entity	A real-world object or concept that a database table represents	"Customer", "Product", "Order", and "Invoice" are entities in an e-commerce database
Entity-Relationship Diagram (ERD)	A visual diagram that shows the tables in a database, their columns, and how they relate to each other	A diagram showing that one "customer" can have many "orders", and each "order" contains many "products"
Execution Plan	The step-by-step strategy a database engine uses to run a query — reveals how it accesses data and whether indexes are being used	Running EXPLAIN on a slow query to discover the database is scanning 2 million rows when an index could narrow it to 10

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F

Term	Definition	Example
Field	Another word for a column — the smallest unit of data storage in a relational table	The "email" field in the "users" table
File-Based Database	An early form of data storage where data is kept in flat files without a DBMS managing relationships or integrity	Old systems that stored all customer records in a single .csv or .txt file
First Normal Form (1NF)	The first rule of database normalisation — every column must hold atomic (indivisible) values and every row must be unique	Splitting "John, Jane" in a single "names" cell into two separate rows — one per person
Foreign Key	A column in one table that references the primary key of another table, creating a link between the two	An "orders" table with a "customer_id" column that refers to the "id" column in the "customers" table
Full Table Scan	A query operation where the database reads every single row in a table to find matching records — slow on large tables	A query with no WHERE clause, or a WHERE clause on a column with no index, triggers a full table scan
Function (SQL)	A reusable block of SQL code that accepts inputs, performs an operation, and returns a value	UPPER('hello') returns 'HELLO' — a built-in scalar function

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G

Term	Definition	Example
Graph Database	A database that stores data as nodes and edges — designed for highly connected data with complex relationships	Neo4j modelling a social network where people are nodes and friendships are edges
GROUP BY	An SQL clause that groups rows sharing a common value so aggregate functions can be applied to each group	GROUP BY country with SUM(revenue) to get total revenue per country
GRANT	An SQL command that gives a user or role permission to perform specific operations on database objects	GRANT SELECT ON customers TO analyst — allowing a user to read but not modify the table

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H

Term	Definition	Example
HAVING	An SQL clause that filters grouped results after GROUP BY — like WHERE but applied after aggregation	HAVING SUM(revenue) > 100000 — showing only countries with total revenue above ₹1 lakh
Heap	The unsorted physical storage of table rows on disk, with no particular order — the default when no clustered index exists	A table where rows are stored in the order they were inserted, with no logical sort
Horizontal Scaling (Sharding)	Distributing a database across multiple servers by splitting the data — each server holds a subset of the rows	Users with IDs 1–1,000,000 on Server A; users 1,000,001–2,000,000 on Server B

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I

Term	Definition	Example
Index	A data structure that speeds up data retrieval by creating a quick lookup path — like the index at the back of a book	An index on the "email" column of a "users" table lets the database find a user by email instantly instead of scanning all rows
Inner Join	An SQL join that returns only rows where there is a matching value in both tables	Joining "orders" and "customers" with INNER JOIN returns only orders that have a matching customer — orphan orders are excluded
INSERT	The SQL command used to add new rows of data into a table	INSERT INTO students (name, age) VALUES ('Priya', 21)
Instance	A running copy of a database management system on a server	Two departments running separate MySQL instances on the same machine, each managing their own databases
Isolation Level	A database setting that controls how much one transaction can see of another's uncommitted changes	READ COMMITTED prevents dirty reads; SERIALIZABLE prevents all concurrency anomalies but is slower

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J

Term	Definition	Example
Join	An SQL operation that combines rows from two or more tables based on a related column	Combining "orders" and "products" to show what items are in each order — linking them via "product_id"
JSON (in Databases)	A flexible data format used to store semi-structured data inside a column — supported natively by many modern databases	Storing a product's variable attributes (colour options, sizes, specs) as a JSON column instead of creating dozens of columns

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K

Term	Definition	Example
Key	A column or set of columns used to identify rows uniquely or to create relationships between tables	Primary key, foreign key, candidate key, and composite key are the main types
Key-Value Store	A simple NoSQL database that stores data as pairs of keys and their associated values — extremely fast for lookups	Redis storing session tokens where the key is the session ID and the value is the user's data

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L

Term	Definition	Example
Latency	The time delay between sending a query to the database and receiving the result	A query taking 3 seconds to return results has high latency — a sign of a missing index or poor schema design
Left Join (Left Outer Join)	An SQL join that returns all rows from the left table, and matching rows from the right table — unmatched rows show NULL	Joining "customers" and "orders" with LEFT JOIN returns all customers, including those who have never placed an order
Lock	A mechanism that prevents two transactions from modifying the same data simultaneously, avoiding corruption	Transaction A locks a row while updating a bank balance — Transaction B must wait until the lock is released
Log (Transaction Log)	A file that records every change made to the database, used for recovery if the system crashes	If a server crashes mid-transaction, the database replays the transaction log to restore a consistent state

