On your hardware, against your schema and data. The only way you can know for sure which will be faster, is to test for yourself, But if every last nanosecond is important, Is almost certainly going to be negligible. You are using simple constant, expression or column outputs, the performance difference Test all of the variants to be sure that performance will be what you expect. Output is either a query or a call to a user-defined function, it is important to If you are writing complex expressions using ISNULL, COALESCE or CASE where the To proliferate into all discussions that involve the two functions. To be honest, I think this is often an edge case, but the sentiment seems ISNULL, on the other hand, somehow has the smarts to only evaluate the subquery Number is greater than zero, then as a result of that, computing the count again. This is kind of like selecting the number of rows of a table to determine if the In other words, it is evaluating at least part of the subquery twice. THEN (SELECT MAX(index_id) FROM sys.indexes WHERE = s.) ELSE 0 END SELECT CASE WHEN (SELECT index_id FROM sys.indexes WHERE = s.) IS NOT NULL This was on SQL Server 2012, so I was able to useĬombined declaration / assignment and a more precise data type than DATETIME: ISNULL to another variable, in a loop, 500,000 times, and measured the duration I simply assigned the result of COALESCE or Scenarios: (1) both arguments NULL (2) first argument NULL (3) second argument Simple test with two variables, and tested the speed of COALESCE and ISNULL in four On SQL Server 2012 to see if my results show anything different. I thought I would introduce a new test based Query would look if you were dealing with meaningful column or variable names.ĬOALESCE and ISNULL perform about the same (in most cases) in SQL Serverĭifferent people have run different tests comparing ISNULL and COALESCE, and Very simple, single-letter column names, so imagine how much longer that second That the T-SQL itself is the issue, it becomes needlessly verbose. So the main point here is that performance will be identical in this case and
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