Is the projection operation traditional?

1. Selection: select tuples that meet the given conditions in relation r to form a new relation.

2. Projection: Select some attributes from the relational schema to form a new relational scale.

Second, the content is different.

1. Selection: Finding those tuples that meet the given conditions in the relationship is called selection. The condition is given by a logical expression, and a tuple with a true value will be selected. This operation extracts tuples from the horizontal direction. In FOXPRO, FOR and WHILE are equivalent to selection operations. For example, the publishing unit in the list =' Higher Education Press' and unit price.

2. Projection: operating from the perspective of columns is equivalent to the vertical decomposition of relations. In FOXPRO, the word FIELDS is equivalent to projection operation. Such as list field unit and name.

Extended data:

Other relational operations include:

1, connection operation

The join operation is to select tuples whose attributes meet certain conditions from the Cartesian products of two relations.

2. Division operation

In relational algebra, division can be understood as the inverse of cartesian product.

3. Traditional set operation

UNION has 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; 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.

Baidu encyclopedia-relationship operation

Baidu Encyclopedia-Selected (Relational Operation)