"Database Review" relational algebra __ Database

Relational Algebra

Q: Why is it called algebra?

A: Because there are operators and operands, the operand is a table, the operator is the intersection, and so on;

Relational algebra is divided into set-based relational algebra and packet-based relational algebra;

The basic operation of relational algebra: and, Selection, projection, Cartesian product, difference, renaming;

The basic operation means that other operators can be launched by basic operation;

Note: If we use the and, intersection, and difference operations, we must ensure that the properties set of R and S are the same, and of course, if they are different, they can also be renamed;

Add:

1. Natural Connection

  

2.θ Connection

3. renaming

An expression tree

Packet-based relational algebra

1. and

If the tuple T appears in R for M times and N times in S, it appears m+n times in R∪s;

2. Pay

If the tuple T appears in R for M times and N times in S, then min (m,n) appears in R∩s;

3. Poor

If the tuple T appears in R for M times and N times in S, then the r-s appears (1) if m>n, the m-n occurs; (2) if m<n, 0 times;

4. Eliminate duplicate operators

Δ (R) can eliminate repeating tuples on relation R;

5. Aggregation Operators

SUM (A)

AVG (A)

MIN (A)

MAX (A)

COUNT (A)

Where A is a property;

6. Grouping operators

Gamma

Contains a property as a grouping attribute, and an aggregation operator;

7. Sorting tuples

Τa (R) means sorting according to the A attribute;

8. External connection

left outer connection:

right outer connection:

9. Extended Projection Operations

Step representations of relational algebraic expressions

Because it is not clear that a complex algebraic expression is represented directly by an expression, we can express it in steps.

Like what:

Can be expressed as: