How to Retrieve Related Records from Multiple Database Tables in a Single Query?

Retrieving Multiple Records Based on Relations

When dealing with database tables with relationships, a common task is to retrieve information from multiple tables based on a specific relation. For instance, consider two tables: Organization and Employee, where one Organization can have multiple Employees. The goal is to retrieve all information about a particular organization, including the first name of all its employees, in a single record set.

Database-Specific Solutions

The appropriate approach for this task varies depending on the database system being used. Some popular database systems offer specific functions or features that can facilitate Group Concatenation, the process of combining multiple values into a single string:

• MySQL: Use the GROUP_CONCAT function to concatenate the first names of employees based on the Organization's ID.

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 provides the STRING_AGG function for Group Concatenation, which can be used to append the employees' first names with a comma-space delimiter.

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 offers the LISTAGG function for concatenating values.

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: MS SQL Server 2017 and later introduced the STRING_AGG function 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 Solutions

In database systems that lack native Group Concatenation functions or for older versions of supported databases, fallback solutions can be employed:

• Create a stored procedure that accepts the Organization's ID and returns the concatenated employee names. Integrate this stored procedure into the query.

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

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