Question

Can you let me know if my interpretation is correct (the last AND part)?

$q = "SELECT title,name,company,address1,address2 
      FROM registrations 
      WHERE title != 0 AND id IN (
          SELECT registrar_id 
          FROM registrations_industry 
          WHERE industry_id = '$industryid'
      )";

Below was really where I am not sure:

... AND id IN (select registrar_id from registrations_industry where industry_id='$industryid')

Interpretation: Get any match on id(registrations id field) equals registrar_id(field) from the join table registrations_industry where industry_id equals the set $industryid

Is this select statement considered a sub routine since it’s a query within the main query?

So an example would be with the register table id search to 23 would look like:

registrations(table)

id=23,title=owner,name=mike,company=nono,address1=1234 s walker lane,address2

registrations_industry(table)

id=256, registrar_id=23, industry_id=400<br>
id=159, registrar_id=23, industry_id=284<br>
id=227, registrar_id=23, industry_id=357

I assume this would return 3 records with the same registration table data And of course varying registrations_industry returns.

Answer 1

For a given test data set your query will return one record. This one:
id=23,title=owner,name=mike,company=nono,address1=1234 s walker lane,address2
To get three records with the same registration table data and varying registrations_industry you need to use JOIN.

Something like this:

SELECT r.title, r.name, r.company, r.address1, r.address2 
FROM registrations AS r
LEFT OUTER JOIN registrations_industry AS ri
ON ri.registrar_id=r.id
WHERE r.title!=0 AND ri.industry_id={$industry_id}