SELECT based on foreign key, to change reference - mysql

Question

I have 2 tables: Equipment, and Equipment_Type:
+-------------------------------+
| EqId [PK] | ETId [FK] | EqNum |
+-------------------------------+
| 1 | 1 | ABC |
| 2 | 1 | DEF |
| 3 | 3 | GHI |
+-------------------------------+
| ETId [PK] | Code | Discipline |
+-------------------------------+
| 1 | MOT | ELEC |
| 2 | MOT | MECH |
| 3 | SW | ELEC |
So from this example, we can see that both of our equipment are electrical motors.
However, due to a misunderstanding in the initial population, all of the equipment types were identified a ELEC disciplines. Since then, the MECH equipment has been identified, and I have to find all of the equipment that has been duplicated in the Equipment_Type table, and change them to reference the MECH equipment types instead.
I tried this:
SELECT * FROM Equipment EQ
INNER JOIN Equipment_Type ET on ET.ETId = EQ.ETId
WHERE ET.Discipline = 'MECH';
Which (obviously) returns no results - as with all the other JOIN queries.
What I want to achieve is a search that will select only the Equipment that has an ELEC Equipment Type that is also a MECH equipment type. I realise this requires a nested query, but I'm not sure where to place it.
So the search should return:
+---------------------------+
| EqNum | ETId | Discipline |
+---------------------------+
| DEF | 1 | ELEC |
Because that entry needs to be changed to the MECH discipline (i.e. ETId = 2 instead of 1)

Answer 1

Here is one method that aggregates the types to get the codes that have both disciplines:
select e.*
from equipment e join
equipment_type et
on e.etid = et.etid join
(select et.code
from equipment_type et
group by et.code
having sum(discipline = 'MECH') > 0 and sum(discipline = 'ELEC') > 0
) ett
on ett.code = et.code;
Another method would use two joins:
select e.*
from equipment e join
equipment_type ete
on e.etid = ete.etid and ete.discipline = 'ELEC' join
equipement_type etm
on ete.code = etm.code and etm.discipline = 'MECH';
This version might be faster with the right indexes.

Answer 2

Do it like this:
select eq_id, eq_name,et_id,description from
(select eq_id,eq_name from equipment) as a
left join
(select et_id,description from equipment_type) as b
on a.eq_id = b.et_id where description = 'ELEC';
IF YOU WANT TO INCLUDE 'MECH';
select eq_id, eq_name,et_id,description from
(select eq_id,eq_name from equipment) as a
left join
(select et_id,description from equipment_type) as b
on a.eq_id = b.et_id where description = 'ELEC' or description = 'MECH';
CHANGE 'ELEC' TO 'MECH':
select eq_id, eq_name,et_id,
case when description = 'ELEC' then 'MECH' else description end as description,
'MECH' as MECH_FIELD from
(select eq_id,eq_name from equipment) as a
left join
(select et_id,description from equipment_type) as b
on a.eq_id = b.et_id where description = 'ELEC' or description = 'MECH';