MySQL pivot table and JOINS - mysql

Question

I have a mysql db with three tables
student
student_intervention
intervention details
I'm trying to do a pivot table view that shows all the students and has columns for each intervention type totalling up the different types of intervention for each student.
So far I have
SELECT t.`first_name`, t.`last_name`, t.`student_id`,
Count(IF(t.`intervention_details_id` = 1, 1, null)) AS Intervention1,
Count(IF(t.`intervention_details_id` = 0, 1, null)) AS Intervention2
FROM (
SELECT student.`student_id`, student.`first_name`,
student.`last_name`,
`student_intervention`.`intervention_details_id`
FROM student, student_intervention
WHERE student_intervention.student_id = student.`student_id`
) t
GROUP BY t.student_id
This works but it only shows data for students who have an intervention. I want a full list of students including those without an intervention. I think I need a JOIN but cannot figure out the right one.
Can anyone help?

Answer 1

use LEFT JOIN instead
SELECT a.`student_id`,
a.`first_name`,
a.`last_name`,
SUM(IF(COALESCE(b.`intervention_details_id`,0) = 1, 1, 0)) Intervention1,
SUM(IF(COALESCE(b.`intervention_details_id`,0) = 0, 1, 0)) Intervention2
FROM student a
LEFT JOIN student_intervention b
ON b.student_id = a.`student_id`
GROUP BY a.`student_id`, a.`first_name`, a.`last_name`
if you want prepared statement
SET #sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'SUM(CASE WHEN COALESCE(b.intervention_details_id ,0) = ',
COALESCE(b.intervention_details_id ,0),
' THEN 1 ELSE 0 END) AS ',
COALESCE(b.intervention_details_id ,0)
)
) INTO #sql
FROM student a
LEFT JOIN student_intervention b
ON b.student_id = a.student_id;
SET #sql = CONCAT('SELECT a.student_id , a.first_name , a.last_name , ', #sql, '
FROM student a
LEFT JOIN student_intervention b
ON b.student_id = a.student_id
GROUP BY a.student_id , a.first_name , a.last_name');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;