The three basic relational operations that a relational database management system can implement are selection (the operation of finding tuples that meet given conditions from a relation), projection (from a relation Specify several attributes in the schema to form a new relationship), connection (which is a horizontal combination of relationships).
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 the logical expression from the relationship R. Formula F is a true tuple, which is an operation from the perspective of rows.
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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