How to Efficiently Count Results from Multiple Database Joins?

How to Get Counts from Multiple Joins with Optimal Efficiency

In database queries, minimizing the number of joins is crucial for performance. However, there may be scenarios where it's tempting to perform multiple joins at once to obtain multiple counts. Let's explore an example and discuss optimization strategies.

Problem Statement

The task is to count the rows returned by multiple joins on the same primary key. A straightforward approach involves creating individual subqueries for each join. However, the question arises: is there a more efficient way to achieve the same result using a single query?

Initial Attempt

One approach is to attempt to count the results of multiple joins using case statements, as demonstrated below:

select alb.titreAlb as "Titre",
sum(case when alb.idAlb=payalb.idAlb then 1 else 0 end) "Pays",
sum(case when alb.idAlb=peralb.idAlb then 1 else 0 end) "Personnages",
sum(case when alb.idAlb=juralb.idAlb then 1 else 0 end) "Jurons"
from album alb
left join pays_album payalb using ( idAlb )
left join pers_album peralb using ( idAlb )
left join juron_album juralb using ( idAlb )
where alb.titreAlb = "LES CIGARES DU PHARAON"
group by alb.titreAlb

However, this method counts the total number of rows across all joined tables, resulting in incorrect results.

Optimization Solution

To optimize the query and obtain separate counts for each join, you can leverage DISTINCT in a single query. A unique key is required for each table involved, and the idalb column must be a unique key for the album table. The revised query is as follows:

select alb.titreAlb as "Titre",
count(distinct payalb.idAlb, payalb.PrimaryKeyFields) "Pays",
count(distinct peralb.idAlb, peralb.PrimaryKeyFields) "Personnages",
count(distinct juralb.idAlb, juralb.PrimaryKeyFields) "Jurons"
from album alb
left join pays_album payalb using ( idAlb )
left join pers_album peralb using ( idAlb )
left join juron_album juralb using ( idAlb )
where alb.titreAlb = "LES CIGARES DU PHARAON"
group by alb.titreAlb

In this query, distinct removes the effects of other joins on the count. However, it's important to remember that distinct doesn't necessarily eliminate the cost of the joins. If covering indexes are available for all (idAlb PrimaryKeyFields)-fields of the tables, this approach might yield comparable speed to the original solution, where each join was performed separately. Nonetheless, testing with EXPLAIN can help determine the optimal strategy.

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