A Friendly Reminder: One Plus NULL Equals NULL

Whenever you perform arithmetic across SQL columns, take care to handle NULL values appropriately.

June 6, 2019

With the unveiling of Fivetran Transformations, you now have even more reason to perform arithmetic operations across columns within your data warehouse. Unfortunately, a case may arise where a value is missing, but its absence should not invalidate an entire record.

Suppose you have sales records from a variety of jurisdictions and product categories. Not all jurisdictions have sales taxes and some product categories come with mandatory hazardous materials fees:

item	price	tax	hazmat_fee
oat_bran	1.00	0.06	NULL
scalpel	5.50	NULL	NULL
mercury_fulminate	200.00	NULL	35.00
cesium_iodide	450.00	45.00	35.00

If you tried to calculate the total cost for each record using the following query:

SELECT item, price, tax, hazmat_fee, price + tax + hazmat_fee as total_cost FROM sales.records

Your results will look like so:

item	price	tax	hazmat_fee	total_cost
oat_bran	1.00	0.06	NULL	NULL
scalpel	5.50	NULL	NULL	NULL
mercury_fulminate	200.00	NULL	35.00	NULL
cesium_iodide	450.00	45.00	35.00	530.00

What you really want in this case is to default to a value of 0 whenever a NULL is encountered. You can do this using the COALESCE function, which takes the first non-NULL argument from a list):

SELECT item, price, tax, hazmat_fee, price + COALESCE(tax, 0) + COALESCE(hazmat_fee, 0) as total_cost FROM sales.records

Returning:

item	price	tax	hazmat_fee	total_cost
oat_bran	1.00	0.06	NULL	NULL
scalpel	5.50	NULL	NULL	NULL
mercury_fulminate	200.00	NULL	35.00	NULL
cesium_iodide	450.00	45.00	35.00	530.00

Remember - when adding across columns, one plus NULL equals NULL! This is true for subtraction, multiplication, and division, too.

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A Friendly Reminder: One Plus NULL Equals NULL

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