An Entity Relationship Diagram conveys a lot of information with a very concise notation. The important part to keep in mind is to limit what you're reading using the following technique:
Choose two entities (e.g. Company and Employee)
Pick one that you're interested in (e.g. how a single Company relates to employees)
Read the notation on the second entity (e.g. the crow's feet with the O above it next to the Employee entity).
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