What are the basic operations in relational algebra and what are their basic functions?

Five basic operations: union, difference, product, selection and casting; Form a complete set of relational algebra operations. Other non-basic operations: All operations synthesized by the above five basic operations can be bai. Union (u), intersection (⌒), projection (π), selection (σ) and Cartesian product (×).

Traditional set operation:

1 and UNION have two relations, R and S, which have the same structure. The union of R and S is a set of tuples belonging to R or S, and the operator is ∨. Write t = r ∪ s.

2. The difference between R and S is a set of tuples belonging to R but not S, and the operator is-. Write it down as t = r-s.

3. Intersection The intersection of R and S is a set of tuples belonging to both R and S, and the operator is ∩. Write it as t = r ∩ s. R∩S=R-(R-S).

Extended data:

1, let the relation pattern be R(A 1, A2, …, An). One of its relations is set to r, T∈R means that t is a tuple of r, and T[Ai] means that tuple t corresponds to the component of attribute Ai.

2. if A={Ai 1, Ai2, …, Aik}, where Ai 1, Ai2, …, Aik is a part of A 1, A2, …, An, then a is called an attribute column or a domain column. フA represents the remaining attribute group after removing {A 1, A2, …, An}. T [a] = (t [ai 1], t [ai2], ..., t [aik]) represents the component set of tuple T on attribute column A.

3.r is an n-mesh relation and s is an m-mesh relation. Let tr∈R(r is subscript) and ts∈S(s is subscript), then trts (adding a half arc above the whole formula, r and s are subscript) is called tuple concatenation. It is a tuple with (n+m) columns. The first n components are n tuples in R, and the last m components are m tuples in S..

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