بررسی اصول جبر راطه ای در sql Server

 در این بخش به بررسی چند مثال از جبر راطه ای می پردازیم.

مثال از Projection

مثال: جدول با عنوان E (for EMPLOYEE)

select salary from E 

select nr, salary from E 

بدون رکورد تکراری

Selection

The same table E (for EMPLOYEE) as above.

select * from E where salary < 200 

select * from E where salary < 200 and nr >= 7 

Note that the select operation in relational algebra has nothing to do with the SQL keyword select. Selection in relational algebra returns those tuples in a relation that fulfil a condition, while the SQL keyword select means "here comes an SQL statement".

Relational algebra expressions

select name, salary from E where salary < 200