For example:
DEPARTMENTMSK TENK
MONHOC
one many
Belong
MSSV MSK STUDENTS HOT NOW
much
MSMH TENMH LT
TH
much
Register
Many-to-many relationship (nn) is converted into 2 consecutive numbers: 0,n/1,n and 0,n/1,n
16
MSK TENK FACULTY
1,1 1,n
MSSV MSK STUDENTS HOT NOW
1.1
MONHOC | ||
MSMH TENMH LT TH | ||
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0,n | DK | 0,n |
MSSV MSMH | ||
1.1
17
3) Inheritance relationship :
Key of the relationship entity “Parent”is the key
foreign and primary keyof the “Child” relationship entities.
Symbol : Similar to MHYNDL:
Father
KA
...
TTQH CON1
KA
TTTinhDacThu_C1
TTQH CONm | |
KA | |
TTTinhDacThu_Cm | |
… 18
2. Rules for converting from MHYNDL to MHLDL:
Review some conventions about symbols in
MHYNDL entity-combination form :
A
KA
IA
Represents entity A with the identifying key attribute KA ( KA can be one or a combination of many attributes), the remaining attributes are IA
R
The associative representation R has no attributes.
calculate
R
IR
The associative representation R has the following properties:
IR.19
Step 1: Transfer entity
Rule 1 : Entity will be converted to relational entity , entity attribute will be converted to
attribute of the relational entity . Key recognition
The entity type will be converted to the primary key of the
relational entity
A
KA
IA
A
KA
IA
MHDL
MHLDL is represented as a relational schema:
A( KA , IA)
20
Example 1 : Given a conceptual data model ( SV entity ) , converting it to relational MHLDL according to rule 1 , we get the relational entity SV :
SV
SV
MSSV HOTEN GT NgaySinh
MSSV HOTEN GT NgaySinh
MHDL
SV ( MSSV , HOTEN, GT, NgaySinh)
21
Example 2: There is a conceptual model of the data to be transferred.
MHLDL as follows:
HD
SHD
MHDL | HD SHD | |
NOWHD DIENGIAI QUANTITY | NgayHD DIENGIAI SOTIEN |
HD ( SHD , NOWHD , DIENGIAI, SOTIEN)
22
Step 2: Transfer combination
A. SIMPLE COMBINATION
23