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# interpolate()

> Fill in missing values by linear interpolation

<Icon icon="tag" iconType="duotone" /> Since [1.1.1][tsdb-1.1.1]

Fill in missing values by linear interpolation.
Use in the same query as [`time_bucket_gapfill`][time_bucket_gapfill].
`interpolate` cannot be nested inside another function call.

## Arguments

The syntax is:

```sql theme={"dark"}
SELECT interpolate(
    value = <anyelement>,
    prev = <anyelement>,
    next = <anyelement>
);
```

| Name    | Type                                                      | Default | Required | Description                                                                                                                                                                                                                                                                                                    |
| ------- | --------------------------------------------------------- | ------- | -------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `value` | SMALLINT \| INTEGER \| BIGINT \| REAL \| DOUBLE PRECISION | -       | ✔        | The value to interpolate                                                                                                                                                                                                                                                                                       |
| `prev`  | EXPRESSION                                                | -       | -        | If no previous value is available for gapfilling, use the `prev` lookup expression to get a previous value. For example, you can use `prev` to fill in the first bucket in a queried time range. The expression must return a `(time, value)` tuple with types corresponding to the bucketed times and values. |
| `next`  | EXPRESSION                                                | -       | -        | If no next value is available for gapfilling, use the `next` lookup expression to get a next value. For example, you can use `next` to fill in the last bucket in a queried time range. The expression must return a `(time, value)` tuple with types corresponding to the bucketed times and values.          |

## Returns

| Column        | Type                                                 | Description                                                          |
| ------------- | ---------------------------------------------------- | -------------------------------------------------------------------- |
| `interpolate` | SMALLINT, INTEGER, BIGINT, REAL, or DOUBLE PRECISION | The gapfilled value. The return type matches the input `value` type. |

[time_bucket_gapfill]: /api-reference/timescaledb/hyperfunctions/time_bucket_gapfill/time_bucket_gapfill

[tsdb-1.1.1]: https://github.com/timescale/timescaledb/releases/tag/1.1.1