Tag Archives: Algebra

SELECT-FROM-WHERE • Basic structure of an SQL query: SELECT attributes FROM tables WHERE tests over rows SELECT X FROM T WHERE C Aggregation • Aggregation functions are functions that produce a single value over a relation. – SUM, MAX, MIN, AVG, COUNT SELECT MAX(nrSeats) MAX actually has FROM Rooms; SELECT COUNT(*) FROM Lectures WHERE room = ’VR’; lecture8.pdf dosyayı indir

Summary so far • SQL is based on relational algebra. – Operations over relations • Operations for: – Selection of rows (
) – Projection of columns () – Combining tables • Cartesian product (x) • Join, natural join (⋈C, ⋈) lecture7.pdf dosyayı indir

Goals of database design • ”Map” the domain, find out what the database is intended to model. – The database should accept all data that is possible in reality. – The database should agree with reality and not accept impossible or unwanted data. • We accomplish this by making sure that our database captures all the constraints of the domain. …