What makes a SQL statement sargable?

By definition (at least from what I’ve seen) sargable means that a query is capable of having the query engine optimize the execution plan that the query uses. I’ve tried looking up the answers, but there doesn’t seem to be a lot on the subject matter. So the question is, what does or doesn’t make an SQL query sargable? Any documentation would be greatly appreciated.

For reference: Sargable




Answer

The most common thing that will make a query non-sargable is to include a field inside a function in the where clause:

SELECT ... FROM ...
WHERE Year(myDate) = 2008

The SQL optimizer can’t use an index on myDate, even if one exists. It will literally have to evaluate this function for every row of the table. Much better to use:

WHERE myDate >= '01-01-2008' AND myDate < '01-01-2009'

Some other examples:

Bad: Select ... WHERE isNull(FullName,'Ed Jones') = 'Ed Jones'
Fixed: Select ... WHERE ((FullName = 'Ed Jones') OR (FullName IS NULL))

Bad: Select ... WHERE SUBSTRING(DealerName,4) = 'Ford'
Fixed: Select ... WHERE DealerName Like 'Ford%'

Bad: Select ... WHERE DateDiff(mm,OrderDate,GetDate()) >= 30
Fixed: Select ... WHERE OrderDate < DateAdd(mm,-30,GetDate())