Selection and projection
The vocabulary of relational algebra (relation, tuple, degree, cardinality) and the two operators that filter a relation: selection σ keeps rows, projection π keeps columns and removes duplicates.
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Relation: what is it?
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A set of tuples with the same schema, drawn as a table with no duplicate row and no order among the rows.
Since it is a set, two identical tuples cannot coexist in it: that is why projection can reduce the number of rows.
On this relation, what does
π nom(σ ville = 'Paris'(CLIENT))return?
```text
CLIENT(id, nom, ville, age)
1 Durand Lyon 34
2 Morel Paris 28
3 Petit Lyon 45
4 Bernard Lille 28
5 Roux Paris 51
6 Faure Lyon 28
7 Lopez Nantes 39
8 Girard Paris 22`- Morel, Roux and Girard
- Paris, Paris and Paris
- Durand, Petit and Faure
- Morel and Roux only
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Morel, Roux and Girard
σ first keeps the three tuples whose city is Paris, then π keeps only the nom column. Read from the inside out.
A relation is a … of tuples: it therefore never contains two … tuples.
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A relation is a set of tuples: it therefore never contains two identical tuples.
SQL, for its part, accepts duplicates in its results; it is
DISTINCTthat restores the behaviour of projection.Tuple: what is it?
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One row of a relation: one value for each of the schema's attributes.
(1, Durand, Lyon, 34)is a tuple of CLIENT; it carries as many values as the schema has attributes.