如何使用MYSQL创建3向左联接？

Question

I'm attempting to create a 3-way LEFT JOIN using MYSQL and I'm having difficulty accomplishing it so I figured this would be the place to figure it out.

I have a three tables as I'll display below. The first contains a list of items that a user has added to their queue to be processed by the game. The other two contain details about each of the items such as their strengths, points to completion, etc. In the queued table, I have two types of items, units and research. The unit details are found in table 2 and the research details are found in table 3.

Table 1: The first table (core_queued_units) contains the following fields: id, unit_id, name, location, class(unit or research),sort.

Table 2: The second table (core_available_units) contains the following fields: id, name, description, etc.

Table 3: The third table (core_available_tech) contains the following fields: id, name, description, etc.

FROM core_queued_units
LEFT JOIN core_available_units
ON core_queued_units.unit_id = core_available_units.id
AND core_queued_units.class='Unit'
LEFT JOIN core_available_tech
ON core_queued_units.unit_id = core_available_tech.id
AND core_queued_units.class='Research'
WHERE core_queued_units.location = '1'
AND core_queued_units.user_id ='".$GLOBALS['self']['usrID']."'
ORDER BY core_queued_units.sort ASC

Answer 1

If fields in core_available_units and core_available_tech are equal, you can try this:

SELECT *
  FROM core_queued_units cq
  LEFT JOIN (
     select cau.*, 'Unit' class from core_available_units cau
      union all
     select cat.*, 'Research' class from core_available_tech cat) c
    ON cq.unit_id = c.unit_id and c.class = cq.class
WHERE cq.location = '1'
  AND cq.user_id ='".$GLOBALS['self']['usrID']."'
ORDER BY cq.sort ASC

Answer 2

If you are trying to join multiple tables, you can do as follows. I am taking Table1 as base table.

SELECT columnname1, columnname2......
FROM Table1 
LEFT OUTER JOIN Table2 ON Table1.PK = Table2.FK (here Table2.FK is the Primary key of Table1 which is used as Foreign key in Table2)
LEFT OUTER JOIN Table3 ON Table1.PK = Table3.FK (same condition as above) 
WHERE insert your conditions here 

using this sample, columns from Table2 & Table3 will added at the right most side of Table1. Please inform if this helps.

Answer 3

Can you try to swap the arguments in the On-clause

core_queued_units.unit_id = core_available_tech.id ?