Introduction

SQL queries can be hard to understand for people who are just starting out. This is especially true when it comes to knowing the difference between nested subqueries, correlated subqueries, and join operations.

Each method has its pros and cons, and picking the right one for a given task is key to getting the best performance and most accurate results. This article will go into detail about each method, give examples, and talk about when to use each one. It aims to allow readers to understand three important SQL concepts and write better queries.

What is a Subquery?

A subquery is a query that is inside of another query. It's a way to get information from one table based on what's in another. A subquery is run first, and then its results are used as a filtering condition in the main query.

Different parts of a query, like the SELECT, FROM, WHERE, and HAVING clauses, can use subqueries. They can also be used with other SQL clauses like EXISTS, NOT EXISTS, IN, NOT IN, ANY, ALL, and some aggregate functions.

There are two types of subqueries: nested and correlated.

Nested Subqueries

A nested subquery is a query that is written inside another query, and the result of the inner query is used to run the outer query. The inner query runs first, and the outer query uses its result. Nested subqueries help developers get information that depends on other information in the same table or other tables.

The following example aims to explain this idea in-depth. Here are two tables that one can consider:

Now, suppose one wants to find out who works in the Sales department. This can be achieved with a subquery that is inside another one. The inner query gets the ID for the Sales department, which is then used by the outer query to identify all the employees who work in this department.

Correlated Subqueries

A correlated subquery is a query that is inside of another query and uses the results of the first query. The inner query is run for each row that the outer query brings back. Correlated subqueries help developers identify information that depends on other information in the same table or other tables.

Here's an example that dives deep into this idea. Suppose there is a manager called Jane Smith and the task is to find all employees who work under her. A subquery that is linked to the main query can help developers achieve this task.

SELECT first_name, last_name FROM employees e1 WHERE department_id =     (SELECT department_id      FROM employees e2      WHERE e2.first_name = 'Jane' AND e2.last_name = 'Smith'        AND e2.department_id = e1.department_id);

The department ID from the outer query is used by the inner query to find all the employees who work in the same department as Jane Smith.

What is a JOIN?

Join is a SQL method for putting together information from two or more tables. It makes a new table by putting together rows from different tables that have the same values in the columns you choose.

A join operation is done with a join clause that says which columns from each table are used to match records. There are different kinds of joins, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

For example, the following SQL query can be used to combine the employee's table and the department's table:

SELECT e.emp_id, e.first_name, e.last_name, d.department_nameFROM employees eJOIN departments d ON e.department_id = d.dept_id;

This query will output a table with four columns: emp_id, first_name, last_name, and department_name. It will include all employees, even if they don't have a matching department record.

As can be seen, the JOIN query combines data from both tables based on a common field (department_id and dept_id) and includes all employees, even those who do not have a matching department record. The LEFT JOIN keyword ensures that all records from the employee's table are included in the output table.

Differences between Nested Subquery, Correlated Subquery, and Join Operation

When the result of the inner query needs to be compared to the result of the outer query, using a nested subquery is the solution. On the other hand, when the inner query needs to be run for every row of the outer query, using a correlated subquery is the way forward. Join operations are used when one needs to combine data from more than one table or when the WHERE clause is not enough. To get the best performance, it's important to choose the method that fits the use case best.

Conclusion

Nested subquery, correlated subquery, and join operations each have their pros and cons. Hence, the decision about which one to choose depends on the use case and the user's needs. Nested and correlated subqueries can be very helpful when one needs to do complex calculations or filtering. On the other hand, join operations are the best way to combine data from multiple tables.

By understanding the difference between these methods, users can make better decisions when designing and running SQL queries. ‍

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