求一个比较难的sql语句

[qq_38741240]

有下面A ，B，C三个表

A表
id,itemId,keyWords

b表
id,itemId,data

c表（ c表table_from字段记录了数据来源对应关系，通过的是itemId做关联，如table_from为1代表的是a表，2代表了b表）
id,table_from,itemId

现在需求是 做链接查询，以c表为准做左链接（left join），c表的itemId要等于A表或者B表的itemId
如果c表的table_from字段为1，那么就是查询a表和c表，且c表的itemId等于a表的itemId.
如果c表的table_from字段为2，那么就是查询b表和c表，且c表的itemId等于b表的itemId
一共需要查询20条数据且只能用一条sql实现（数据库是mysql，没有外键等之类的关联，都是独立表）

[陶太郎]

引用 1 楼 hongmei85 的回复:

没验证过，试下 select c.id, c.table_from, c.itemId, a.keyWords as kkk from c left join a on c.itemId=a.itemId where c.table_from=1 union select c.id, c.table_from, c.itemId, b.data as kkk from c left join b on c.itemId=b.itemId where c.table_from=2

没毛病 我验证过了

[hongmei85]

没验证过，试下 select c.id, c.table_from, c.itemId, a.keyWords as kkk from c left join a on c.itemId=a.itemId where c.table_from=1 union select c.id, c.table_from, c.itemId, b.data as kkk from c left join b on c.itemId=b.itemId where c.table_from=2

[cnaaa8]

select c.id, c.table_from, c.itemId, a.keyWords as kkk from c left join a on c.itemId=a.itemId where c.table_from=1 union select c.id, c.table_from, c.itemId, b.data as kkk from c left join b on c.itemId=b.itemId where c.table_from=2