## key()


A primary key: at most one row per combination of `cols`, and no NULL


Usage

``` python
key(*cols)
```


in any of them. `sv.key("id")` is `sv.unique("id")` and `'"id" IS NOT NULL'`, which it stands for in the contract and the report.


## Parameters


`*cols: str`  
The names of the key's columns.


## Returns


`Key`  
A `Key`, for `requires` or `ensures`.