SQL過濾和分類現實示例

發佈於2025-03-25

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.

Tables Structure
WHERE Clause
GROUP BY Clause
HAVING Clause
ORDER BY Clause
LIMIT Clause
DISTINCT Clause
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;

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;

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;

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;

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;

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;

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;

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;

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;

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  7000;

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 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;

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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