The start with connect by clause can be used to select data that has a hierarchical relationship (usually some sort of parent->child (boss->employee or thing->parts).
It is also being used when an sql execution plan is explained.

In the following example, the table from which that data is selected consists of just these attributes: parent and child. We make sure (by means of a unique constraint) that the child is uniqe within the table. This is just like in the real life where (as of yet) a child cannot have two different mothers.

The data filled into the table is such that a the sum over the children with the same parent is the value of the parent:

 

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