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Nested Query

Nested queries, also known as subqueries, are SQL queries that are embedded within another SQL query. They can be used in various parts of the main query, including the SELECT, FROM, WHERE, and HAVING clauses.

Subqueries enable complex operations and can be crucial for performing advanced data manipulations and analyses directly within the database.

Example

Consider we have students and exam_results table with the following data, and we want to fetch the names of students who scored above 92 in at least 1 subject.

Query

SELECT student_name
FROM Students
WHERE student_id IN (
    SELECT student_id
    FROM Exam_Results
    WHERE score > 92
);

java

SELECT student_name
FROM Students
WHERE student_id IN (
    SELECT student_id
    FROM Exam_Results
    WHERE score > 92
);

Result

Executing the above query will fetch the names of students who scored more than 92 marks in any subject.

Usage of Nested Queries

WHERE Clause: To filter records from the main query by comparing them against the results of the subquery. For example, find employees whose salary is above the average salary:
```
SELECT Name, Salary
FROM Employees
WHERE Salary > (SELECT AVG(Salary) FROM Employees);
```
SELECT Clause: Subqueries in the SELECT clause can provide details on each record. For instance, to find the total number of orders per customer:
```
SELECT Name, (SELECT COUNT(*) FROM Orders WHERE Orders.CustomerID = Customers.CustomerID) as OrderCount
FROM Customers;
```

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