A database is a collection of organized information that can be accessed and managed efficiently. Clinical databases aim to accurately capture and store patient data to facilitate analysis and reporting. Relational databases are commonly used as they allow data to be organized into tables and linked together through common identifiers. Data entry involves transferring paper records into electronic format in the database. Double data entry checks for errors by having two people enter the same data, while single entry relies more on in-built validation checks. Databases must be designed carefully to collect only necessary variables and ensure high data quality.