Contact and understanding in PowerDesigner CDM

1.CDM There are three kinds of connections in Relationship( Relationship ) Inheritance( Inherit ) Association( relation )

2. The main understanding here is Relationship( Relationship )

1. Create two new entities Schools and principals , Connect them with relationships

2. Double click relationship Enter the property panel , choice Cardinalities

one-one one-on-one one-many One to many many-one For one more many-many Many to many
Dependent rely on mandatory mandatory Cardinality Number

1.： one-on-one 、 One to many 、 For one more 、 Many to many Well understood.
One school corresponds to one principal 、 One school corresponds to multiple principals 、 Or multiple schools correspond to one principal Or multiple schools correspond to multiple principals

2.Dependent rely on
Dependency correlation Entity B Dependent on entities A be Entity A Of Identifiers Entry entity B To entity B Of iddentifiers Together form an entity B Of identifiers
understand ： Master slave table relationship
A Main table B For the slave watch The primary key of the secondary table depends on the primary key of the primary table The primary key of the secondary table consists of the primary table's primary key and its own primary key
for example The school depends on the principal ： From this conceptual model Generated entity model The last generated table creation statement is


You can see the primary key of the principal entering the school , The primary key of the school is composed of the original primary key
3.madatory mandatory
Whether it's one-on-one Or one to many Or many to many , Each other's primary key will enter itself to form a foreign key
Compulsion means non emptiness
for example ：School to Headmaster Check force
A school corresponds to one or more principals => The foreign key of the principal in the school is not empty
Headmaster to School Do not check force
It means that a headmaster can correspond to 0 A school
The corresponding table creation statement is

3.Cardinality Number
0.1 =》 0 A or a
1.1 =》 1 individual
0.n =》 0 One or more
1.n =》 1 One or more

for example ：School to Headmaster The quantitative relationship of
One school corresponds to 0 One or more 1 One or more 0 A or a

Be careful ： When you're down Cardinality When changes above one-one one-on-one one-many One to many many-one For one more many-many Many to many Will change accordingly