As an example I have such records in my DB:

```
product_id
------------------------------------
887349c7-699e-4792-adde-3a44acd2d768
887349c7-699e-4792-adde-3a44acd2d768
887349c7-699e-4792-adde-3a44acd2d768
ecdcf200-6bc0-4b67-b47a-5946a1ab1dcf
ecdcf200-6bc0-4b67-b47a-5946a1ab1dcf
01a5419d-aaf8-4524-99ce-9bf5c1836823
01a5419d-aaf8-4524-99ce-9bf5c1836823

```

How can I get (using `MySQL` or `Knex.js` syntax) every `product_id`, but only 1, because I have copies (it's okay, this is how it has to be):

```
product_id
------------------------------------
887349c7-699e-4792-adde-3a44acd2d768
ecdcf200-6bc0-4b67-b47a-5946a1ab1dcf
01a5419d-aaf8-4524-99ce-9bf5c1836823

```