Logical Processing order of a SQL query

The logical processing order of a SQL query can be understood by diving deeper into each step. Though SQL queries are written in a particular order, the database engine executes them differently. Below is a detailed breakdown of the logical order with a sample query for clarity:

Logical Order of SQL Execution:

1. FROM: Identify and Load the Data

2. JOIN: Combine Tables (if applicable)

3. WHERE: Filter Rows Based on Conditions

4. GROUP BY: Group Rows for Aggregation

5. HAVING: Filter Groups Based on Aggregate Conditions

6. SELECT: Select Columns or Expressions

7. DISTINCT: Remove Duplicates (if requested)

8. ORDER BY: Sort the Result Set

9. LIMIT/OFFSET: Limit the Number of Rows (Pagination)

Now, let's look at each step in more detail:

1. FROM: Identify and Load the Data

• The query execution begins with identifying the source of the data, i.e., the table(s) that will be queried.
• If subqueries or views are involved, the engine executes those first.

Example:

FROM employees

• Here, the data comes from the employees table.

2. JOIN: Combine Tables

• If multiple tables are involved, the database will determine how to combine the data. The different types of joins (INNER, LEFT, RIGHT, FULL OUTER) define how rows are matched between tables.
• JOIN operations are performed before filtering in the WHERE clause, ensuring all necessary rows are available.

Example:

FROM employees e
JOIN departments d ON e.department_id = d.department_id

• The employees and departments tables are joined using the department_id column.

3. WHERE: Filter Rows Based on Conditions

• After combining tables, the WHERE clause filters out rows that do not meet specific conditions. These conditions are typically based on individual column values.
• This is a row-level filtering.

Example:

WHERE e.hire_date > '2020-01-01'

The query filters employees who were hired after January 1, 2020.

4. GROUP BY: Group Rows for Aggregation

• Once the data is filtered, the GROUP BY clause groups rows that have the same values in specified columns. Aggregations (like COUNT, SUM, AVG) are then applied to these groups.
• Without a GROUP BY clause, aggregate functions like COUNT() would consider the entire result set as a single group.

Example:

GROUP BY e.department_id

• The query groups employees by department_id, preparing for aggregation (such as counting employees per department).

5. HAVING: Filter Groups Based on Aggregate Conditions

• HAVING works like WHERE, but it filters groups instead of individual rows.
• This clause is typically used to filter based on aggregate functions, which cannot be used in the WHERE clause.

Example:

HAVING COUNT(e.employee_id) > 5

• The query filters out departments that have fewer than or equal to 5 employees, keeping only departments with more than 5 employees.

6. SELECT: Select Columns or Expressions

• Once the data is grouped and filtered, the SELECT clause specifies the actual columns or expressions that will be returned in the final output.
• You can include aggregate functions or calculations in the SELECT clause, along with regular column values.

Example:

SELECT e.department_id, COUNT(e.employee_id) AS total_employees

• The query returns the department_id and the count of employees (total_employees) for each department.

7. DISTINCT: Remove Duplicates (if applicable)

• If the DISTINCT keyword is used, it eliminates duplicate rows from the result set.
• It is applied after the SELECT clause but before any ordering.

Example:

SELECT DISTINCT department_id

• This would remove any duplicate department IDs from the result, though it's not needed in the given query.

8. ORDER BY: Sort the Result Set

• The ORDER BY clause sorts the result set based on one or more columns.
• Sorting can be done in ascending (ASC) or descending (DESC) order.

Example:

ORDER BY total_employees DESC

• The query orders departments by the number of employees in descending order, meaning the department with the highest number of employees appears first.

9. LIMIT/OFFSET: Limit the Number of Rows (Pagination)

• The LIMIT clause restricts the number of rows returned by the query.
• OFFSET can be used in combination with LIMIT to skip a specified number of rows (for example, for pagination).

Example:

LIMIT 10

• The query limits the output to the top 10 rows based on the number of employees.

Final Sample Query

SELECT e.department_id, COUNT(e.employee_id) AS total_employees
FROM employees e
WHERE e.hire_date > '2020-01-01'
GROUP BY e.department_id
HAVING COUNT(e.employee_id) > 5
ORDER BY total_employees DESC
LIMIT 10;

Step-by-Step Breakdown of Execution:

• 1. FROM employees: The query starts by identifying the employees table.
• 2. WHERE hire_date > '2020-01-01': Filters out all employees who were hired before January 1, 2020.
• 3. GROUP BY department_id: Groups the remaining employees by their department_id.
• 4. HAVING COUNT(employee_id) > 5: Retains only those departments that have more than 5 employees.
• 5. SELECT department_id, COUNT(employee_id): Selects the department ID and the total number of employees for each department.
• 6. ORDER BY total_employees DESC: Sorts the departments in descending order based on the number of employees.
• 7. LIMIT 10: Returns only the top 10 departments.

Conclusion:

SQL queries are written in a "high-level" format that is more intuitive for users (starting with SELECT), but the database engine executes them in a very logical and systematic order to retrieve the correct data efficiently.

Understanding the logical order of query execution is essential for writing complex queries that work as expected, especially when using JOIN, GROUP BY, and HAVING.