what is the differences between that line in oracle ERD?

There are several notations to represent an entity-relationship model. The one you are looking at is called Barker's notation where the relationship(line) is divided into two halves. Each half can be either solid (relationship is mandatory) or dashed (relationship is optional) and you can read those relationships as follows:

1. Solid half and solid half - relationship is mandatory.

   Each computer must have a hard drive, and one hard drive must be installed into one computer.

2. Dashed half and solid half - relationship is mandatory on the right and optional on the left:

   Each computer must have a hard drive, and one hard drive may or may not be installed into one computer. (will be vise verse if right side of the relationship is dashed and the left one is solid )

3. Dashed half and dashed half - relationship is optional.

   Each computer may or may not have a hard drive, and one hard drive may or may not be installed into one computer.

Same goes for one to many and many to many relationships.

Dashed line indicates minimum cardinality of relation can be zero, continuos line indicates minimum cardinality of relation is at least one.

Show this interesting document