How to query child nodes in oracle

In Oracle database, querying the child nodes of a node is a common requirement, especially when dealing with large hierarchical data. In this article, we will introduce how to use Oracle's CONNECT BY statement and SYS_CONNECT_BY_PATH function to query child nodes.

First, let’s take a look at the CONNECT BY statement. The CONNECT BY statement is a recursive query statement of Oracle, which allows querying tree structure or hierarchical data. There are two ways to use CONNECT BY: one is to use the START WITH clause to specify the starting node of the query, and the other is to omit START WITH, in which case a recursive query will be performed from the entire table.

Below we give a simple example to illustrate this query method. Suppose we have a table named "EMPLOYEE", which contains employee information. There is a "DEPT_ID" field representing the department to which the employee belongs, and an "EMP_ID" field representing the employee's unique ID.

We can query all employees whose "DEPT_ID" is 100 through the following statement:

SELECT * FROM EMPLOYEE START WITH DEPT_ID = 100 CONNECT BY PRIOR EMP_ID = MANAGER_ID;

This query statement means to recursively query their subordinate employees starting from the employees whose "DEPT_ID" is 100. , where "PRIOR" is a keyword, indicating the employee of the upper level recursion, and "MANAGER_ID" is a field, indicating the ID of the employee's upper level manager.

In the query results, we can see the information of all employees whose "DEPT_ID" is 100 and their subordinate employees.

However, this method does not provide the level information of each employee, that is, the level of each employee. At this time, we need to use the SYS_CONNECT_BY_PATH function to get the full path of each node.

The following is an example:

SELECT EMPLOYEE.*, LEVEL, SYS_CONNECT_BY_PATH(EMP_ID, '/') AS PATH
FROM EMPLOYEE
START WITH DEPT_ID = 100
CONNECT BY PRIOR EMP_ID = MANAGER_ID;

In the query results, we can see the complete path information of each employee, such as "/100/101/103". Here, "/" is the path separator, and "LEVEL" is a system variable indicating the layer level of the current node.

In addition, we can also use other methods to query child nodes. For example, we can use the "WITH" keyword to create a public expression and use it in the query. This method is more readable and makes it easier to combine query conditions.

In short, you can use the CONNECT BY statement and the SYS_CONNECT_BY_PATH function to query the child nodes of a node in the Oracle database. In practical applications, different query methods can be selected according to specific needs to achieve better results.

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