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Relational algebra of tyut Taiyuan University of technology 2022 database
2022-07-06 13:12:00 【Lala Lulu^_^】
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 :
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