Inline View

 Inline View is essentially a subquery in the FROM clause of a SQL statement, treated as a "virtual" table. It allows you to perform complex queries without creating temporary tables or views in the database. Instead, the subquery in the FROM clause acts as an intermediate result set that is used by the outer query.

Inline views are particularly useful when:

• You need to perform complex filtering, aggregation, or transformations on data.
• You want to avoid creating permanent views or temporary tables.
• You need a "virtual" table for joining or filtering data in your query.

Key Characteristics of Inline Views:

1.     Subquery in the FROM Clause:
An inline view is a subquery that appears in the FROM clause of the outer query. It returns a result set that can be treated like a table, allowing you to reference the output of the subquery as a virtual table.

2.     Temporary and Virtual:
An inline view is not a physical object like a table or a view; it exists only during the execution of the query. Once the query finishes, the inline view disappears.

3.     Aliases:
You must assign an alias (a name) to the inline view, which allows you to reference its columns and treat the subquery as a "table."

4.     Can Be Used for Complex Queries:
Inline views are often used for complex queries where you need to filter, aggregate, or join data before performing additional operations.

Syntax for Inline View:

SELECT column1, column2, ...

FROM (SELECT subcolumn1, subcolumn2, ...

      FROM table_name

      WHERE conditions) AS inline_view_alias

WHERE outer_query_conditions;

In this syntax:

• The inner SELECT statement is the inline view that generates a result set.
• The alias (inline_view_alias) is required to refer to this result set in the outer query.
• The outer query can perform operations like filtering, sorting, or joining on the result set from the inline view.

Examples of Using Inline Views:

1. Using Inline Views for Aggregation:

In this example, we calculate the total salary per department and then find the employees who earn more than the average salary of their respective departments:

SELECT employee_id, name, salary, department_id

FROM employees e

JOIN (SELECT department_id, AVG(salary) AS avg_salary

      FROM employees

      GROUP BY department_id) AS dept_avg

ON e.department_id = dept_avg.department_id

WHERE e.salary > dept_avg.avg_salary;

• The inline view (SELECT department_id, AVG(salary) FROM employees GROUP BY department_id) calculates the average salary for each department.
• The outer query then joins this result with the employees table and filters employees who earn more than the average salary of their department.

2. Using Inline Views for Complex Filtering:

You can use inline views to filter results based on complex conditions. For example, to get the highest-paid employees from each department:

SELECT e.employee_id, e.name, e.salary, e.department_id

FROM employees e

JOIN (SELECT department_id, MAX(salary) AS max_salary

      FROM employees

      GROUP BY department_id) AS dept_max

ON e.department_id = dept_max.department_id

AND e.salary = dept_max.max_salary;

• The inline view computes the maximum salary per department.
• The outer query filters employees whose salary matches the computed maximum salary for their department.

3. Using Inline Views for Row Numbering:

If you want to select the top 3 highest-paid employees from each department, you can use an inline view with ROW_NUMBER() to rank employees by salary:

SELECT employee_id, name, salary, department_id

FROM (

  SELECT employee_id, name, salary, department_id,

         ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank

  FROM employees

) AS ranked_employees

WHERE rank <= 3;

• The inline view ranks employees within each department by salary, using the ROW_NUMBER() function.
• The outer query filters the top 3 highest-paid employees in each department by checking the rank.

4. Using Inline Views with Joins:

Inline views can be helpful when you need to join multiple datasets that require transformations or filtering before the join operation.

Example: Join employees with departments and calculate their performance ratings:

SELECT e.employee_id, e.name, e.salary, d.department_name, p.performance_score

FROM employees e

JOIN (SELECT employee_id, performance_score

      FROM performance_reviews

      WHERE review_year = 2024) AS p

ON e.employee_id = p.employee_id

JOIN departments d

ON e.department_id = d.department_id;

• The inline view (SELECT employee_id, performance_score FROM performance_reviews WHERE review_year = 2024) fetches the performance scores of employees for the year 2024.
• The outer query joins this inline view with employees and departments tables to retrieve complete employee details along with their performance score.

Advantages of Inline Views:

1.     No Need for Temporary Tables:
Inline views allow you to perform complex operations without creating temporary tables or permanent views, making queries more efficient.

2.     Encapsulation of Complex Logic:
Complex filtering, aggregation, or data transformations can be encapsulated within the inline view, simplifying the outer query and improving readability.

3.     Avoiding Duplicate Data:
You can perform aggregation or filtering inside the inline view to reduce the amount of data returned to the outer query, which can improve performance.

4.     Flexibility and Reusability:
Inline views can be easily modified or reused within the same query without affecting the underlying database schema.

Disadvantages of Inline Views:

1.     Potential Performance Overhead:
While inline views are useful for temporary data manipulation, if the subquery is complex or involves large tables, it may lead to performance issues due to repeated execution of the subquery.

2.     Complexity in Nested Queries:
Using inline views in deeply nested queries can make the SQL harder to understand and maintain, especially if multiple inline views are involved.

3.     Lack of Indexing:
Inline views cannot benefit from indexing, which might make queries slower compared to using indexed tables or views directly.

Performance Considerations for Inline Views:

1.     Subquery Optimization:
If the inline view involves aggregation or sorting operations, ensure that the underlying tables are indexed appropriately (e.g., indexing the columns used in JOIN, WHERE, or GROUP BY clauses).

2.     Avoid Complex Queries in Inline Views:
Complex queries within inline views (especially involving joins or aggregations) can significantly impact performance. Use them sparingly and try to optimize them wherever possible.

3.     Evaluate with and without Inline Views:
Sometimes, breaking down an inline view into a common table expression (CTE) or using a temporary table might be more efficient, especially for very large result sets.

Best Practices for Using Inline Views:

1.     Use Descriptive Aliases:
Assign meaningful aliases to the inline view to make the query more readable. Avoid generic names like subquery1 or t.

2.     Keep Inline Views Simple:
Try to keep the logic inside an inline view as simple as possible to improve query performance and maintainability.

3.     Consider Performance Implications:
Test queries with inline views against large datasets to ensure they perform optimally. Sometimes using a JOIN or temporary tables might be more efficient than relying on complex inline views.

4.     Use Inline Views for Complex Filtering:
If you need to filter or transform data before using it in the outer query (such as applying ROW_NUMBER(), aggregating, or filtering by a computed condition), inline views can make your queries more readable and maintainable.