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Millie Molotov
Millie Molotov

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SQL Filtering and Sorting with Real-life Examples

This blog explains the SQL clauses like WHERE, HAVING, ORDER BY, GROUP BY, and other related clauses using real-life examples with the employees and departments tables.

Table of Contents

  1. Tables Structure
  2. WHERE Clause
  3. GROUP BY Clause
  4. HAVING Clause
  5. ORDER BY Clause
  6. LIMIT Clause
  7. DISTINCT Clause
  8. AND, OR, NOT Operators

Tables Structure

employees Table

emp_id name age department_id hire_date salary
1 John Smith 35 101 2020-01-01 5000
2 Jane Doe 28 102 2019-03-15 6000
3 Alice Johnson 40 103 2018-06-20 7000
4 Bob Brown 55 NULL 2015-11-10 8000
5 Charlie Black 30 102 2021-02-01 5500

departments Table

dept_id dept_name
101 HR
102 IT
103 Finance
104 Marketing

WHERE Clause

The WHERE clause is used to filter records based on specified conditions.

SQL Query

SELECT name, age, salary
FROM employees
WHERE age > 30;
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Result

name age salary
John Smith 35 5000
Alice Johnson 40 7000
Bob Brown 55 8000

Explanation: The WHERE clause filters the rows to include only employees who are older than 30 years.

Example with AND Operator

SELECT name, age, salary
FROM employees
WHERE age > 30 AND salary > 5000;
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Result

name age salary
Alice Johnson 40 7000
Bob Brown 55 8000

Explanation: The WHERE clause filters employees who are older than 30 and have a salary greater than 5000.


GROUP BY Clause

The GROUP BY clause is used to group rows that have the same values into summary rows, like finding the number of employees in each department.

SQL Query

SELECT department_id, COUNT(*) AS employee_count
FROM employees
GROUP BY department_id;
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Result

department_id employee_count
101 1
102 2
103 1

Explanation: The GROUP BY clause groups employees by department_id and counts the number of employees in each department.


HAVING Clause

The HAVING clause is used to filter groups created by the GROUP BY clause. It works like the WHERE clause but is used after aggregation.

SQL Query

SELECT department_id, AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id
HAVING AVG(salary) > 5500;
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Result

department_id avg_salary
102 5750
103 7000

Explanation: The HAVING clause filters the groups based on the average salary of employees in each department. Only departments with an average salary greater than 5500 are included.


ORDER BY Clause

The ORDER BY clause is used to sort the result set by one or more columns. By default, it sorts in ascending order; to sort in descending order, use DESC.

SQL Query (Ascending Order)

SELECT name, salary
FROM employees
ORDER BY salary;
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Result

name salary
John Smith 5000
Charlie Black 5500
Jane Doe 6000
Alice Johnson 7000
Bob Brown 8000

Explanation: The result is sorted by salary in ascending order.

SQL Query (Descending Order)

SELECT name, salary
FROM employees
ORDER BY salary DESC;
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Result

name salary
Bob Brown 8000
Alice Johnson 7000
Jane Doe 6000
Charlie Black 5500
John Smith 5000

Explanation: The result is sorted by salary in descending order.


LIMIT Clause

The LIMIT clause is used to specify the number of records to return from the result set. This is particularly useful for paging or limiting large result sets.

SQL Query

SELECT name, salary
FROM employees
ORDER BY salary DESC
LIMIT 3;
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Result

name salary
Bob Brown 8000
Alice Johnson 7000
Jane Doe 6000

Explanation: The LIMIT clause restricts the output to only the top 3 highest-paid employees.


DISTINCT Clause

The DISTINCT clause is used to return only distinct (different) values in a result set, removing duplicates.

SQL Query

SELECT DISTINCT department_id
FROM employees;
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Result

department_id
101
102
103

Explanation: The DISTINCT clause returns unique department_id values, eliminating duplicates.


AND, OR, NOT Operators

The AND, OR, and NOT operators are used to combine multiple conditions in the WHERE clause.

AND Operator

The AND operator is used to combine two or more conditions. The result will include only rows where all conditions are true.

SELECT name, age, salary
FROM employees
WHERE age > 30 AND salary > 5500;
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Result

name age salary
Alice Johnson 40 7000
Bob Brown 55 8000

Explanation: The WHERE clause filters rows where both conditions (age > 30 and salary > 5500) are true.

OR Operator

The OR operator is used when only one of the conditions must be true.

SELECT name, age, salary
FROM employees
WHERE age < 30 OR salary > 7000;
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Result

name age salary
Jane Doe 28 6000
Alice Johnson 40 7000
Bob Brown 55 8000

Explanation: The WHERE clause filters rows where either age < 30 or salary > 7000 is true.

NOT Operator

The NOT operator is used to exclude rows where a condition is true.

SELECT name, age, salary
FROM employees
WHERE NOT salary > 6000;
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Result

name age salary
John Smith 35 5000
Charlie Black 30 5500
Jane Doe 28 6000

Explanation: The WHERE clause filters rows where salary > 6000 is false, meaning it returns employees earning 6000 or less.


Conclusion

This blog explains how to filter, group and sort data using SQL’s WHERE, HAVING, ORDER BY, GROUP BY, and other clauses with real-life examples from the employees and departments tables. Understanding these clauses is fundamental for writing efficient SQL queries, analyzing data, and managing databases effectively.

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