database - How to write inner join query in collection method (magento2)? - Magento Stack Exchange
In a SQL database query , a correlated subquery also known as a synchronized subquery is a subquery a query nested inside another query that uses values from the outer query. Because the subquery may be evaluated once for each row processed by the outer query, it can be slow. Here is an example for a typical correlated subquery. In this example, the objective is to find all employees whose salary is above average for their department. In the above nested query the inner query has to be re-executed for each employee.
In this lesson, we will see how SQL subqueries can be incorporated within queries. As their name suggests, subqueries are queries embedded within a query. They are also called inner queries or nested queries , and they are part of another query, called an outer query. Alternative names for these SQL features are inner select and outer select , respectively.
Good news! The database includes three tables: students , teachers , and classes. You can see the tables below:. Certainly, you can use a query to find out:.