Originally, we join a table like this
| zip_code | data_val |
|---|---|
| 94105 | 123.45 |
| 94106 | 100.10 |
with one like this
| zip_code | city |
|---|---|
| 94105 | San Francisco |
| 94106 | San Francisco |
to yield the result
| zip_code | city | data_val |
|---|---|---|
| 94105 | San Francisco | 123.45 |
| 94106 | San Francisco | 100.10 |
...but it's faster (if less easy to maintain) to store that directly