What are the three basic operations of relational databases?

The three basic relational operations that relational databases can implement are: 1. Selection operation, the operation of finding tuples that meet given conditions from a relationship is called selection; 2. Projection operation, from the relational model Specify several attributes to form a new relationship; 3. Connection operation is a horizontal combination of relationships.

Any retrieval operation in a relational database is composed of three basic operations. These three basic operations are: selection operation, projection operation, Join operation.

1. Selection operation. Selection can also be said to be restriction. It is to select tuples that meet given conditions in the relationship R. The selection operation is actually to select from the relationship R so that the logical expression F is True tuples, this is the operation from a row perspective.

2. Projection operation. The projection on the relationship R is to select several attribute columns from R to form a new relationship. ∏A(R) = {t[A]|t∈R}where A is the attribute column in R. The projection operation is performed from the perspective of the column.

3, connection operation. The connection can also be said to be theta connection, which selects tuples that meet certain conditions between attributes from the Cartesian product of two relations. There are two most important and commonly used join operations, namely equivalent join and natural join. The connection when θ is "=" is called an equivalent connection. A natural join is a special type of equal value join.

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