Date range overlapping check constraint

Question

I\'ve a simple table in sql server 2005 with 3 columns: DateStart, DateEnd and Value. I tried to set a \"table check constraint\" to avoid inserting overlapping records. For

Answer 1

The CHECK is being executed after the row has been inserted, so the range overlaps with itself.

You'll need to amend your WHERE to include something like: @MyTableId <> MyTableId.

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BTW, your WHERE expression can be simplified.

Ranges don't overlap if:

• end of the one range is before the start of the other
• or start of the one range is after the end of the other.

Which could be written in SQL like:

WHERE @DateEnd < DateStart OR DateEnd < @DateStart

Negate that to get the ranges that do overlap...

WHERE NOT (@DateEnd < DateStart OR DateEnd < @DateStart)

...which according to De Morgan's laws is the same as...

WHERE NOT (@DateEnd < DateStart) AND NOT (DateEnd < @DateStart)

...which is the same as:

WHERE @DateEnd >= DateStart AND DateEnd >= @DateStart

So your final WHERE should be:

WHERE
    @MyTableId <> MyTableId
    AND @DateEnd >= DateStart
    AND DateEnd >= @DateStart

[SQL Fiddle]

NOTE: to allow ranges to "touch", use <= in the starting expression, which would produce > in the final expression.

Answer 2

I'd just like to add onto the Answer of Branko Dimitrijevic the case where the DateEnd is null since I currently have such a scenario.

This can occur when you're keeping punch in logs and the user is still logged in.

WHERE
    @MyTableId <> MyTableId
    AND @DateEnd >= DateStart
    AND DateEnd >= @DateStart
    OR @DateEnd >= DateStart
    AND DateEnd is null
    OR @DateStart >= DateStart
    AND DateEnd is null

I don't know how well this query is performance wise, I'm sure there are ways to optimize it.