Question

Say I have a table which stores customers order IDs. Such as

| Customer ID | Order ID | Order Date

How can I get all customers who have ordered today?

Also OrderDate would be a DateTime.

Something like

SELECT DISTINCT CustomerID
FROM TableName
Where OrderDate > Today

But the last part is what I can’t figure out.

Answer 1

It’s fairly common to only want a date out of a datetime – you should be able to Google for the specifics of your RDBMS (since you don’t mention it). The important bit is to make your query SARGable by transforming today’s date¹ – not the order date.

For MSSQL, something like

SELECT DISTINCT CustomerID
FROM TableName
--I assume you want midnight orders as well - so use >=
Where OrderDate >= DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE()))

would work by taking number of days today is from Date 0 (DATEDIFF(dd, 0, GETDATE())) and adding them back to Date 0 (DATEADD(dd, 0, x)). That’s T-SQL specific, though.

¹ If you were searching for an arbitrary date, you’d still transform both arguments:

SELECT DISTINCT CustomerID
FROM TableName
Where 
    OrderDate >= DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE()))
    --You *do not* want midnight of the next day, as it would duplicate orders
    AND OrderDate < DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE()) + 1)