Subqueries can be categorized based on where they appear in an SQL statement and their purpose. They are categorized as,

1. Nested Scalar Subquery
2. Single-Row Subquery
3. Multiple-Row Subquery
4. Single-Column Subquery
5. Correlated Subquery

Example Data

The given table will be used for the demonstration of subtypes of nested queries.

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1. Nested Scalar Subquery

A Nested Scalar Subquery returns a single value and is often used in a comparison. In this example, we retrieve the student_name where the student_id is the maximum.

Query

SELECT student_name
FROM students
WHERE student_id = (SELECT MAX(student_id) FROM students);

Database Exercise

Database Schema:

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Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Result

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2. Single-Row Subquery

A Single-Row Subquery is used to fetch a specific row from the database table, in this case, retrieving the student_name and age where the student_id is the maximum in the students table.

Query

Database Exercise

Database Schema:

-- Database schema would be rendered here

Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Result

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3. Multiple-Row Subquery

A Multiple-Row Subquery filters results based on a condition involving multiple rows. Here, we retrieve student_name for those in departments where at least one student is older than 21.

Query

Database Exercise

Database Schema:

-- Database schema would be rendered here

Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Result

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4. Single-Column Subquery

A Single-Column Subquery is used to extract a single column’s value from the database. In this example, we fetch the student_name where the student_id is less than the maximum.

Query

Database Exercise

Database Schema:

-- Database schema would be rendered here

Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Result

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5. Correlated Subquery

A Correlated Subquery references columns from the outer query, creating a connection. Here, we fetch student_name for those whose age is greater than the average age in their respective departments.

Query

Database Exercise

Database Schema:

-- Database schema would be rendered here

Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Result

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