Saturday, January 12, 2019

Retrieve all the employees under a manager from a self-referencing table in MySQL

In previous post, we discussed how we can manage to store the hierarchical data in MySQL using self-referencing relationship.
Now, we will see how we can retrieve the data back.
Here, we have manager-employees relationship. One manager has other employees (maybe manager or non-manager) working under him. We will retrieve the whole list of employees under a manager.

SQL Code
SET @id := '2';
SELECT  id, name, managerid, @id
FROM (SELECT * FROM employee ORDER BY managerid, name) E 
WHERE FIND_IN_SET(managerid, @id) > 0
AND @id := CONCAT(@id, ',', id);

Let's assume the managerid to search is "2".
We first need to sort employee table (derived table E) by managerid to make sure that records are in correct order. (This is required as we iterate the record from beginning and add to manager list if conditions matched).
FIND_IN_SET returns the position of a string within a list of strings. (Greater than zero means this employee is under him).
If current record work under this manager(s), CONCAT the id of current record into manager list. (He may have other employees under him).


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