In a hierarchical query, one expression in the CONNECT BY condition must be qualified by the PRIOR operator. If the CONNECT BY condition is compound, then only one condition requires the PRIOR operator, although you can have multiple PRIOR conditions. PRIOR evaluates the immediately following expression for the parent row of the current row in a hierarchical query.

PRIOR is most commonly used when comparing column values with the equality operator. (The PRIOR keyword can be on either side of the operator.) PRIOR causes Oracle to use the value of the parent row in the column. Operators other than the equal sign (=) are theoretically possible in CONNECT BY clauses. However, the conditions created by these other operators can result in an infinite loop through the possible combinations. In this case Oracle detects the loop at run time and returns an error. Refer to Hierarchical Queries for more information on this operator, including examples.

CONNECT_BY_ROOT is a unary operator that is valid only in hierarchical queries. When you qualify a column with this operator, Oracle returns the column value using data from the root row. This operator extends the functionality of the CONNECT BY [PRIOR] condition of hierarchical queries.

Restriction on CONNECT_BY_ROOT

You cannot specify this operator in the START WITH condition or the CONNECT BY condition.