How to efficiently retrieve data in one to many relationships

Question

I am running into an issue where I have a need to run a Query which should get some rows from a main table, and have an indicator if the key of the main table exists in a subtable (relation one to many).

The query might be something like this:

select a.index, (select count(1) from second_table b where a.index = b.index) 
from first_table a;

This way I would get the result I want (0 = no depending records in second_table, else there are), but I'm running a subquery for each record I get from the database. I need to get such an indicator for at least three similar tables, and the main query is already some inner join between at least two tables...

My question is if there is some really efficient way to handle this. I have thought of keeping record in a new column the "first_table", but the dbadmin don't allow triggers and keeping track of it by code is too risky.

What would be a nice approach to solve this?

The application of this query will be for two things:

1. Indicate that at least one row in second_table exists for a given row in first_table. It is to indicate it in a list. If no row in the second table exists, I won't turn on this indicator.
2. To search for all rows in first_table which have at least one row in second_table, or which don't have rows in the second table.

Another option I just found:

select a.index, b.index 
from first_table a 
left join (select distinct(index) as index from second_table) b on a.index = b.index

This way I will get null for b.index if it doesn' exist (display can finally be adapted, I'm concerned on query performance here).

The final objective of this question is to find a proper design approach for this kind of case. It happens often, a real application culd be a POS system to show all clients and have one icon in the list as an indicator wether the client has open orders.

Answer 1

Try using EXISTS, I suppose, for such case it might be better then joining tables. On my oracle db it's giving slightly better execution time then the sample query, but this may be db-specific.

SELECT first_table.ID, CASE WHEN EXISTS (SELECT * FROM second_table WHERE first_table.ID = second_table.ID) THEN 1 ELSE 0 END FROM first_table