Question

Slightly unusual requirement here, which unfortunately is down to a poor table design a long way back down the development path!

I have 3 tables, repairs, staff, technicians

The repairs table contains all the information on repair tasks booked in to my system. This contains a field “Technician” this field will contain the ID of either a staff member (from the table staff) or an ID of an outsourced (offsite) technician (from the table technicians), in this latter case the ID will have a prefix of “T”

So, due to this latter case prefix, I need to be able to grab that T and use it to determine whether my SQL query needs to JOIN table staff or table technicians

So, I have a fairly simple SQL Query:

SELECT technicians.screenName,

    repairs.turnAround, repairs.technician, repairs.dateIn, repairs.Type, 
    invoices.status, invoices.grossTotal 

    FROM repairs 
    LEFT JOIN invoices ON repairs.invNo=invoices.id 
    LEFT JOIN technicians ON technicians.id = REPLACE(repairs.technician, 'T','')

WHERE repairs.id ='REQUIRED JOB ID' ORDER BY repairs.dateIn DESC

This will work fine, and overcomes the “T” prefix for all cases where I have an outsourced technician.

BUT…

The IDs will mix up if I try to JOIN the staff table.

So I need a conditional join, such as:

  LEFT JOIN
      WHEN instr(repairs.technican,'T') > 0 THEN 
          JOIN TECHNICIANS TABLE
      ELSE
          JOIN STAFF TABLE
      END 

The further issue I can see here is that the field technicans.screenName being included in the field list will not work if I am not joining the technicians table, however, as the staff table includes a field screenName which I’d need if I joined that table, if I had an ambiguous field name screenName with no table prefix it should work shouldn’t it?

EDIT: I should probably add that the conditional join example above does NOT work!

Answer 1

You can join both tables with including/excluding INSTR condition, and show not null value then:

SELECT COALESCE(technicians.screenName, staff.screenName) AS screenName,

repairs.turnAround, repairs.technician, repairs.dateIn, repairs.Type, 
invoices.status, invoices.grossTotal 

FROM repairs 
LEFT JOIN invoices ON repairs.invNo=invoices.id 
LEFT JOIN technicians ON instr(repairs.technican,'T') > 0 AND technicians.id = REPLACE(repairs.technician, 'T','')
LEFT JOIN staff ON instr(repairs.technican,'T') = 0 AND staff.id = repairs.technician

WHERE repairs.id ='REQUIRED JOB ID' ORDER BY repairs.dateIn DESC