如何检查数据库中是否已存在值对？

Question

In VB.Net, I am trying to check if a certain value pair already exists in the database. If not, it needs to be added:

Public Shared Sub updateChoice(ByVal dbConn As NS.DBConn, ByVal Y As String, ByVal Z As Int32)
    'check if the Y <=> Z pair already exists
    Dim sqlText As String = "select from table where X = " & CStr(Y)" and Y =" &CStr(Z)
    Dim pairExists = dbConn.ExecuteAsDataSet(sqlText)

    If (pairExists <= 0) Then
        Dim sqlText As String = "insert into table ..." ' Insert pair into DB
        dbConn.ExecuteNonQuery(sqlText)
    End If
End Sub

Visual Studio gives me an error on If (pairExists <= 0) Then : Operator <= is not defined for types System.Data.DataSet and Integer

I am having a hard time understanding this error. What am I doing wrong?

Answer 1

It would be more efficient to do the check and the insert in the same statment, it also significantly reduces the chance of meeting a race condition by reducing the time between the check and the insert. The statement you would want is:

MERGE Table WITH (HOLDLOCK) AS t
USING (VALUES (@X, @Y)) AS s (X, Y)
    ON s.X = t.X
    AND s.X = t.Y
WHEN NOT MATCHED THEN INSERT (X, Y) VALUES (X, Y)
OUTPUT $action;

Using MERGE with HOLDLOCK is the best way I know of to avoid a race condition, however it is not a substitute for a unique constraint. If the values should be unique in the table then this should still be enforced with a constraint. You can then simply check the return value of ExecuteNonQuery() , it will return 1 if a row is inserted, 0 if the pair already exists.

You should also be using parameterised queries!