Resolving Foreign Key References from the Same Table in SQL

As a technical blogger, I’ve encountered numerous questions on Stack Overflow regarding complex SQL queries. One such question caught my attention recently: “How to resolve foreign key references from the same table?” In this article, we’ll delve into the intricacies of joining tables with identical columns and explore various solutions using SQL.

Understanding Foreign Key References

A foreign key is a column or set of columns in a database table that refers to the primary key values in another table. The purpose of a foreign key is to establish relationships between two tables and enforce data consistency by ensuring that only valid values can be inserted into the referencing table.

In the given example, we’re dealing with three tables: orgunit, department, and OrgUnitParent. Each of these tables contains columns with similar names, which makes it challenging to join them using a standard SQL query.

The Problem

The problem lies in the fact that we have two foreign key references in the same table (orgunit) that point to different parent tables: OrgUnitParent and another instance of orgunit. To solve this issue, we need to create a temporary join or use a more advanced SQL technique.

The Current Query

The provided query attempts to join the three tables but fails due to the duplicate foreign key references:

select 
    orgunit.shortname       AS OnsiteDept, 
    department.shortname    AS DeptName, 
    OrgUnitParent.orgunitid AS ParentID,
    Orgunit.xrefcode        AS OrgXref,
    Orgunit.departmentid    AS DeptID,  
    Orgunit.shortname       AS Parentshotname,
    Orgunit.orglevelid      AS OrgLevel

from orgunit
left join   department
on orgunit.departmentid = department.departmentid

left join   OrgUnitParent
on orgunit.orgunitid = OrgUnitParent.orgunitid
AND OrgUnitParent.orgunitid = orgunit.orgunitid

where orgunit.shortname;

This query will not produce the desired results because of the duplicate foreign key references.

The Solution

To resolve this issue, we can use a combination of left joins and subqueries to create a temporary join table that contains all the required information.

Using a Temporary Join Table

One possible solution is to create a temporary join table by joining orgunit with OrgUnitParent and then joining the result with department. Here’s an example query:

with temp_join as (
  select 
    ou.orgunitid AS parent_id, 
    oup.orgunitid AS child_id, 
    ou.shortname AS shortname,
    d.shortname AS dept_name
  from orgunit ou left join OrgUnitParent oup on ou.orgunitid = oup.orgunitid
  left join department d on ou.departmentid = d.departmentid
)
select * from temp_join;

This query creates a temporary join table temp_join that contains the parent and child foreign key references. We can then use this table to select the required columns.

Using a Common Table Expression (CTE)

Another approach is to use a common table expression (CTE) to define a temporary join table:

with temp_join as (
  select 
    ou.orgunitid AS parent_id, 
    oup.orgunitid AS child_id, 
    ou.shortname AS shortname,
    d.shortname AS dept_name
  from orgunit ou left join OrgUnitParent oup on ou.orgunitid = oup.orgunitid
  left join department d on ou.departmentid = d.departmentid
)
select * from temp_join;

This CTE is similar to the previous example but uses a different syntax to define the temporary join table.

Using a Hierarchy ID

For more complex hierarchical relationships, we can use a hierarchy ID data type. This approach requires additional setup and maintenance but provides a flexible way to manage complex tree-like structures.

create hierarchy orgunit_hierarchy (
  orgunitid int,
  parent_orgunitid int
);

with temp_join as (
  select 
    ou.orgunitid AS parent_id, 
    oup.orgunitid AS child_id, 
    ou.shortname AS shortname,
    d.shortname AS dept_name
  from orgunit ou left join OrgUnitParent oup on ou.orgunitid = oup.orgunitid
  left join department d on ou.departmentid = d.departmentid
)
select * from temp_join;

This example creates a hierarchy ID orgunit_hierarchy and uses it to define the temporary join table.

Conclusion

Resolving foreign key references from the same table in SQL can be challenging, but there are several techniques to achieve this. By using temporary join tables, common table expressions (CTEs), or hierarchy IDs, we can create complex queries that retrieve the required information. When working with hierarchical relationships, it’s essential to understand the different data types and their applications.

Additional Considerations

When dealing with foreign key references from the same table, consider the following best practices:

• Use temporary join tables or CTEs to define complex relationships.
• Avoid using SELECT * queries; instead, specify only the required columns.
• Optimize your queries by reducing the number of joins and using efficient data types.

By applying these techniques and best practices, you’ll be able to write more effective SQL queries that resolve foreign key references from the same table.

Last modified on 2023-05-29