Tag Archives: (and Construction)

Setting • DBMS must allow concurrent access to databases. – Imagine a bank where account information is stored in a database not allowing concurrent access. Then only one person could do a withdrawal in an ATM machine at the time – anywhere! • Uncontrolled concurrent access may lead to problems. lecture11.pdf dosyayı indir

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