Glossary

Term	Definition
OLTP	Online Transaction Processing. Databases optimised for fast row-level reads and writes. Your app's production database.
OLAP	Online Analytical Processing. Systems optimised for complex aggregations across large datasets. Data warehouses.
ETL	Extract, Transform, Load. Data is transformed before loading into the target system.
ELT	Extract, Load, Transform. Data is loaded raw first, then transformed using SQL (the dbt approach).
Star Schema	A data warehouse schema with a central fact table surrounded by denormalised dimension tables.
Snowflake Schema	A star schema where dimension tables are further normalised into sub-dimension tables.
Fact Table	Stores measurable, quantitative data (ratings, counts, amounts). Contains foreign keys to dimensions.
Dimension Table	Stores descriptive context (movie titles, genres, user details, dates).
ERD	Entity-Relationship Diagram. A visual map of database tables and their relationships.
dbt	Data Build Tool. Runs SQL transformations and manages the dependency graph between models.
dbt model	A SQL SELECT statement saved as a .sql file. dbt compiles it into a table or view.
ref()	dbt function that references another dbt model. Builds the dependency graph automatically.
source()	dbt function that references a raw source table. Enables source freshness checks.
DAG	Directed Acyclic Graph. In Airflow, a DAG defines the tasks in a pipeline and their dependencies.
Operator	An Airflow class that defines what a task does. BashOperator, PythonOperator, etc.
DataFrame	A 2D tabular data structure in Pandas or PySpark. Like a spreadsheet in code.
Schema (dbt)	A dbt YAML file (schema.yml) that defines tests and documentation for models.
DAX	Data Analysis Expressions. The formula language used in Power BI for measures and calculated columns.
Measure (Power BI)	A dynamic calculation evaluated at query time based on filter context. Always use for aggregations.
Star Schema (Power BI)	The recommended data model layout in Power BI: fact table in centre, dimensions surrounding it.
Window function	SQL function that operates across a set of rows related to the current row. RANK, LAG, LEAD, SUM OVER.
CTE	Common Table Expression. A named subquery defined using WITH. Makes SQL modular and readable.
Cardinality	The uniqueness of data values in a column, or the type of relationship between tables (1:1, 1:N, N:M).
Data Lineage	The path data takes from source to final model. Visible in the dbt docs lineage graph.