How to Efficiently Concatenate Data from One-to-Many Relationships in SQL?

Mastering Group Concatenation for One-to-Many Relationships in SQL

In relational databases, it is often necessary to retrieve information from tables that have one-to-many relationships. A common scenario is to gather data about a parent record and the associated child records.

Consider the example of an Organization table and an Employee table, where each organization can have multiple employees. To retrieve all information about a particular organization, along with the first names of all its employees, we explore different approaches.

Standard SQL Techniques

Unfortunately, standard SQL-92 and SQL-99 do not include built-in functions for group concatenation. Vendor-specific solutions are required.

MySQL

MySQL offers the GROUP_CONCAT function:

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 versions 9.0 and later provide 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 utilizes the LISTAGG function:

SELECT
  o.ID,
  o.Address,
  o.OtherDetails,
  LISTAGG(e.FirstName || ' ' || e.LastName, ', ') WITHIN GROUP (ORDER BY e.FirstName) AS Employees
FROM
  Employees e
INNER JOIN
  Organization o
ON
  o.Org_ID = e.Org_ID
GROUP BY
  o.Org_ID;

Fallback Solution

If vendor-specific functions are unavailable, stored procedures can be created to perform the concatenation.

Stored Procedure Example (Generic)

CREATE PROCEDURE MY_CUSTOM_GROUP_CONCAT_PROCEDURE
(
  @Org_ID INT
)
AS
BEGIN
  DECLARE @Employees NVARCHAR(MAX) = '';
  SELECT @Employees = @Employees + FirstName + ' ' + LastName + ', '
  FROM Employees
  WHERE Org_ID = @Org_ID;
  
  SELECT @Employees = LEFT(@Employees, LEN(@Employees) - 2);
  RETURN @Employees;
END
GO;

Usage

SELECT
  o.ID,
  o.Address,
  o.OtherDetails,
  MY_CUSTOM_GROUP_CONCAT_PROCEDURE(o.ID) AS Employees
FROM
  Organization o;

By leveraging these techniques, developers can efficiently retrieve multi-record information associated with one record, providing a holistic view of data in one-to-many relationships.

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