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M

Term	Definition	Example
Many-to-Many Relationship	A relationship where multiple rows in Table A relate to multiple rows in Table B — requires a junction table	Students can enrol in many courses; courses can have many students — linked via an "enrolments" junction table
Materialized View	A saved query result stored as a physical table that can be refreshed periodically — faster than running the query each time	A daily sales summary view that is pre-calculated at midnight so reports load instantly all day
Migration	A versioned, scripted change to a database schema — adding a column, renaming a table, or changing a data type	Deploying version 2.0 of an application that adds a "loyalty_points" column to the "customers" table
Multi-Version Concurrency Control (MVCC)	A method that allows multiple transactions to read and write simultaneously by keeping multiple versions of a row	PostgreSQL using MVCC so readers never block writers — each transaction sees a consistent snapshot of data

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N

Term	Definition	Example
NoSQL Database	A category of databases that store data in formats other than relational tables — document, key-value, graph, or column-family	MongoDB (documents), Redis (key-value), Cassandra (column-family), Neo4j (graph)
Normalisation	The process of organising a database to reduce data redundancy and improve data integrity by applying a series of formal rules	Splitting a table that stores customer name, customer city, and order details into separate "customers" and "orders" tables
NOT NULL	A constraint that prevents a column from being left empty — a value must always be provided	Applying NOT NULL to the "email" column so every user must have an email address on record
NULL	A special marker indicating that a value is unknown, missing, or not applicable — it is not zero or an empty string	A "middle_name" column showing NULL for customers who have no middle name

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O

Term	Definition	Example
OLAP (Online Analytical Processing)	Database systems optimised for complex analytical queries across large historical datasets	A business analyst running a query across 5 years of sales data to identify seasonal trends
OLTP (Online Transaction Processing)	Database systems optimised for fast, high-volume, everyday transactions — insert, update, delete	A retail point-of-sale system processing hundreds of transactions per second
One-to-Many Relationship	A relationship where one row in Table A relates to multiple rows in Table B	One customer can have many orders — but each order belongs to exactly one customer
One-to-One Relationship	A relationship where each row in Table A corresponds to exactly one row in Table B	A "users" table and a "user_profiles" table — each user has exactly one profile
ORM (Object-Relational Mapping)	A programming technique that lets developers interact with the database using objects in their code instead of writing SQL	Django's ORM letting a Python developer write User.objects.filter(age__gt=18) instead of SELECT * FROM users WHERE age > 18

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P

Term	Definition	Example
Partition	Dividing a large table into smaller, more manageable pieces while still treating it as one logical table	Partitioning an "orders" table by year — 2022, 2023, 2024 each stored separately — so queries for one year don't scan all years
Phantom Read	A problem in concurrent databases where a transaction re-runs a query and sees new rows that appeared from another transaction	Transaction A counts 50 orders; Transaction B inserts 5 more; Transaction A counts again and gets 55 — it "sees" phantom rows
Primary Key	A column (or combination of columns) that uniquely identifies each row in a table — no two rows can share the same primary key value, and it cannot be NULL	"customer_id" in a customers table — every customer has a different ID, and no customer can exist without one
Procedure — see Stored Procedure

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Q

Term	Definition	Example
Query	A request sent to the database to retrieve, insert, update, or delete data — usually written in SQL	SELECT name, email FROM customers WHERE city = 'Nagpur' — retrieving all Nagpur customers
Query Optimiser	The component inside a DBMS that analyses a query and determines the most efficient way to execute it	Choosing to use an index rather than a full table scan to run a WHERE clause faster
Query Plan — see Execution Plan

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R

Term	Definition	Example
RDBMS (Relational Database Management System)	A DBMS that organises data into tables with rows and columns, and uses SQL to manage and query it	MySQL, PostgreSQL, Oracle Database, and Microsoft SQL Server are all RDBMSs
Record	A single row in a database table — represents one complete instance of the entity the table describes	One row in the "students" table represents one student with all their details
Referential Integrity	The rule that a foreign key value in one table must always match an existing primary key value in the referenced table	An "orders" table cannot have a customer_id that does not exist in the "customers" table
Replication	Copying data from one database server to one or more other servers to improve availability and fault tolerance	A primary database in Mumbai continuously copying its data to a replica in Bengaluru — if Mumbai fails, Bengaluru takes over
REVOKE	An SQL command that removes previously granted permissions from a user or role	REVOKE DELETE ON orders FROM intern — preventing an intern from deleting order records
Right Join (Right Outer Join)	An SQL join that returns all rows from the right table and matching rows from the left	Joining "employees" and "departments" with RIGHT JOIN returns all departments, including those with no employees yet
Roll Back	Undoing all changes made by a transaction that failed or was explicitly cancelled	A failed payment transaction rolling back so the customer's balance is not deducted
Row	A horizontal record in a table — one complete set of data values for a single entity	One row in the "products" table holds all the details of one product

