Jeremy Smith on March 28, 2017

Handy SQL Functions For Self-Referencing Tables

I have a client that uses NetSuite, a cloud-based CRM/ERP system. NetSuite records customer information in a datbase table called (surprise!) customers. The table is self-referencing, so you can have customers with many subcustomers by setting the parent_id on the subcustomer records to the customer_id on the customer record. When we originally set up my client’s usage of the system, we decided to make company accounts the top-level customers and all the individuals at each company the subcustomers under the customer record. Company records had certain attributes (like a company name, and details about products owned by the company) that were hidden and left blank on the individual records. And individual records had attributes (like first name, and role) that were hidden and left blank on the company record. But both would make use of other attributes (like email and whether or not the record was active). This solution wasn’t ideal, but given other constraints at the time of implementation, it made the most sense.

Recently, I needed a way to extract that customer data from NetSuite with all company and individual records. But on the individual records, I needed to display the company-level attributes that were only available on their parent records. I also needed to know when any record was last modified, whether the change was on the individual or associated company record. (If I simply joined the child to the parent, I wouldn’t know which last modified date would be the right one to use in my SELECT statement.)

Thankfully, there are two nice SQL functions that come in handy for this: IFNULL and GREATEST.

IFNULL takes two arguments. If the first argument is not NULL, it will return it. Otherwise it will return the second argument. This is helpful because I can do a LEFT OUTER JOIN on the self-referencing table, and use IFNULL to join each record to it’s parent record (if the parent exists) or back to itself (if it doesn’t), like this:

SELECT * FROM customers AS c LEFT OUTER JOIN customers AS p
ON IFNULL(c.parent_id, c.customer_id) = p.customer_id;

That gives us all the company-level attributes on the individual records, but we still need a last modified date that accounts for changes on either the child or parent record. That’s where GREATEST comes in. If you pass two or more arguments to the GREATEST function, it will return the largest argument. (Watch out, though: if any argument is NULL it will return NULL.) So, if I know that my last modified date should really be the greater of the child record’s last modified and the parent record’s last modified, I can use this in my SELECT statement:

SELECT GREATEST(c.last_modified_date, p.last_modified_date) ...

Altogether, that gives us a query that looks something like the following. We’re grabbing all the individual-level attributes from the child (aliased AS c) and all the company-level attributes from the parent (aliased AS p), and getting the greatest available last modified date.

GREATEST(c.last_modified_date, p.last_modified_date)
FROM customers AS c LEFT OUTER JOIN customers AS p
ON IFNULL(c.parent_id, c.customer_id) = p.customer_id;

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