How to use connect by in oracle

CONNECT BY is used to perform hierarchical queries in Oracle. It traverses the data by specifying the hierarchical structure. The specific steps include: creating a hierarchical structure and specifying the connected columns to define the hierarchical structure; setting the connection conditions and using priority and current keywords specify the relationship between the previous level and the current level. The common condition is prior column_name = current column_name; use advanced functions such as START WITH, CONNECT_BY_ROOT and LEVEL to perform more complex hierarchical queries.

Usage of CONNECT BY in Oracle

Brief description

CONNECT BY is a keyword used in Oracle to perform hierarchical queries, which can traverse data by specifying a hierarchical structure. It is useful for querying tree-shaped data or data with hierarchical relationships.

Syntax

<code>SELECT columns
FROM table_name
CONNECT BY prior column_name = current column_name</code>

Where:

• columns: The columns to be selected
• table_name: The name of the table to be queried
• prior column_name: The column name of the previous level
• current column_name: The current level Column names

Usage

1. Create a hierarchy

First, you need to specify the connection Column, which is the column that defines the hierarchy. It can be a foreign key column in a table that represents parent and child relationships.

2. Specify connection conditions

The CONNECT BY clause uses the prior and current keywords to specify connection conditions. The prior keyword refers to columns at the previous level, while the current keyword refers to columns at the current level. The connection condition is usually:

<code>prior column_name = current column_name</code>

This means that the column value of the upper level of the current row is equal to the column value of the current row.

Example

Suppose we have a table called Employee with the following columns:

• Employee_ID
• Manager_ID

The Manager_ID column represents the employee's manager and can be used to create hierarchies.

To get all employees as well as their managers, we can use the following query:

<code>SELECT Employee_ID, Manager_ID, Employee_Name
FROM Employee
CONNECT BY prior Manager_ID = current Employee_ID;</code>

Advanced Features

In addition to basic usage, CONNECT BY also Several advanced functions are provided:

• START WITH: Specify the starting point for hierarchical traversal.
• CONNECT_BY_ROOT: References the root node of the hierarchy.
• LEVEL: Gets the level of the current row in the hierarchy.

These functions allow you to perform more complex hierarchical queries, such as finding all child nodes of a specific node or calculating the distance between nodes.

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