How to Efficiently Retrieve Multiple Related Records in SQL Databases?

How to Retrieve Multiple Records Related to a Single Record

In database management, it is often necessary to fetch data from multiple tables that are connected through relationships. This article demonstrates an efficient method for retrieving all information about a specific organization as well as the first names of its employees.

The concept of obtaining multiple related records requires the use of group concatenation, which is not standardized in SQL-92 or SQL-99. Thus, database-specific solutions are necessary.

MySQL

MySQL provides the GROUP_CONCAT function for group concatenation. To retrieve the desired data, execute the following query:

select 
  o.ID, o.Address, o.OtherDetails,
  GROUP_CONCAT( concat(e.firstname, ' ', e.lastname) ) as Employees
from 
  employees e 
  inner join organization o on o.org_id=e.org_id
group by o.org_id

PostgreSQL

PostgreSQL 9.0 introduced the STRING_AGG function:

select 
  o.ID, o.Address, o.OtherDetails,
  STRING_AGG( (e.firstname || ' ' || e.lastname), ', ' ) as Employees
from 
  employees e 
  inner join organization o on o.org_id=e.org_id
group by o.org_id

Oracle

Oracle provides LISTAGG for group concatenation:

select 
  o.ID, o.Address, o.OtherDetails,
  LISTAGG(e.firstname || ' ' || e.lastname, ', ' ) as Employees
from 
  employees e 
  inner join organization o on o.org_id=e.org_id
group by o.org_id

MS SQL Server

Similar to PostgreSQL, MS SQL Server uses STRING_AGG for group concatenation:

select 
  o.ID, o.Address, o.OtherDetails,
  STRING_AGG(e.firstname || ' ' || e.lastname, ', ' ) as Employees
from 
  employees e 
  inner join organization o on o.org_id=e.org_id
group by o.org_id

Fallback Solution

For older database versions or non-compliant databases, a fallback method exists:

1. Create a stored procedure that takes the organization ID as input and outputs the concatenated employee names.
2. Use this stored procedure in the query:

select 
  o.ID, o.Address, o.OtherDetails,
  MY_CUSTOM_GROUP_CONCAT_PROCEDURE( o.ID ) as Employees
from 
  organization o

By implementing these database-specific solutions or using the fallback method, you can retrieve the desired information efficiently and accurately.

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