ER diagram, is this allowed?

I have to create an ER diagram based on a relational schema.

There is a table of players, and a table of zones. A player can 'live' in many zones, and each zone is owned by one or more players.

I've come up with this simple ER diagram but I'm n开发者_JAVA技巧ot sure having relationships going each way is allowed?

Cheers

Yes, that is a perfectly good Entity Relation Diagram. (I am not responding as to whether it makes sense or not: you still need to resolve the Relations and Cardinality.)

Using the correct terms helps people understand exactly what you are discussing, and which level you are discussing. Loose talk results in much more volume in the discussion, and time wasted in clarifying what you meant by which term. Not good for productive technical endeavours.

1. At this early stage, it is normal to model Entities and Relations (not Attributes), that's why it is called an ER diagram; we are nowhere near modelling the data. The Relations are relevant, and that's why you are detailing and evaluating their nature in the diamonds and Cardinality. The goal is to clarify the true Entities, and their Relations to each other. Many-to-many relations remain as relations. The ERD is purely Logical, there is no Physical.

2. Once you have some confidence with that, that you have gotten the Entities and Relations right, you move onto a Data Model (which includes Attributes). Still at a Logical level, the n::n relations remain as relations.

   • As you progress, you may show further detail, such as Domain for each Attribute. That's the DataType, but at the Logical level, just as the terms are Entity = Table and Attribute = Column, Domain = DataType.
     .

3. When you get to the Physical level, the Data Model has Tables; Columns; DataTypes.

   • And n::n Relations are manifested as the Associative Tables.
     .
4. The idea is, as long as you are working through the prescribed steps, at (1), the content in the diamonds will determine (expose) if they need to be stored, and the diamond is thus promoted to an Entity; otherwise it remains a Relation.

There is a junction table called lives-in in the relational schema I've been given. However, I thought when mapping a relational schema [back] to an ER diagram a junction table becomes a relationship?

• The Relational term is Associative table.

• Yes. If it is a pure n::n Table (containing nothing but the two FKs to the PKs of the parent Tables), at the ERD level, which is Logical only, it is a Relation.

• If it has Columns other than the two FKs, it is an Entity.

Since there's a many-to-many relationship between [Players] and [Zones] you have to add a junction table (called for ex. [PlayersZones]). The notation itself is correct (Chen notation), though I prefer the Crow's Foot Notation.

I am not able to see your images (blocked!) so I'll just try to describe the "correct" design. If a player living in a zone doesn't necessarily mean they own it, you should have four tables:

PLAYER (playerid, <other fields>)
ZONE (zoneid, <other fields>
PLAYER_ZONE(playerid, lives_in_zoneid)
ZONE_OWNER (zoneid, owner_playerid)

Otherwise three tables would suffice.