Working with the database is about retrieving data, adding data, modifying, deleting and preserving the integrity of the data, backing up the database, allowing simultaneous access to the database for multiple users, verifying the identity of users, etc. The database in Croatian has been created entirely by author named: "Applying SQL Complex Queries to Access Data in a Database Using MS SQL Server 2019". The goals of the paper are: (1) to show certain capabilities of MS SQL Server 2019, (2) to show ways of querying using aggregate functions, (3) to show how to set nested queries i.e. subqueries, (4) to warn about possible database corruption, (5 ) present the ability to collect data from multiple relationships, (6) show and explain, by way of example, basic concepts in databases such as relation, relational schema, RMP model, etc., (7) present more complex queries to quickly find specific data in a database, to (8) demonstrate how to delete data on MS SQL Server 2019, (9) explain ways to protect data from unauthorized and simultaneous access to data.