Inline View Method and the Correlated Subquery in SQL

Let's dive into the execution flow for both methods: the Inline View Method and the Correlated Subquery Method. Understanding the step-by-step execution of these queries will help clarify how SQL processes each scenario to get the desired result.

1. Inline View Method Execution Flow

The Inline View Method uses a subquery in the FROM clause to create a temporary result set (inline view) and then processes it in the outer query.

Query:

SELECT DepartmentId, MAX(Salary) AS HighestSalary
FROM (
SELECT DepartmentId, Salary
FROM Employee
) AS InlineView
GROUP BY DepartmentId;

Execution Steps:

1. Inner Query Execution (Inline View):

The inner query (SELECT DepartmentId, Salary FROM Employee) is executed first.

It selects the DepartmentId and Salary columns from the Employee table.

This step essentially creates a temporary result set (inline view) that includes all the rows from the Employee table but only with the DepartmentId and Salary columns.

Sample result of the inner query:

1. Outer Query Execution (Aggregation and Grouping):

After the inline view (temporary result set) is created, the outer query takes this result set and performs aggregation using the MAX(Salary) function.

The GROUP BY DepartmentId ensures that the maximum salary is calculated for each department.

The outer query scans through the rows from the inline view and groups the rows based on DepartmentId. Then, it computes the maximum salary for each group.

1. Return the Result:

The query returns the highest salary for each department.

Final result:

Execution Flow Summary:

1. Execute the inner query to create the inline view.

2. The outer query aggregates (using MAX) and groups the results by DepartmentId.

3. Return the final result.

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1. Correlated Subquery Method Execution Flow

In the Correlated Subquery Method, the inner subquery is executed for each row in the outer query, making it more dynamic but potentially slower than the inline view method.

Query:

SELECT DepartmentId, EmployeeId, Salary AS HighestSalary
FROM Employee e
WHERE Salary = (
SELECT MAX(Salary)
FROM Employee
WHERE DepartmentId = e.DepartmentId
);

Execution Steps:

1. Outer Query Execution (Row-by-Row Processing):

The outer query begins by reading each row from the Employee table.

For each row in the Employee table, the query retrieves the DepartmentId, EmployeeId, and Salary.

Sample row-by-row processing from the outer query:

1. Inner Query Execution (Correlated Subquery):

For each row from the outer query, the inner query (SELECT MAX(Salary) FROM Employee WHERE DepartmentId = e.DepartmentId) is executed.

The subquery calculates the maximum salary in the department corresponding to the current row's DepartmentId.

The correlation between the outer query and the subquery happens through the DepartmentId condition (WHERE DepartmentId = e.DepartmentId).

For example:

For the row with EmployeeId = 1, the subquery computes the maximum salary for DepartmentId = 101, which is 6000.

For the row with EmployeeId = 3, the subquery computes the maximum salary for DepartmentId = 102, which is 7500.

Sample execution of subquery for each row:

1. Filtering Rows:

After calculating the maximum salary for the corresponding DepartmentId, the outer query compares the Salary of the current row with the maximum salary returned by the subquery.

If the Salary matches the maximum salary for the department, the row is retained.

Rows that do not match are filtered out.

For example:

The row for EmployeeId = 2 will be retained because its salary of 6000 matches the maximum salary in department 101.

The row for EmployeeId = 1 will be filtered out because its salary of 5000 is less than the maximum salary in department 101.

1. Return the Result:

The outer query returns the rows where the employee's salary matches the maximum salary for their department.

Final result:

Execution Flow Summary:

1. For each row in the outer query, execute the inner correlated subquery.

2. The subquery calculates the maximum salary for the current DepartmentId.

3. Compare the employee's salary with the result of the subquery.

4. Filter and return rows where the employee's salary matches the maximum for their department.

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Key Differences in Execution Flow:

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Performance Considerations:

Inline View Method is more efficient when you are only interested in the highest salary for each department, without needing detailed employee information. It performs the aggregation in a single pass over the data.

Correlated Subquery Method can be slower for large datasets because it executes the subquery for every row in the outer query, potentially leading to many redundant calculations. However, it allows you to retrieve both the department’s highest salary and detailed employee information (e.g., EmployeeId, Name).