Say you have 1’ or 5’ contours with an elevation field. Instead of calculating a new field or making selections, you can use the mod function. I find it extremely useful.
In the definition query tab of your layer properties:
The mod expression above divides the Contour value by 25 and throws out any values that don’t have a remainder of 0.
This will return only Contour values divisible by 25 – you can also use this to make quicker selections if you do need to calculate a field. This can also be used in labelling class definition queries, so if you want to show all contours but only label 50’ contours, use the expression