[csnote] ER diagram

【ER chart 】
entity-relationship, There are three components ： Entity 、 attribute 、 Chain system
Used for relational dbs The conceptual design of

【 Three connections of entities 】
Including one-on-one 、 One to many 、 Many to many three

If A To B It's a one to many relationship , Then draw a line with an arrow pointing to B
If it's one-on-one , Draw two lines with arrows
If it's many to many , Draw two lines without arrows
（ Pointed at 1？）

【 It means that there are many relationships 】
How many times does an entity appear in the chain , It's going to take a couple of lines to connect .

The following figure shows the prerequisite relationship of a course , There are two in the priori relationship Course Entity , The first is to repair first, and the second is to repair later , Two lines should be used to represent this relationship ：

【 The multidirectionality of connections 】
Although teachers can offer many courses , And teach multiple students , For specific students and courses, only one teacher teaches , This is enough to become a ternary connection

【 Denotes a subclass 】
Use a triangle and two lines to represent classes and subclasses , The attributes and connections related to subclasses are connected , And related to parent and child classes, even the parent class