Why can DNA tests with mixed DNA of several people not be used to detect a criminal in a database?

Scenario: Two males attack another pair of two males with a weapon. The attackers are fought off and the weapon remains. Why does the police require DNA samples of the two who have been attacked to be able to find out if the two attackers are in the national criminal database?

Or without the specific scenario: Can a DNA profile be created from a sample where several individuals have been involved and it is not possible to separate the cells and assign them clearly so that this profile can then be searched for in a database?

Disclaimer: This happened to me and several years later, I’m simply curious why I had to give a sample to help police find the perpetrators.