Mysql trigger with joins - mysql

Question

I have three MYSQL tables that all relate to each other as you can see below.
I'm currently storing the dateofjob twice because i do not know how to create table joins in a trigger. Currently i'm using this trigger on the specimen details table
CREATE TRIGGER `NewAge_update` BEFORE UPDATE ON `Speciemn_Details`
FOR EACH ROW BEGIN
SET NEW.Due_Date = DATE_ADD(NEW.CastDate,INTERVAL NEW.Age DAY);
END
And this is working perfectly, but i want to be able to remove the column dateofjob from the specimen details table and calculate the duedate from the Job_Details table to save myself from having double data.
I will have to join the job_details.ID with the Test_Details.PARENTID and the Test_Details.TESTID with the specimen_details.TESTID, I just don't know how to do this inside a trigger. any help would be appreciated greatly
I've attempted to be as detailed as possible BUT If anymore information is required please ask me
Job_Details
+----+-----------+
| ID | dateofjob |
+----+-----------+
| 1 | 1/01/2015 |
| 2 | 1/01/2016 |
| 3 | 1/01/2017 |
+----+-----------+
Test_Details
+----------+--------+--------------+
| ParentID | TestID | TestLocation |
+----------+--------+--------------+
| 2 | 2154 | Barn |
| 2 | 2155 | Barn |
| 1 | 8055 | Yard |
+----------+--------+--------------+
Specimen_Details
+--------+----------+---------+-----------+-----------+
| TestID | Specimen | TestAge | dateofjob | Duedate |
+--------+----------+---------+-----------+-----------+
| 2154 | A | 3 | 1/01/2016 | 4/01/2016 |
| 2154 | B | 8 | 1/01/2016 | 9/01/2016 |
| 8055 | A | 2 | 1/01/2015 | 3/01/2015 |
+--------+----------+---------+-----------+-----------+

Answer 1

Joins inside trigger code really aren't any different from joins in any other procedural code, except that you can't query the triggering table and you instead have the OLD and NEW pseudorecords. If I've understood your description, you want to do something like
CREATE TRIGGER `NewAge_update` BEFORE UPDATE ON `Specimen_Details`
FOR EACH ROW
BEGIN
DECLARE jobdate DATE;
SELECT dateofjob
INTO jobdate
FROM job_details j, test_details t
WHERE j.ID = t.ParentID
AND t.TestID = NEW.TestID;
SET NEW.Duedate = DATE_ADD(jobdate, INTERVAL NEW.TestAge DAY);
END;
(Don't forget that trigger bodies are procedural, so you don't have to do everything in a single statement.)