What is different between ER Diagram and Database Schema?

Question

What is the difference between ER Diagrams and Database Schema? MySQL Workbench has facility to draw ER diagrams, but the symbols for ER diagrams different in other drawing tools than MySQL Workbench method.

Answer 1

A database schema is usually a relational model/diagram. it shows the link between tables: primary keys and foreign keys.

In database diagram the relation between an apple and a apple tree would be: A foreign key "ID__TRE" which cannot be null in the table "APPLE" is linked to a primary key "ID_TRE" in the table "TREE".

An entity relationship diagram. Shows links between the entities and the kind of relation between them. We are not talking about tables or keys there! Usually the entity relationship diagram follows Merise model. Database manager and developer as myself usually build an entity relationship model before conceiving the relational model/diagram.
The set of symbol in Merise are:(0-1, 0-n, 1-1, 1-n). The first number 0 or 1 describes whether the other part of the association is required for an object to exist. If it is zero, it means it can exists without being associated. If it is One it means that the object only exist in relation with an other object (e.g an apple need a tree to exist --> 1, a tree needn't apple to exists -->0) The second character tell us how many objects are accepted in the other part of the association. If it is 1, then only one object can exists in the relation, if it is n, a infinite number of object can be linked (e.g.: an apple can have one tree --> 1, a tree can have multiples apples --> n)

With Entity relationship the relationship will be described as : An apple has to belong to at least one tree to exists and can belong to only one tree(1-1). A tree needn't an apple to exist but it can have an infinite number of apples (0-n).

In fact both description mean the same but one is database oriented while the other is modelling oriented. Some modelling software such as DB-MAIN convert automatically an ER diagram to the relational diagram.