Relational Algebra

Six operators, each taking relations in and giving one relation out. Compose them into a tree and evaluate it from the leaves up — the same shape as every query plan.

σ π ρ — one input

⋈ ∪ − — two inputs

Set semantics

Expression

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The Six Operators

Selection WHERE

keeps tuples that satisfy a condition

Projection SELECT DISTINCT

keeps some attributes, drops duplicates

Rename AS

renames attributes (or the relation)

⋈

Natural join NATURAL JOIN

pairs tuples that agree on shared attributes

∪

Union UNION

tuples in either input

−

Difference EXCEPT

tuples in the left but not the right

Knowledge Check

Question 1 of 3

Employee has 6 tuples in 3 departments. How many tuples does π_did(Employee) contain?

DSAverse

Relational Algebra

The Six Operators

Knowledge Check

See also

Relational Algebra

The Six Operators

Knowledge Check

See also