mysql - How to get multiple records against one record based on relation?

Question

I have two tables Organisation and Employee having one to many relation i.e one organisation can have multiple employees. Now I want to select all information of a particular organisation plus first name of all employees for this organisation. What’s the best way to do it? Can I get all of this in single record set or I will have to get multiple rows based on no. of employees? Here is a bit graphical demonstration of what I want:

Org_ID      Org_Address    Org_OtherDetails    Employess

1           132A B Road    List of details     Emp1, Emp2, Emp3.....

Answer 1

The original question was database specific, but perhaps this is a good place to include a more generic answer. It's a common question. The concept that you are describing is often referred to as 'Group Concatenation'. There's no standard solution in SQL-92 or SQL-99. So you'll need a vendor-specific solution.

• MySQL - Use the built-in GROUP_CONCAT function. In your example you would want something like this:

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 is equally simple now that string_agg(expression, delimiter) is built-in. Here it is with 'comma-space' between elements:

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

PostgreSQL before 9.0 allows you to define your own aggregate functions with CREATE AGGREGATE. Slightly more work than MySQL, but much more flexible. See this other post for more details. (Of course PostgreSQL 9.0 and later have this option as well.)

• Oracle - same idea using LISTAGG.

• MS SQL Server - same idea using STRING_AGG

• Fallback solution - in other database technologies or in very very old versions of the technologies listed above you don't have these group concatenation functions. In that case create a stored procedure that takes the org_id as its input and outputs the concatenated employee names. Then use this stored procedure in your query. Some of the other responses here include some details about how to write stored procedures like these.

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