what is? SQL is not formal language, relational algebra is formal language of RDB
SQL is developed after RDB
An algebra in relation
what should in it?
what can input to algebra?
input a relation, output a relation
RA is procedural in nature,
a way to think to split query and most efficient
optimize the query in system
Key Relational Algebra Operators
unary: 一元的
Projection: which attribute we want to take out for the output
SELECT in SQL
Selection: select the tuple in this attribute when constraint is true
WHERE in SQL
Product: cross join
Join: matching rows on 2 relations
Set:
UNION, INTERSECT
Renaming: AS, rename the relation or attribute