## references()


A foreign key: every non-NULL value of `col` occurs in the column `to`


Usage

``` python
references(
    col,
    *,
    to=None,
)
```


of another table.

Only for `requires`, where it is checked on the data like [unique](unique.md#sverify.unique) (the referenced table must be bound to the same backend). With a key on the referenced column it lets `Rel.optimize` turn a left join on it into an inner join (`Report.optimized`). Not supported by incremental views.


## Parameters


`col: str`  
The referencing column of the input the condition is on.

`to: ir.Column | tuple[str, str] | None = None`  
The referenced column, keyword-only: an Ibis column of a named table (`customers.id`), or a `(table, column)` pair of names as [verify](verify.md#sverify.verify) and the backend know them.


## Returns


`References`  
A `References` condition, for `requires`.


## Raises


`TypeError`  
`to` is missing, or is not a column of a named table.


## Examples

``` python
sv.references("customer_id", to=customers.id)
sv.references("customer_id", to=("customers", "id"))
```