複数テーブルの結合について

マスタ1とマスタ2をLEFT JOINしてNULLにならないフィールドの方をIFで使う、っていうのでどうですか？

こんな感じでしょうか？

create table tbl1(id int,item_code varchar(20),name varchar(20));
insert into tbl1 values
('1','a123','りんご'),
(2,'b123','白菜'),
(3,'c123','みかん'),
(4,'d123','青葉'),
(5,'e123','メロン');

create table tbl2(id int,item_code varchar(20),name varchar(20),makercode varchar(20),subcode varchar(20));
insert into tbl2 values
(1,'a123','りんご','aaa','aa1'),
(2,'b123','白菜','aaa','ds2'),
(3,'c123','みかん','bbb','ed3'),
(4,'d123','青葉','ccc','vf4');

create table tbl3(id int,item_code varchar(20),name varchar(20),makercode varchar(20),lavel varchar(20));
insert into tbl3 values
(1,'d123','青葉','ccc','wnsd'),
(2,'e123','メロン','ccc','asde');

select t1.id,t1.item_code,t1.name
,coalesce(t2.makercode,t3.makercode) as makercode
from tbl1 as t1
left join tbl2 as t2 on t1.item_code=t2.item_code
left join tbl3 as t3 on t1.item_code=t3.item_code

UNION ALLでFULL JOINのまねをしてみました。

SELECT * FROM `買い物かご` t1
JOIN (
    SELECT coalesce(m1.id,m2.id) id, coalesce(m1.item_code,m2.item_code) item_code, coalesce(m1.name,m2.name) name,
    coalesce(m1.makercode,m2.makercode) makercode, coalesce(subcode,label) subcode
    FROM
    `マスタ１` AS m1
        LEFT OUTER JOIN
    `マスタ２` AS m2 ON m1.item_code = m2.item_code
    UNION ALL
    SELECT coalesce(m1.id,m2.id), coalesce(m1.item_code,m2.item_code), coalesce(m1.name,m2.name),
    coalesce(m1.makercode,m2.makercode), coalesce(subcode,label)
    FROM
    `マスタ１` AS m1
        RIGHT OUTER JOIN
    `マスタ２` AS m2 ON m1.item_code = m2.item_code
    WHERE
    m1.item_code IS NULL
) m3
ON t1.item_code=m3.item_code
;


id item_code name    id      item_code name makercode subcode
4       d123    青葉    4       d123    青葉    ccc     vf4
1       a123    りんご  1       a123    りんご  aaa     aa1
2       b123    白菜    2       b123    白菜    aaa     ds2
3       c123    みかん  3       c123    みかん  bbb     ed3
5       e123    メロン  2       e123    メロン  ccci    asde