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Subqueries in SQL can be used in **several places** inside the main query:

### 1. In `WHERE`

- To filter records based on the result of another query.
**Example:**

```sql
SELECT Name, Age
FROM Employees
WHERE DepartmentID = (SELECT ID FROM Departments WHERE Name = 'Sales');
```

### 2. In `FROM`

- The subquery creates a **temporary table**, which you can then work with.
**Example:**

```sql
SELECT AVG(Age) AS AverageAge
FROM (SELECT Age FROM Employees WHERE DepartmentID = 101) AS Subtable;
```

### 3. In `SELECT`

- To compute a value right inside the list of selected columns.
**Example:**

```sql
SELECT Name,
       (SELECT Name FROM Departments WHERE Departments.ID = Employees.DepartmentID) AS Department
FROM Employees;
```

### 4. In `HAVING`

- To filter groups after aggregate functions.
**Example:**

```sql
SELECT DepartmentID, COUNT(*) AS EmployeeCount
FROM Employees
GROUP BY DepartmentID
HAVING COUNT(*) > (SELECT AVG(EmployeeCount) FROM Employees GROUP BY DepartmentID);
```

> Put simply, subqueries can be used **anywhere SQL lets you insert an expression**, to dynamically get data for filtering, calculations, or aggregation.

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