1

Can anyone explain the links between the tables? like sometimes there's a bubble, other times a rectangle appears and so on.

enter image description here

Christophe
  • 68,716
  • 7
  • 72
  • 138
Tasbeeh
  • 45
  • 4

2 Answers2

0

That’s called Crow’s Foot notation and is a standard way of indicating the cardinality of a relationship between two entities

NickW
  • 8,430
  • 2
  • 6
  • 19
0

This is an entity-relationship diagram (ERD) using Crow's foot notation. The lines represent relationships between tables. In this variant of the notation, the * identifies the primary key of a table.

At each end of a relationship, you will find two symbols:

  • closest to the table: a | or 3 lines joining. The first means a cardinality of maximum 1 and the second a cardinality maximum N (i.e. "many").
  • next to the first symbol, either a | or a O : The first means a mandatory participation (i.e. minimum 1) and the second an optional participation (i.e. minimum is 0).

So, combining the two gives 4 possibilities:

  • enter image description here means "none or 1" (aka 0..1),
  • enter image description here means "1 and exactly 1" (aka 1..1),
  • ![enter image description here means "none, one or many" (aka 0..n), and
  • the last combination, which is not on your diagram, would mean "1 or more" (i.e. 1..n)
Christophe
  • 68,716
  • 7
  • 72
  • 138