使用来自2个数据库的表进行查询

Question

I have 3 tables, t1,t2, and t3. ti and t2 are in databases D1 . But table t3 is contained in both databases D1 AND D2 like some 5 records are in D1,t3 and rest 5 in D2.t3.

the full table t3 is actually combined of records of the same table (ie, t3) from D1 AND D2

the query should be like;

select t1.*,t2.* from D1.t1,D1.t2 where t1.aid=t3.cid and t3.id IN(1,2,4) and  t1.aid=t2.bid

the problem is how can i provide 't3.cid and t3.id IN(1,2,4)' IN THE QUERY since they are from different databases

Answer 1

MySQL databases are mostly for organization and permission sharding. There is very little overhead joining tables from multiple databases into a single query.

SELECT *
    FROM D1.t1 as d1t1
    INNER JOIN D1.t2 as d1t2 ON d1t1.aid = d1t2.bid
    INNER JOIN D2.t3 as d2t3 ON <your join condition here>
    INNER JOIN D1.t3 as d1t3 ON <another join condition>
WHERE
    d1t3.id IN (1,2,3)

You didn't specify the join condition in the original question but this query should get you started.

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After reading the comments, what you need is a UNION:

(SELECT t3.* FROM D1.t3) UNION (SELCT t3.* FROM D2.t3)

You will need to repeat the join conditions above in each UNION clause.

You should probably look at your database structure, though. The UNION will give you the data you need, but at the expense of performance. If you have two tables with identical structures, you should see if there is a way to migrate your data into a single table.