Selecting records where all joined rows match - mysql

Question

Given the following schema:
employees
id | name
employee_attributes
id | employee_id | key | value
I would like to select all employees that have the provided attributes.
The following statement works:
SELECT employees.* FROM employees
INNER JOIN employee_attributes ON employee_attributes.employee_id = employees.id
WHERE employee_attributes.key = 'foo' AND employee_attributes.value = 'bar'
but only allows me to find an employee by one attribute. How can I adapt this to retrieve employees by more than one attribute?
To be clear, if I supply two sets of attributes to match against, the query should only return employees that have at least those two attributes.
For example, if Bob has just one attribute:
key | value
===========
foo | bar
But I supply two attributes to the query (foo and bar, bin and baz), Bob should not be returned.

Answer 1

Following should work:
SELECT employees.id, employees.name, count(employee_attributes.id) as attribute_count FROM employees
INNER JOIN employee_attributes ON employee_attributes.employee_id = employees.id
WHERE (employee_attributes.key = 'foo' AND employee_attributes.value = 'bar') OR (employee_attributes.key = 'bin' AND employee_attributes.value = 'baz')
group by employees.id, employees.name
having attribute_count >= 2;

Answer 2

You can get the employee ids using aggregation:
SELECT ea.employee_id
FROM employee_attributes.employee_id
WHERE (ea.key = 'foo' AND ea.value = 'bar') OR
(ea.key = 'bin' AND ea.value = 'baz')
GROUP BY ea.employee_id
HAVING COUNT(DISTINCT ea.key) = 2;
For the full information, you can use a JOIN:
SELECT e.*
FROM employee e JOIN
(SELECT ea.employee_id
FROM employee_attributes.employee_id
WHERE (ea.key = 'foo' AND ea.value = 'bar') OR
(ea.key = 'bin' AND ea.value = 'baz')
GROUP BY ea.employee_id
HAVING COUNT(DISTINCT ea.key) = 2
) ea
ON ea.employee_id = e.id;

Answer 3

Use conditional aggregation:
SELECT employees.*
FROM employees
INNER JOIN employee_attributes
ON employee_attributes.employee_id = employees.id
GROUP BY employee_attributes.employee_id
HAVING SUM(CASE WHEN employee_attributes.key = 'foo' AND
employee_attributes.value = 'bar' THEN 1 ELSE 0 END) > 0 AND
SUM(CASE WHEN employee_attributes.key = 'bin' AND
employee_attributes.value = 'baz' THEN 1 ELSE 0 END) > 0