Arithmetic and Comparison Expressions in SQL
Arithmetic and comparison expressions are the building blocks of computed values and conditions in SQL. Their behavior is governed entirely by the types of their operands.
Before this Literal Values, Data Types, and Type Casting
SQL evaluates arithmetic the same way regular math does: multiplication and division before addition and subtraction.
That might sound obvious, but it's easy to forget when you're writing a multi-step calculation in one expression. 500 + 12 * 9 doesn't give you 4608. It gives you 608, because the multiplication runs first. If you want addition to run first, you need parentheses.
This matters every time you build a formula with more than one operation. Budget calculations, price adjustments, commission breakdowns — any time the order affects the answer, parentheses make your intent explicit:
SELECT (400 + 150) * 3 AS campaign_costAdd the two components first, then multiply the total by three. Without the parentheses, SQL multiplies 150 by 3 and then adds 400 — a completely different number.
SQL also has a fifth arithmetic operator you may not have encountered: %, called modulo. It returns the remainder after division:
275 divided by 8 is 34 with 3 left over. The result is 3. Modulo comes up whenever you need to know what's left after dividing a quantity evenly — items remaining after packing into boxes, positions in a sequence, that kind of thing.
One piece from casting to keep in mind here: dividing two integers still truncates the decimal. 1000 / 3 returns 333, not 333.333.... Cast one side to ::numeric when you need the full result: 1000::numeric / 3 returns 333.333....
The one thing that trips people up: NULL in arithmetic.
Any arithmetic involving NULL produces NULL. 5000 * NULL returns NULL, not zero. A missing value in any part of a formula makes the whole result unknown. If a calculation returns unexpected NULLs, a NULL in one of the inputs is usually why.
A calculation uses: 5000 * NULL. What does SQL return?
9 Arithmetic and Comparison Expressions practice problems
Write a query to return the number of leftover items in a single column named `remaining_items`.
Write a query to return the total licensing cost in a single column named `total_cost`.
Write a query to return the total campaign cost in a single column named `campaign_cost`.
Write a query to return each warehouse's whole-unit share in a single column named `units_per_warehouse`. The leftover units are out of scope here.
Write a query to return each employee's precise share in a single column named `exact_share`.
Write a query to return the total order cost in a single column named `total_order_cost`.
Write a query to return what the calculation `base * multiplier` produces in its current state, in a single column named `payout`.
Write a query to return both values in a single row.
Write a query to return the tax amount owed in a single column named `tax_amount`.
These problems are part of the Arithmetic and Comparison Expressions lesson in SQLMaxx, with instant grading and a worked solution on each.
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