How do you handle NULLs in a DATEDIFF comparison? - sql

Question

I have to compare 2 separate columns to come up with the most recent date between them. I am using DATEDIFF(minute, date1, date2) to compare them, however, in some records the date is Null, which returns a null result and messes up the CASE.
Is there a way around this, or a way to predetermine which date is null up front?
(psudocode)
UPDATE TABLE
SET NAME = p.name,
NEW_DATE = CASE WHEN DATEDIFF(minute,d.date1,d.date2) <= 0 THEN d.date
ELSE d.date2
END
FROM TABLE2 d
INNER JOIN TABLE3 p
ON d.ACCTNUM = p.ACCTNUM

Answer 1

You can just add extra logic into your CASE:
UPDATE TABLE
SET NAME = p.name,
NEW_DATE = CASE
WHEN d.date1 IS NULL THEN -- somewhat
WHEN d.date2 IS NULL THEN -- somewhat
WHEN DATEDIFF(minute,d.date1,d.date2) <= 0 THEN d.date
ELSE d.date2
END
FROM TABLE2 d
INNER JOIN TABLE3 p
ON d.ACCTNUM = p.ACCTNUM

Answer 2

I would use ISNULL.
UPDATE TABLE
SET NAME = p.name,
NEW_DATE = CASE WHEN ISNULL(DATEDIFF(minute,d.date1,d.date2), 0) <= 0 THEN d.date
ELSE d.date2
END
FROM TABLE2 d
INNER JOIN TABLE3 p
ON d.ACCTNUM = p.ACCTNUM
or maybe
ISNULL(DATEDIFF(minute,d.date1,d.date2), 1)
if you want to handle the null values the other way around.

Answer 3

You can try some think like this. You can use Is Null to check null.
UPDATE TABLE
SET NAME = p.name,
NEW_DATE = CASE Case When date2 Is Null Then GetDate()
Case When date1 Is Null Then GetDate()
WHEN DATEDIFF(minute,d.date1,d.date2) <= 0 THEN d.date
ELSE d.date2
END
FROM TABLE2 d
INNER JOIN TABLE3 p
ON d.ACCTNUM = p.ACCTNUM
Microsoft's ISNULL() function is used to specify how we want to treat
NULL values.
In this case we want NULL values to be zero.
Below, if "UnitsOnOrder" is NULL it will not harm the calculation,
because ISNULL() returns a zero if the value is NULL:
SQL Server / MS Access
SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0)) FROM Products
SQL NULL Functions

Answer 4

This should give you what you want with minimal processing required.
UPDATE TABLE
SET NAME = p.name,
NEW_DATE = CASE WHEN COALESCE(date1, date2)>COALESCE(date2, date1)
THEN COALESCE(date1, date2)
ELSE COALESCE(date2, date1)
END
FROM TABLE2 d
INNER JOIN TABLE3 p
ON d.ACCTNUM = p.ACCTNUM
WHERE NOT (date1 is null and date2 is null);

Answer 5

I personally use this:
IIF([yourvariable] is null, expression_when_true, expression_when_false)
This actually works well if you have more complex datatypes like DATE. I've looked for the solution also and lots of people asked how to substract null date or why simple 0000-00-00 doesn't work. The IIF can avoid substraction when minuend or subtrahend is null.
Example:
IIF([ReturnDate] is null,
datediff(day,[BookDate],getdate())*CostPerDay,
datediff(day,[BookDate],[ReturnDate])*CostPerDay)