CONNECT_BY_ISLEAF

CONNECT_BY_ISLEAF is a pseudo column in Oracle that is used in hierarchical queries (using CONNECT BY) to identify whether a row is a leaf node in the hierarchy. A leaf node is a node that does not have any children. This can be particularly useful in situations where you want to distinguish between rows that are leaf nodes (the last nodes in the hierarchy) and those that have child nodes.

Key Points:

• Definition: CONNECT_BY_ISLEAF returns 1 if the current row is a leaf node (i.e., has no children), and 0 if the current row has one or more children.
• Common Use Case: It's commonly used in hierarchical queries to filter or identify leaf nodes, such as finding items in a category tree that do not have further subcategories, or finding employees who do not manage anyone else.

Syntax:

CONNECT_BY_ISLEAF

It is used as part of a SELECT statement in hierarchical queries that use CONNECT BY. This pseudo column does not require a parameter and simply returns 1 or 0 depending on whether the current node is a leaf or not.

Example:

Consider a table employees with columns emp_id, emp_name, and manager_id, where manager_id is used to establish a hierarchical relationship between employees and their managers. The query below shows how to use CONNECT_BY_ISLEAF to identify the leaf nodes (employees with no subordinates).

SELECT emp_id, emp_name,

       CONNECT_BY_ISLEAF AS is_leaf

FROM employees

START WITH manager_id IS NULL

CONNECT BY PRIOR emp_id = manager_id;

Explanation:

• CONNECT_BY_ISLEAF: This column will return 1 for rows that are leaf nodes (employees who don’t manage anyone), and 0 for rows that have subordinates.
• START WITH manager_id IS NULL: This sets the root of the hierarchy, which in this case is the top-level employees who don't have managers.
• CONNECT BY PRIOR emp_id = manager_id: Defines the hierarchical relationship, where the emp_id of a manager is linked to the manager_id of an employee.

Sample Output:

emp_id	emp_name	is_leaf
1	Alice	0
2	Bob	0
3	Charlie	1
4	David	1
5	Eve	1
6	Frank	1

Explanation of Output:

• Alice and Bob are managers, so they are not leaf nodes (is_leaf = 0).
• Charlie, David, Eve, and Frank are employees who don’t manage anyone, so they are leaf nodes (is_leaf = 1).

 

Use Cases of CONNECT_BY_ISLEAF:

1. Identify Leaf Nodes: CONNECT_BY_ISLEAF is most useful when you need to identify or filter out leaf nodes in a hierarchical query. For example, in a tree of categories, leaf nodes represent the final items (products) in the hierarchy that do not have further subcategories.

Example:

SELECT category_id, category_name,

       CONNECT_BY_ISLEAF AS is_leaf

FROM categories

START WITH parent_category_id IS NULL

CONNECT BY PRIOR category_id = parent_category_id

WHERE CONNECT_BY_ISLEAF = 1;

2. Filter Out Leaf Nodes: If you're interested only in non-leaf nodes (e.g., categories that have subcategories), you can use CONNECT_BY_ISLEAF to exclude the leaf nodes.

Example:

SELECT category_id, category_name

FROM categories

START WITH parent_category_id IS NULL

CONNECT BY PRIOR category_id = parent_category_id

WHERE CONNECT_BY_ISLEAF = 0;

3. Count Leaf Nodes: You can use CONNECT_BY_ISLEAF to count the number of leaf nodes in a hierarchy. This is useful when you need to perform a report on how many final items exist in a category tree or an employee reporting structure.

Example:

SELECT COUNT(*) AS leaf_count

FROM employees

START WITH manager_id IS NULL

CONNECT BY PRIOR emp_id = manager_id

WHERE CONNECT_BY_ISLEAF = 1;

4. Calculate Hierarchy Depth Excluding Leaf Nodes: You can use CONNECT_BY_ISLEAF in combination with the LEVEL pseudo column to calculate the depth of a hierarchy excluding leaf nodes.

Example:

SELECT emp_id, emp_name, LEVEL

FROM employees

START WITH manager_id IS NULL

CONNECT BY PRIOR emp_id = manager_id

WHERE CONNECT_BY_ISLEAF = 0;

 

Limitations and Considerations:

1. Performance:
• CONNECT_BY_ISLEAF is computationally efficient for small to medium hierarchies but may have performance impacts on large hierarchies. Proper indexing on parent-child relationships (e.g., emp_id and manager_id) can help improve performance.
2. Handling Cycles:
• When working with hierarchical data that may contain cycles (e.g., an employee who manages themselves), you can use the NOCYCLE keyword to prevent infinite recursion. Cycles can interfere with the accurate identification of leaf nodes.

Example:

CONNECT BY NOCYCLE PRIOR emp_id = manager_id

3. Use in Complex Queries:
• CONNECT_BY_ISLEAF can be combined with other pseudo columns like LEVEL, CONNECT_BY_ROOT, and SYS_CONNECT_BY_PATH to provide more detailed insights into the hierarchy. However, combining multiple pseudo columns in a complex query may affect readability and performance.
4. Root Node Handling:
• The root node of a hierarchy will never be a leaf node, as it typically has children. However, if your data structure includes cases where the root itself could be a leaf (for example, if the root node has no children), CONNECT_BY_ISLEAF can still return 1 for such a root node.