IFF and IsNull

Use of the Format function in a SELECT will cause you to lose rows in your results if the value you are formatting is NULL.

Using IsNull function and IIF construct in MS Access

FORMAT(IIF(IsNull (O.Tax),0,O.Tax),"Currency") as TAX

Syntax for MS SQL

Coalesce could also be used in place of IsNull below.

CAST(IsNull(TAX,0)AS money) as TAX