SQL check if record exists - sql

Question

So I spent some time to research what the best way was to check if a record exist was. Ended up with this.
var checkExistance = "SELECT TOP 1 exerVariName FROM exerciseVariants WHERE exerVariName = '" + exerVariName + "'";
However I failed many times to make this work while using it on my page!
var exerVariName = Request.Form["exerVariName"];
var checkExistance = "SELECT TOP 1 exerVariName FROM exerciseVariants WHERE exerVariName = '" + exerVariName + "'";
if (IsPost && Validation.IsValid()) {
if (ModelState.IsValid) {
foreach (var c in db.Query(checkExistance)) {
if (c.exerVariName != exerVariName) {
var insertData = "INSERT INTO exerciseVariants (exerVariName, exerVariNameID) " +
"VALUES (#0, #1)";
db.Execute(insertData, exerVariName, exerciseID);
Response.Redirect("~/insertexervariname");
}
}
}
}
So the variable I put into the SQL line is a request form thing so its a user input that I want to check if it exists in the database, if it already exists I dont want it to be posted. And the above is what i tried with the foreach in the if ispost.
How would one go about to achieve this? (c# razor/cshtml)

Answer 1

Your code has race conditions.
The best way to do what you want is to have the database enforce the constraint, using a unique index/constraint:
create unique index unq_exerciseVariants_exerVariName on exerciseVariants(exerVariName);
If you want to avoid the error on updates, then you can check as you update as well:
INSERT INTO exerciseVariants (exerVariName, exerVariNameID)
SELECT exerVariName, exerVariNameID
FROM (SELECT #0 as exerVariName, #1 as exerVariNameID
) x
WHERE NOT EXISTS (SELECT 1
FROM exerciseVariants
WHERE ev.exerVariName = x.exerVariName
);
Note that this check may not be sufficient due to race conditions, if two different threads attempt to insert the same name at the same time. That is why it is best to have the database enforce the uniqueness.

Answer 2

I am no C# person by any means, but I do see one logical problem with your current code. Currently your EXISTS and INSERT queries are being run separately, separated by several lines of .NET code and perhaps many more actual machine instructions. The net (no pun intended) result of this is that you might end up inserting when the existence check appears true but is no longer true. To avoid this, the existence check should appear in the WHERE clause of the INSERT. Something like this should work:
INSERT INTO exerciseVariants (exerVariName, exerVariNameID)
VALUES (#0, #1)
WHERE EXISTS
(
SELECT * exerVariName
FROM exerciseVariants
WHERE exerVariName = '" + exerVariName + "'";
)
Now the INSERT should happen atomically, meaning your check and the actual insert will all be done at the same time, regardless of what other threads might be doing.