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What are the relational operations in a relational database? Many novices are not very clear about this. In order to help you solve this problem, the following editor will explain it in detail. People with this need can come and learn. I hope you can gain something.

1. Selection operation to select all tuples that meet a given condition from a relation R

2. Projection operation to select some specified attributes from all attributes of a relation R.

3. Join operation is to select tuples that satisfy certain conditions from the generalized Cartesian product of two relations to form a new relationship.

Relational operation

There are two kinds of basic operations of relations: one is traditional set operations (union, difference, intersection, etc.), and the other is special relation operations (selection, projection, join, division, outer join, etc.). Some queries need a combination of several basic operations. it takes several steps to complete.

1. Traditional set operation

1. 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 it down as Tanner R ∪ S.

2. The difference between DIFFERENCE R and S belongs to R but not to R

A collection of tuples in S, with the operator -. Record it as T=R-S.

3. The intersection of (INTERSECTION) R and S is a set of tuples belonging to both R and S, and the operator is ∩. Write it down as Tanner R ∩ S. R ∩ Shear R-(Rmurs).

2. Selection operation

Finding those tuples from a relationship that meet a given condition is called selection. The conditions are given in logical expressions, and tuples with true values are selected. This operation extracts tuples from the horizontal direction. The phrases FOR and WHILE in FOXPRO are equivalent to selection operations.

For example: LIST FOR publisher = 'higher Education Press' AND unit price

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