Defining the Parent/Child Relationship v12

For any given row, its parent and its children are determined by the CONNECT BY clause. The CONNECT BY clause must consist of two expressions compared with the equals (=) operator. In addition, one of these two expressions must be preceded by the keyword, PRIOR.

For any given row, to determine its children:

1. Evaluate parent_expr on the given row.

2. Evaluate child_expr on any other row resulting from the evaluation of table_expression.

3. If parent_expr = child_expr, then this row is a child node of the given parent row.

4. Repeat the process for all remaining rows in table_expression. All rows that satisfy the equation in step 3 are the children nodes of the given parent row.

By iteratively repeating this process treating each child node found in the prior steps as a parent, an inverted tree of nodes is constructed. The process is complete when the final set of child nodes has no children of their own - these are the leaf nodes.

A SELECT command that includes a CONNECT BY clause typically includes the START WITH clause. The START WITH clause determines the rows that are to be the root nodes - i.e., the rows that are the initial parent nodes upon which the algorithm described previously is to be applied. This is further explained in the following section.