TSQL divide by zero encountered despite no columns

Question

I've been trying to understand why I get a "divide by zero encountered" (Msg 8134) with my SQL query, but I must be missing something. I would like like to know the why for the specific case below, I am not looking for NULLIF, CASE WHEN... or similar as I already know about them (and can of course use them in a situation as the one below).

I have an SQL statement with a computed column similar to

SELECT
    TotalSize,
    FreeSpace,
    (FreeSpace / TotalSize * 100)
FROM
    tblComputer
...[ couple of joins ]...
WHERE
    SomeCondition = SomeValue

Running this statement errors with the above mentioned error messages, which, in itself, is not the problem - obviously TotalSize might well be 0 and therefore cause the error.

Now what I don't understand is that I do not have any rows where the TotalSize column is 0 when I comment the computed column out, I double checked that this isn't the case.

Then I thought that for some reason the column computation would be performed on the whole result set before actually filtering with the conditions of the where clause, but this a) wouldn't make sense imho and b) when trying to reproduce the error with a test set-up everything works fine (see below):

INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0001',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0002',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0003',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0004',0)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0005',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0006',0)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0007',1)

INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (1,100,21)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (2,100,10)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (3,100,55)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (4,0,10)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (5,100,23)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (6,100,18)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (7,100,11)

-- This statement does not throw an error as apparently the row for ComputerID 4 
-- is filtered out before computing the (FreeSpace / TotalSize * 100)
SELECT 
TotalSize,
FreeSpace,
(FreeSpace / TotalSize * 100)
FROM 
tblComputer
JOIN
tblHDD ON
tblComputer.ID = tblHDD.ComputerID
WHERE
IsServer = 1

I am quite stumped and would like to know what the reason is.

Any ideas or pointers into the right direction are very welcome, thanks in advance

Update

Thank you so far for your input, but unfortunately I seem not to be getting closer to the root of the problem. I managed to strip the statement down a little bit and now have the case that I can execute it without errors if one JOIN is removed (I would need it for additional columns in the output which I temporarily removed).

I do not understand, why using the JOIN leads to the error, shouldn't a standard INNER JOIN always either return the same number of rows or less, but never more?

Working code

SELECT 
TotalSize,
FreeSpace
((FreeSpace / TotalSize) * 100)
FROM 
MyTable1
INNER JOIN 
MyTable2 ON
MyTable1.ID = MyTable2.Table1ID
WHERE 
SomeCondition

Error causing code

SELECT 
TotalSize,
FreeSpace
((FreeSpace / TotalSize) * 100)
FROM 
MyTable1
INNER JOIN 
MyTable2 ON
MyTable1.ID = MyTable2.Table1ID
-- This JOIN causes "divide by zero encountered" error
INNER JOIN 
MyTable3 ON
MyTable2.ID = MyTable3.Table2ID
WHERE 
SomeCondition

I also tried my luck using a cursor and looping over the result row by row, but in that case no error occurred (no matter, which of the two statements above I tried).

Sorry for the messy code indentation, somehow the correct formatting doesn't seem to be applied.

G.

Answer 1

SQL is a declarative language; you write a query that logically describes the result you want, but it is up to the optimizer to produce a physical plan. This physical plan may not bear much relation to the written form of the query, because the optimizer does not simply reorder 'steps' derived from the textual form of the query, it can apply over 300 different transformations to find an efficient execution strategy.

The optimizer has considerable freedom to reorder expressions, joins, and other logical query constructions. This means that you cannot, in general, rely on any written query form to force one thing to be evaluated before another. In particular, the rewrite given by Lieven does not force the WHERE clause predicate to be evaluated before the expression. The optimizer may, depending on cost estimations, decide to evaluate the expression wherever it seems most efficient to do so. This may even mean, in some cases, that the expression is evaluated more than once.

The original question considered this possibility, but rejected it as 'not making much sense'. Nevertheless, this is the way the product works - if SQL Server estimates that a join will reduce the set size enough to make it cheaper to compute the expression on the result of the join, it is free to do so.

The general rule is to never depend on a particular evaluation order to avoid things like overflow or divide-by-zero errors. In this example, one would employ a CASE statement to check for a zero divisor - an example of defensive programming.

The optimizer's freedom to reorder things is a fundamental tenet of its design. You can find cases where it leads to counter-intuitive behaviours, but overall the benefits far outweigh the disadvantages.

Paul

Answer 2

What rows are returned when you run:

SELECT
   TotalSize
FROM
   tblComputer
   ...[ couple of joins ]...
WHERE
   SomeCondition = SomeValue
   and ((TotalSize * 100) = 0)

This might give you a clue as to how SQL Serve ris evaluating (TotalSize * 100) to be zero.

Another idea, is there anything in your where statement which might also be the problem?
You're assuming it's the TotalSize, but it might be somewhere else.

Answer 3

I was running into the same issue. In my case NULLs were acceptable so I was able to fix it this way:

Select Expression1 / Expression2 -- Caused Division By 0
Select Expression1 / NULLIF(Expression2,0) -- Causes result to be NULL

If you need other handling, you can wrap the entire expression in an ISNULL function like this:

Select ISNULL(Expression1 / NULLIF(Expression2,0)-5) -- Returns -5 instead of null or divide by 0

Answer 4

The basic steps that SQL Server uses to process a single SELECT statement include the following

1. The parser scans the SELECT statement and breaks it into logical units such as keywords, expressions, operators, and identifiers.
2. A query tree, sometimes referred to as a sequence tree, is built describing the logical steps needed to transform the source data into the format required by the result set.
3. The query optimizer analyzes different ways the source tables can be accessed. It then selects the series of steps that returns the results fastest while using fewer resources. The query tree is updated to record this exact series of steps. The final, optimized version of the query tree is called the execution plan.
4. The relational engine starts executing the execution plan. As the steps that require data from the base tables are processed, the relational engine requests that the storage engine pass up data from the rowsets requested from the relational engine.
5. The relational engine processes the data returned from the storage engine into the format defined for the result set and returns the result set to the client.

My interpretation of things is that there is no guarantee that your where clause get's evaluated before evaluating the computed column for all rows.

You could verify that assumption by changing you query like below and forcing the where clause to be evaluated before the computation.

SELECT
    TotalSize,
    FreeSpace,
    (FreeSpace / TotalSize * 100)
FROM (
  SELECT
      TotalSize,
      FreeSpace,
  FROM
      tblComputer
  ...[ couple of joins ]...
  WHERE
      SomeCondition = SomeValue
  ) t