Notes on SQL Select Statement Execution Sequence

Overview

• Focus on the execution sequence of the SELECT statement in SQL.
• Example involves retrieving department names and maximum salaries for departments with specific criteria.

Query Requirements

• Display: Department name and maximum salary.
• Conditions: Average salary for the department must be > 1200.
• Sorting: Results should be sorted by department name.

Data Sources

• Tables Involved: Employee and Departments.
• Join Condition: Employee department number must match Departments department number.

SQL Clauses Used

1. SELECT

   • Specify the columns to return (e.g., department name, maximum salary as 'Max').

2. FROM

   • Identify source tables: Employee and Departments.
   • A JOIN condition is necessary for multiple tables.

3. GROUP BY

   • Aggregate data, grouping by department name.
   • It divides data into groups based on department.

4. HAVING

   • Set a condition to filter groups, where the average salary > 1200.

5. ORDER BY

   • Sort results by department name (Dname).

Execution Sequence

1. FROM Clause

   • DBMS retrieves data from the specified tables first.
   • Combines data into a temporary larger table based on JOIN conditions.

2. WHERE Clause

   • Applies filters to the temporary table to exclude records that don't meet the conditions.
   • Only records that meet the conditions are retained.

3. GROUP BY Clause

   • Groups remaining records according to department name (Dname).
   • DBMS can only access and manipulate data within these groups going forward.

4. Aggregates Calculation

   • Calculate aggregate functions (e.g., MAX, AVG) for each group.
   • Each group retains references to its maximum and average values.

5. HAVING Clause

   • Evaluates the condition for averages, removing groups that do not meet the criteria (> 1200).

6. SELECT Clause

   • Confirms which data to return based on available data in the groups.
   • Returns department name and calculated maximum salary.

7. ORDER BY Clause

   • Sorts the resulting dataset ascendingly by department name (Dname).
   • Ensures that the required data is accessible for sorting.

Important Notes

• Data displayed must be available in the group created by GROUP BY.
• Items in ORDER BY must also be present in the groups.
• Additional filters can be added in the WHERE clause, alongside JOIN conditions.

Example of Result Set

• Display result includes department names and maximum salaries which meet the specified conditions, sorted alphabetically.

• Final Warning: Ensure any data you wish to display or sort by is accounted for in the GROUP BY clause when using aggregation operations.