Use of fetchOne() method in sql query Magento 2.

You can run a direct SQL query for fetching only first value using fetchOne() method.

fetchOne() Fetches the first column of the first row of the SQL result as output.

You can use direct SQL query for the fetch the first column of the first row using below way, Get Product id by Product SKU,

Output is 1. //entity_id for SKU 24-MB01

Check for other Direct SQL Query in Magento 2