How to avoid multiple data in SQL table

Question

I am developing Web application and i used SQL server for database storing (backend).

In every form will have submit button and record will be stored into database.

sometimes, end user clicking "Submit" button at multiple times.

so, many duplicate entry will be occupying in a table.

To avoid this, i am using IF NOT EXISTS statement.

Will the below query work to avoid duplicate entry even if clicks multiple times "Submit" button.

BEGIN
   IF NOT EXISTS (SELECT * FROM Employee 
                   WHERE ENo = @ENo
                   AND JoinNo = @JoinNo
                   AND JoinDate = JoinDate)
   BEGIN
       INSERT INTO EmailsRecebidos (ENo, JoinNo, JoinDate,DOB,Address)
       VALUES (@ENo, @JoinNo, @JoinDate,@DOB,@Address)
   END
END

Answer 1

Hi @BeUnique

Not sure why you check data exists from table Employee, while insert into another table EmailsRecebidos.

To avoid duplicate, shouldn't the logic be both the same table?

IF NOT EXISTS 
(SELECT * FROM YourTable WHERE column1 = @value1 AND column2 = @value2)
BEGIN
  INSERT INTO YourTable (column1, column2) VALUES (@value1, @value2);
END

Best regards,

Cosmog

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Answer 2

When the 3 columns in the WHERE clause are the primary key for the table, then yes, it will work.