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

Retrieving Related Records in a Single Query

In the realm of relational databases, it's often necessary to query multiple records from different tables that share a relationship. A common scenario is retrieving all the information about a specific organization along with the first names of all its employees.

To achieve this, we can leverage various database-specific techniques, as described below:

MySQL and PostgreSQL:

Utilizing the built-in GROUP_CONCAT function in MySQL or string_agg() in PostgreSQL, we can aggregate all the first names of employees related to a given organization and combine them into a single string:

SELECT
  o.ID,
  o.Address,
  o.OtherDetails,
  GROUP_CONCAT(e.firstname) AS Employees
FROM organization AS o
JOIN employee AS e
  ON o.org_id = e.org_id
GROUP BY
  o.org_id;

PostgreSQL 9.0 and Later:

PostgreSQL 9.0 and later introduced the STRING_AGG function, which allows for more flexibility in concatenation:

SELECT
  o.ID,
  o.Address,
  o.OtherDetails,
  STRING_AGG(e.firstname || ' ' || e.lastname, ', ') AS Employees
FROM organization AS o
JOIN employee AS e
  ON o.org_id = e.org_id
GROUP BY
  o.org_id;

Oracle:

Oracle provides the LISTAGG function for this purpose:

SELECT
  o.ID,
  o.Address,
  o.OtherDetails,
  LISTAGG(e.firstname, ', ') AS Employees
FROM organization AS o
JOIN employee AS e
  ON o.org_id = e.org_id
GROUP BY
  o.org_id;

MS SQL Server:

MS SQL Server offers the STRING_AGG function:

SELECT
  o.ID,
  o.Address,
  o.OtherDetails,
  STRING_AGG(e.firstname, ', ') AS Employees
FROM organization AS o
JOIN employee AS e
  ON o.org_id = e.org_id
GROUP BY
  o.org_id;

Fallback Solution for Other Databases:

If your database does not support any of these built-in functions, you can opt for a fallback solution by creating a stored procedure that takes the organization ID as an input and concatenates the employee names accordingly:

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

By employing these techniques, you can efficiently retrieve multiple records from related tables and present them in a consolidated form, eliminating the need for multiple queries or row-by-row data assembly.

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