Page 482 - Computer_Science_F5

(b) JOIN
The ‘JOIN’ clause merges rows from multiple tables by using a common column
that links them. For example,
SELECT employees.name, departments.department_name
FROM employees Chapter Eight: Databases and Database Management Systems
FOR ONLINE READING ONLY
JOIN departments ON employees.department_id = departments.department_id;

(c) WHERE
The ‘WHERE’ clause is used to filter records. For example,

SELECT name, age
FROM employees
WHERE age > 50;

(d) ORDER BY
The ‘ORDER BY’ clause sorts the result set in either ascending or descending order.
For example,
SELECT name, salary
FROM employees
ORDER BY salary ASC;

(e) GROUP BY
The ‘GROUP BY’ clause organises rows with identical values in specified columns
into summary rows. For example,
SELECT department_id, COUNT(*)
FROM employees
GROUP BY department_id;

COUNT(*) in this statement, tallies the number of records associated with each
unique department_id in the employees table, summarizing the total rows per
department_id group. This query outputs a list of department_id values alongside
the corresponding count of employees within each department.

(f) HAVING
The ‘HAVING’ clause was introduced in SQL to enable filtering based on aggregate
functions, which the ‘WHERE’ keyword cannot do. For example,
SELECT department_id, COUNT(*)
FROM employees

GROUP BY department_id
HAVING COUNT(*) > 10;

473
Student’s Book Form Five

Computer Science Form 5.indd 473 23/07/2024 12:35