We may sometimes need data which is already present in table but we need that data during insert. If we query data first and then insert the data, it will require running 2 SQL commands. But it will be best to run just one single SQL command that will query the data first and set the data during INSERT Query.

This technique elegantly merges data retrieval and insertion, streamlining the process and reducing the number of queries required.

Let's see an Example:

Consider a website's menu structure stored within a table named menus. Each menu item holds properties such as:

id: A unique identifier
menu_name: The name of the menu item
parent_id: The identifier of the parent menu item (if applicable)

To establish a parent-child relationship between menus, we can effectively leverage a SELECT during INSERT query.

INSERT INTO `menus` (`id`, `menu_name`, `parent_id`)
(SELECT
   NULL,
   'childMenu',
   menu.id <--- setting the parent menu id
FROM `menus` AS menu
WHERE menu.menu_name = 'parentMenu'
LIMIT 1);

Breaking Down the Query:

1. INSERT INTO menus (id, menu_name, parent_id): This segment initiates an INSERT operation into the menus table, targeting the specified columns.
2. SELECT NULL, 'childMenu', menu.id FROM menus AS menu: This represents a nested SELECT query that retrieves the desired data for insertion from menus table.
3. WHERE menu.menu_name = 'parentMenu': This condition filters the results to locate the specific parent menu item.
4. LIMIT 1: This constraint ensures that only a single row is selected, preventing potential duplication issues.

Advantages:

1. Optimized Performance: Combining operations into a single query often enhances execution speed.
2. Enhanced Readability: Nesting queries can improve code organization and clarity.
3. Data Integrity: This approach safeguards data consistency by ensuring parent-child relationships are accurately established during insertion.