What are the basic operations of relational database

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The three basic relational operations that can be realized in a relational database are: 1, selection operation, the operation of finding tuples that meet the given conditions from the relationship is called selection; 2, projection operation, from the relational schema to specify several attributes to form a new relationship; 3, the join operation is the horizontal combination of the relationship.

Any retrieval operation of relational database is composed of three basic operations: selection operation, projection operation and join operation.

1, selection operation, selection can also be said to be a restriction, it is to select tuples in relation R that meet the given conditions, and selection operation is actually to select tuples from relation R to make the logical expression F true, which is carried out from the point of view of rows.

2, projection operation, projection on relation R is to select several attribute columns from R to form a new relation. ∏ A (R) = {t [A] | t ∈ R} where An is the attribute column in R. The projection operation is an operation from the point of view of a column.

3, join operation. A connection can also be called a θ connection, which selects tuples between attributes that satisfy certain conditions from the Cartesian product of two relations. There are two most important and commonly used connections in connection operation, namely, equivalent connection and natural connection. A connection when θ is "=" is called an equivalent connection. Natural connection is a special equivalent connection.

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