Relational algebra of tyut Taiyuan University of technology 2022 database

There are two models for this problem ：

The first is to give you numbers 、 Groups of tables of letters or attributes , Let you use relational algebra

The second is to give “ Relational database schema ”, Let's use it “ Relational algebra expression ” Represents various queries , It is more difficult than the first

Random this year

Let's take a brief look at the concept ：

One 、 Relational database schema ： Short for R（A1,A2,~~~An）,R Name the relationship ,A For the property name

eg: company （ Company number 、 Company name 、 Address ）

eg:S (S#,SNAME,AGE,SEX)( Some relationship models will be in pure English ）

Two . Relational algebra expression ：

hand over n、 and U、 Bad -（ In high school , A little ）、 The cartesian product x、 choice σ、 Projection Π、 Connect

  The cartesian product ：

S:				T:
A	B			A	B
3	4			1	5
7	2			3	6

be SxT=

S.A	S.B	T.A	T.B
3	4	1	5
3	4	3	6
7	2	1	5
7	2	3	6

Cartesian product is an operation in horizontal direction , Each row of the first table is connected to each row of the second table

choice σ：

grammar ：σ+ Conditions （ The table queried ）

eg： goods （ goods ID, Name of commodity , Price , brand , model , Color , manufacturer , Place of Origin ）

         Check the commodity information of Shanghai origin ：σ Place of Origin =‘ Shanghai ’ ( goods )

        choice It is a horizontal query , The result is all the attributes of qualified goods （ Remember to add single quotation marks to the string ）

Projection Π：

eg： goods （ goods ID, Name of commodity , Price , brand , model , Color , manufacturer , Place of Origin ）

         Inquire about the goods produced in Shanghai ID, Name of commodity ：Π goods ID, Name of commodity （σ Place of Origin =‘ Shanghai ’（ goods ））

         Projection is a vertical query , The result is a single attribute

Connect ：

Connections are generally used for tables , There is a natural connection 、 External connection 、 Left connection 、 The right connection . The teacher won't embarrass us , There are only natural connection questions in the question bank , So just talk about the simplest natural connection

 

  As shown above , Connection is to perform Cartesian operations on rows with the same attributes in two relationships

except ：

In the last question R As divisor ,S As divisor

Method ：（1） Find out first R、S Common attributes in B、C

           （2） Then find the dividend R There are attribute values and S There are rows with the same attribute value ,

                    namely R（2）：

A	B	C
a1	b1	c2
a1	b2	c3
a2	b2	c3
a1	b2	c1

            （3） Remove the second step table R（2） Common attribute column in B、C, Just stay A Column

                        namely

A

a1

a1

a2

a1

                         Simplify to

Master these basic concepts , We can do questions ,SQL There are many ways to query , So the answers to many questions are not unique .

example 1： The real question

Explain ：1.

example 2、

Explain ： 

example 3、

 

Explain ：