Order Number | Payment Type Type | Payment Type Key
    1              PaymentType1           5
    2              PaymentType2           5
    3              PaymentType1           5

SELECT
    o.Number,
    COALESCE(pm1.key, pm2.key),
    CASE WHEN o.PaymentTypeType = "PaymentType1"
        THEN pm1.pm1_attr1
        ELSE pm2.pm2.attr3
        END as "Friendly Attribute Name"
FROM ORDER o
   LEFT OUTER JOIN PaymentMethod1 pm1 ON
       o.PaymentTypeType = "PaymentType1" AND
       o.PaymentTypeKey = pm1.key
   LEFT OUTER JOIN PaymentMethod2 pm2 ON
       o.PaymentTypeType = "PaymentType2" AND
       o.PaymentTypeKey = pm2.key