A table is in (1NF) when every value is atomic: one value that you do not take apart in queries.
Two common ways to break 1NF:
member (a list in a cell)
| member_id | name | phones |
|---|---|---|
| 1 | Rina Sato | 555-0101, 555-0199 |
| 2 | Tomas Ferreira | 555-0102 |
member (a repeating group)
| member_id | name | phone1 | phone2 |
|---|---|---|---|
| 1 | Rina Sato | 555-0101 | 555-0199 |
| 2 | Tomas Ferreira | 555-0102 | NULL |
The fix
Phone number is a multivalued attribute (Entities and Attributes). Give it its own table, one row per value:
member_phone
| member_id | phone |
|---|---|
| 1 | 555-0101 |
| 1 | 555-0199 |
| 2 | 555-0102 |
Now "who has 555-0199?" is a plain WHERE phone = '555-0199', and a member can have any number of phones.