## asof_join()


Match each left row with the latest right row at or before it.


Usage

``` python
asof_join(
    left,
    right,
    on,
    by=(),
    tolerance=None,
    *,
    key=None,
    how="left",
)
```


`on` is the time column of both tables; rows match only when their `by` columns are equal and `left.on - tolerance <= right.on <= left.on`. [key](key.md#sverify.key) identifies left rows (default: `by` and `on`); the left table must not repeat it. With `how="left"`, left rows without a match are kept with NULLs. The output has the left columns, then the right ones except `by`, with clashing names suffixed `_right` (so `on_right` is the time of the match).


## Parameters


`left: ir.Table`  
The table whose rows are matched; one output row per left row.

`right: ir.Table`  
The table to take the latest earlier row from. For the result to be determined, `requires` must give it a key on `(*by, on)`.

`on: str`  
The time column, of the same name in both tables.

`by: str | Sequence[str] = ()`  
Columns that must be equal for rows to match, as one name or a sequence. Default: none.

`tolerance: Any = None`  
How much earlier than the left row a match may be (an Ibis interval, or a number for numeric time columns). Default None: no limit. The join computes `on - tolerance`, which overflows at the type's smallest value; a contract infers the lower bound on the left `on` column that rules it out (#244).

`key: Sequence[str] | None = None`  
Columns that identify left rows. Default: `by` and `on`.

`how: Literal[``"left", `<span class="st">`"inner"``]`</span>` = ``"left"`  
`"left"` (the default) keeps left rows without a match, with NULLs; `"inner"` drops them.


## Returns


`ir.Table`  
An Ibis table expression, built from a range join and `ROW_NUMBER`.