Logical operators OR AND in condition and order of conditions in WHERE

  • Let's examine these two statements:


    If CONDITION 1 is TRUE, will CONDITION 2 be checked?
    If CONDITION 3 is FALSE, will CONDITION 4 be checked?

    What about conditions on WHERE: does the SQL Server engine optimize all conditions in a WHERE clause? Should programmers place conditions in the right order to be sure that the SQL Server optimizer resolves it in the right manner?


    Thank to Jack for link, surprise from t-sql code:

    IF  1/0 = 1 OR 1 = 1
          SELECT 'True' AS result
          SELECT 'False' AS result
    IF  1/0 = 1 AND 1 = 0
          SELECT 'True' AS result
          SELECT 'False' AS result

    There is not raise a Divide by zero exception in this case.


    If C++/C#/VB has short-circuiting why can't SQL Server have it?

    To truly answer this let's take a look at how both work with conditions. C++/C#/VB all have short circuiting defined in the language specifications to speed up code execution. Why bother evaluating N OR conditions when the first one is already true or M AND conditions when the first one is already false.

    We as developers have to be aware that SQL Server works differently. It is a cost based system. To get the optimal execution plan for our query the query processor has to evaluate every where condition and assign it a cost. These costs are then evaluated as a whole to form a threshold that must be lower than the defined threshold SQL Server has for a good plan. If the cost is lower than the defined threshold the plan is used, if not the whole process is repeated again with a different mix of condition costs. Cost here is either a scan or a seek or a merge join or a hash join etc... Because of this the short-circuiting as is available in C++/C#/VB simply isn't possible. You might think that forcing use of index on a column counts as short circuiting but it doesn't. It only forces the use of that index and with that shortens the list of possible execution plans. The system is still cost based.

    As a developer you must be aware that SQL Server does not do short-circuiting like it is done in other programming languages and there's nothing you can do to force it to.

    Where is the final quote block from? Could you add a reference?

  • MicSim

    MicSim Correct answer

    9 years ago

    There's no guarantee in SQL Server if or in which order the statements will be processed in a WHERE clause. The single expression that allows statement short-circuiting is CASE-WHEN. The following is from an answer I posted on Stackoverflow:

    How SQL Server short-circuits WHERE condition evaluation

    It does when it feels like it, but not in the way you immediately think of.

    As a developer you must be aware that SQL Server does not do short-circuiting like it is done in other programming languages and there's nothing you can do to force it to.

    For further details check the first link in the above blog entry, which is leading to another blog:

    Does SQL Server Short-Circuit?

    The final verdict? Well, I don't really have one yet, but it is probably safe to say that the only time you can ensure a specific short-circuit is when you express multiple WHEN conditions in a CASE expression. With standard boolean expressions, the optimizer will move things around as it sees fit based on the tables, indexes and data you are querying.

    Apparently there are some edge cases (or a bug) where even `case` is not safe

  • SQL is a declarative programming language. Unlike, say, C++ which is an imperative programming language.

    I.e. you can tell it what you want in the end-result, but you can't dictate how the result is being executed, it's all up to the engine.

    The only true way to guarantee "short-circuiting" (or any other control flow) inside WHERE is to use indexed views, temporary tables and similar mechanisms.

    PS. You can also use execution plan hints (to "hint" the engine how to execute a query, which indexes to use and HOW to use them), just thought I should mention it, while we're on this topic...

  • 1)--OR ( any one or both conditions will be TRUE)

    if condition 1 is TRUE then condition 2 will also checked it can be either TRUE or FALSE

    --AND ( both conditions must be TRUE)

    if condition 1 is FALSE then condition 2 will not be checked

    "if condition 1 is FALSE then condition 2 will not be checked" This is not true. See the answer above. SQL Server may still evaluate condition 2 because it does not perform short-circuit evaluation in `WHERE` clauses.

  • The only way to control how conditions within the WHERE clause is to use brackets to group them together.

    WHERE Col1 = 'Something' AND Col2 = 'Something' OR Col3 = 'Something' and Col4 = 'Something'

    is very different from

    WHERE (Col1 = 'Something' AND Col2 = 'Something') OR (Col3 = 'Something' and Col4 = 'Something')

    Just curious. How are these two conditions different? Different results, performance, execution plan? I thought they would be equivalent.

    With the first one you need to match Col1, Col4 and either Col2 or Col3. In the second line to match Col1 and Col2 or you need to match Col3 and Col4 but Col1 and Col4 will never need to both be evaluated together.

    No, you are wrong. `AND` has higher precedence than `OR`. Both are equivalent. What you say would be true for the `WHERE Col1 = x AND (Col2 = x OR Col3 = x) AND Col4 = x` query. See SQL-Fiddle test

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Content dated before 6/26/2020 9:53 AM