Conditional inner join statements in MySQL

Question

Is there a way I can conditionally change which table i'm inner joining on based on the value of a field in another table? Here's what I got so far (but it errors) out:

SELECT 
    j.jobID, j.jobNumber,
    CASE
        WHEN j.idType = 'dealership' THEN d.dealershipName
        WHEN j.idType = 'Group' THEN g.groupName
        WHEN j.idType = 'Agency' then a.agencyName
    END as dealershipName,
    CASE
        WHEN p.manualTimestamp != '0000-00-00 00:00:00' THEN UNIX_TIMESTAMP(p.manualTimestamp)
        WHEN p.manualTimestamp = '0000-00-00 00:00:00' THEN p.timestamp
    END as checkTS,
    CONCAT_WS(' ', ui.fName, ui.lName) as salesRep
FROM jobs j
LEFT JOIN dealerships d ON j.dealershipID = d.dealershipID
LEFT JOIN dealershipgroups g ON j.dealershipID = g.groupID
LEFT JOIN agencies a ON j.dealershipID = a.agencyID
INNER JOIN payments p ON j.jobID = p.jobID
IF j.idType = 'dealership' THEN 
    INNER JOIN smdealershipjoins smdj ON j.dealershipID = smdj.dealership
    INNER JOIN userinfo ui ON smdj.sm = ui.userID           
ELSEIF j.idType = 'Group' THEN 
    INNER JOIN smgroupjoins gj ON j.dealershipID = gj.groupID
    INNER JOIN userinfo ui ON gj.sm = ui.userID
ELSEIF j.idType = 'Agency' THEN 
    INNER JOIN smagencyjoins aj ON j.dealershipID = aj.agencyID
    INNER JOIN userinfo ui on aj.sm = ui.userID
END IF

So, there's a table (jobs) that has and idType (dealership, group, or agency) and a clientID (called dealershipID). What I need to do it join a certain table (based on idType) to determine which sales manager "owns" the client account. If idType = 'dealership' I need to join smdealershipjoins, if it's 'Group' i need to join smgroupjoins, and if it 'agency' I need to join smagencyjoins so that I can then join that table to userinfo in order to get the sales manager's name.

I've also tried to use case statements to do the inner joining, but that gave an error too.

Answer 1

Left / outer join to each of the three options and the conditional in your select will take care of the rest.

Perhaps something like:

...
LEFT JOIN smdealershipjoins smdj 
    ON j.dealershipID = smdj.dealership
LEFT JOIN smgroupjoins gj 
    ON j.dealershipID = gj.groupID
LEFT JOIN smagencyjoins aj 
    ON j.dealershipID = aj.agencyID
LEFT JOIN userinfo ui 
    ON ui.userID = CASE
        WHEN j.idType = 'dealership' THEN smdj.sm
        WHEN j.idType = 'Group' THEN gj.sm
        WHEN j.idType = 'Agency' then aj.sm
    END