This article will provide an in-depth explanation of WHERE
, HAVING
, ORDER BY
, GROUP BY
and other clauses and related operators in SQL through employee and department table cases, helping you master SQL data filtering and sorting skills.
Table of contents
- Table structure
-
WHERE
clause -
GROUP BY
clause -
HAVING
clause -
ORDER BY
clause -
LIMIT
clause -
DISTINCT
clause -
AND
,OR
,NOT
operators
Table structure
Employee List
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 |
Department list
dept_id | Dept_name |
---|---|
101 | hr |
102 | it |
103 | finance |
104 | Marketing |
WHERE
clause
The WHERE
clause is used to filter records that meet certain criteria.
SQL Query:
<code class="sql">SELECT name, age, salary FROM employees WHERE age > 30;</code>
result:
name | age | Salary |
---|---|---|
john smith | 35 | 5000 |
alice johnson | 40 | 7000 |
bob brown | 55 | 8000 |
Description: Filter out information of employees older than 30 years old.
Example of AND
operator:
<code class="sql">SELECT name, age, salary FROM employees WHERE age > 30 AND salary > 5000;</code>
result:
name | age | Salary |
---|---|---|
alice johnson | 40 | 7000 |
bob brown | 55 | 8000 |
Description: Filter information of employees older than 30 years old and salary greater than 5,000.
GROUP BY
clause
GROUP BY
clause is used to group records with the same value and is often used in statistical aggregation operations.
SQL Query:
<code class="sql">SELECT department_id, COUNT(*) AS employee_count FROM employees GROUP BY department_id;</code>
result:
department_id | Employee_count |
---|---|
101 | 1 |
102 | 2 |
103 | 1 |
Description: Grouped by department ID and count the number of employees in each department.
HAVING
clause
The HAVING
clause is used to filter the results after GROUP BY
grouping.
SQL Query:
<code class="sql">SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id HAVING AVG(salary) > 5500;</code>
result:
department_id | avg_salary |
---|---|
102 | 5750 |
103 | 7000 |
Description: Filter out departments with an average salary of more than 5,500.
ORDER BY
clause
ORDER BY
clause is used to sort the result set.
SQL query (ascending order):
<code class="sql">SELECT name, salary FROM employees ORDER BY salary;</code>
result:
name | Salary |
---|---|
john smith | 5000 |
charlie black | 5500 |
jane doe | 6000 |
alice johnson | 7000 |
bob brown | 8000 |
SQL query (descending order):
<code class="sql">SELECT name, salary FROM employees ORDER BY salary DESC;</code>
result:
name | Salary |
---|---|
bob brown | 8000 |
alice johnson | 7000 |
jane doe | 6000 |
charlie black | 5500 |
john smith | 5000 |
LIMIT
clause
The LIMIT
clause is used to limit the number of records returned.
SQL Query:
<code class="sql">SELECT name, salary FROM employees ORDER BY salary DESC LIMIT 3;</code>
result:
name | Salary |
---|---|
bob brown | 8000 |
alice johnson | 7000 |
jane doe | 6000 |
Description: Only the information of the top 3 employees is returned.
DISTINCT
clause
The DISTINCT
clause is used to remove duplicate values.
SQL Query:
<code class="sql">SELECT DISTINCT department_id FROM employees;</code>
result:
department_id |
---|
101 |
102 |
103 |
Description: Returns the unique department ID.
AND
, OR
, NOT
operators
These operators are used to combine multiple conditions.
(Omit repeated And, Or, Not examples, keep the space simple)
in conclusion
Through actual cases, this article explains in detail the key clauses and operators used in SQL for data filtering and sorting. Proficient in this knowledge will effectively improve your SQL query efficiency and data analysis capabilities.
The above is the detailed content of SQL Filtering and Sorting with Real Life Examples. For more information, please follow other related articles on the PHP Chinese website!

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