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S

Term	Definition	Example
Schema	The blueprint of a database — the definition of its tables, columns, data types, and relationships	A schema defines that the "orders" table has columns: order_id (INT), customer_id (INT), order_date (DATE), and total (DECIMAL)
Second Normal Form (2NF)	A normalisation rule that requires a table to be in 1NF and that every non-key column depends on the entire primary key, not just part of it	Removing "product_name" from an "order_items" table whose key is (order_id, product_id) — product_name depends only on product_id, not the full key
SELECT	The most frequently used SQL command — retrieves data from one or more tables based on specified conditions	SELECT * FROM employees WHERE department = 'Engineering'
Sequence	A database object that generates a series of unique numbers in order, used to populate primary key columns	A sequence that produces 1, 2, 3, 4... automatically as each new order is inserted
Sharding — see Horizontal Scaling
SQL (Structured Query Language)	The standard language used to communicate with relational databases — used to create, retrieve, update, and delete data	Virtually every relational database — MySQL, PostgreSQL, Oracle — uses SQL as its primary interface
SQL Injection	A security attack where malicious SQL code is inserted into an input field to manipulate or destroy the database	Entering ' OR '1'='1 into a login form to bypass authentication — a classic injection attack
Stored Procedure	A pre-written, saved block of SQL code stored in the database that can be called by name — like a function for SQL	A "ProcessPayment" stored procedure that handles all the steps of a payment transaction in one reusable call
Subquery	A query nested inside another query — the inner query's result is used by the outer query	SELECT name FROM employees WHERE salary > (SELECT AVG(salary) FROM employees) — finding employees earning above average
Surrogate Key	An artificial primary key — typically an auto-incremented number — created purely to uniquely identify rows, with no business meaning	Assigning a customer_id of 10042 to a customer purely for database purposes — the number itself means nothing to the business

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T

Term	Definition	Example
Table	The fundamental storage structure in a relational database — data is organised into rows (records) and columns (fields)	A "products" table with columns for product_id, name, price, and stock_quantity
Third Normal Form (3NF)	A normalisation rule that requires every non-key column to depend only on the primary key — not on another non-key column	Removing "city" from a "customers" table where city depends on "zip_code" rather than directly on "customer_id"
Transaction	A group of one or more SQL operations treated as a single unit — either all succeed together, or none are applied	Transferring ₹10,000 between two bank accounts is one transaction — the debit and credit must both succeed or both fail
Trigger	An automated action that the database executes automatically when a specific event occurs on a table	A trigger that automatically logs every DELETE operation on the "customers" table to an audit log table
Truncate	An SQL command that removes all rows from a table quickly without logging individual row deletions — faster than DELETE but cannot be rolled back easily	TRUNCATE TABLE temp_data — clearing a staging table before loading fresh data into it

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U

Term	Definition	Example
Union	An SQL operator that combines the results of two or more SELECT queries into a single result set, removing duplicates	Combining a list of email addresses from "customers" and "newsletter_subscribers" into one deduplicated list
UNION ALL	Like UNION but keeps all rows including duplicates — faster because it skips the deduplication step	Combining two sets of transaction records where duplicates are expected and acceptable
Unique Constraint	A rule that ensures all values in a column (or combination of columns) are distinct across all rows	Applying UNIQUE to the "email" column so no two users can register with the same email address
UPDATE	The SQL command used to modify existing data in a table	UPDATE products SET price = 499 WHERE product_id = 7 — changing the price of one specific product

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V

Term	Definition	Example
View	A virtual table defined by a saved SELECT query — it has no physical data of its own but behaves like a table	A "active_customers" view that automatically shows only customers who placed an order in the last 90 days
Vertical Scaling	Increasing the power of a single database server by adding more CPU, RAM, or storage	Upgrading the database server from 16GB RAM to 128GB to handle heavier query loads

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W

Term	Definition	Example
WHERE	An SQL clause that filters which rows are returned or affected by a query based on one or more conditions	SELECT * FROM orders WHERE status = 'pending' — retrieving only orders that have not been fulfilled
Wildcard	A special character used in SQL queries to match patterns in text data	The % symbol in LIKE 'Na%' matches any value starting with "Na" — e.g. "Nagpur", "Nandita", "Naresh"
Write-Ahead Logging (WAL)	A technique where changes are recorded in a log file before being applied to the database — ensures durability and crash recovery	PostgreSQL writing every committed transaction to the WAL file so changes can be replayed if the system crashes

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X

Term	Definition	Example
XML (in Databases)	A structured data format supported by many databases for storing hierarchical or semi-structured data within columns	Storing a product's full specification tree as an XML column in SQL Server

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Y

Term	Definition	Example
YAML (Database Config)	A human-readable format commonly used to write database configuration files and connection settings	Defining a PostgreSQL connection — host, port, username, password — in a YAML config file for a web application

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Z

Term	Definition	Example
Zero Downtime Migration	A database migration strategy designed to apply schema changes without taking the application offline	Adding a new column to a live production table in stages so users never experience an outage
Zone (Data Zone)	A logical or physical partition of a database or data lake that separates data by sensitivity, source, or processing stage	Separating raw incoming data (Bronze Zone), cleaned data (Silver Zone), and analytics-ready data (Gold Zone) in a data lake

*This glossary covers 100+ terms across the full database landscape, from beginner SQL to advanced concepts like MVCC, WAL, and sharding.

A-Z Database Glossary

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