Some people have the mistaken idea that having a record expunged means it is completely erased, as if the whole thing never happened. This is not the case. In reality, the fact that you were arrested is still visible, as are the charges that you were facing. However, the change is that any conviction is dismissed, and it's marked as such.
Many people want to have their records expunged for the purposes of getting a job. Employers who see a conviction may be hesitant to hire them. Having the record expunged can help you, but it does not mean your potential employer won't see any information when running a background check. He or she will just see the updated information, which says that the conviction was dismissed. This could improve your chances of getting the position, though your employer will still know that an arrest took place.
So, if your employer asks about previous criminal convictions, are you allowed to say you don't have one? Absolutely. You are not lying and can truthfully note that you don't have a conviction on your record.
That being said, you may want to mention that you were arrested and that the case against you was dismissed. This way, your employer won't be surprised to see it on your record. This is up to you, of course, but it's good to know because it ensures that you and your employer are on the same page.
If you want to clean up your record, it's critical to know exactly what paperwork to fill out and what steps to take. Be sure you fully research the process and the impact it could have before you begin.
Source: California Courts: The Judicial Branch of California, "Clearing Your Adult Criminal Record in California," accessed Nov. 22, 2016