[Solved] Oracle SQL select within select
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Oracle SQL select within select

Database type:

So I have reason identified as some kind of ID in table1. I need to write query which writes out the belonging meaning of a reason from table 2. I hope my bad english was enough to explain my problem.

table1

    id  reason
  --------------
    1     1
    2     2
    3     1

table2

domain         value  meaning
------------------------------
table1.reason    1    example1
table1.reason    2    example2

I tried this select but i got error "ORA-01427: single-row subquery returns more than one row" but if I add max() on t2.meaning it only returns me meaning example2 because max value = 2

select 
    t1.id
    (select t2.meaning
    from table2 t2, table1 t1
    where t2.value = t1.reason
    and t2.domain = 'table1.reason') as reason
from table1;

How can i solve this problem?

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
Optimal indexes for this query:
CREATE INDEX table1_idx_reason ON table1 (reason);
CREATE INDEX table2_idx_domain_value ON table2 (domain,value);
The optimized query:
SELECT
        t1.id(SELECT
            t2.meaning 
        FROM
            table2 t2,
            table1 t1 
        WHERE
            t2.value = t1.reason 
            AND t2.domain = 'table1.reason') AS reason 
    FROM
        table1

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* original question posted on StackOverflow here.