# lag

Returns the value from a row at a specified offset before the current row in its ordered partition.

`lag` uses the whole partition, regardless of a `ROWS`, `RANGE` or `GROUPS` frame. To look in the opposite direction, use [`lead`](https://coordinatordb.com/docs/reference/functions/window-functions/lead).

**Syntax**

```text
lag(value [, offset [, default]])
    OVER ([PARTITION BY expression, ...] ORDER BY expression, ...)
```

**Arguments**

* `value` — The expression evaluated at the target row.
* `offset` — The number of rows to move before the current row. Defaults to `1`. It is converted to an integer once for the query; a negative converted offset raises a SQL error. An offset of `0` selects the current row.
* `default` — The value returned when the target is outside the partition. Defaults to `NULL`. A supplied expression is evaluated against the current row.

**Returned value**
The target row's value, preserving its native type, or the fallback when no target row exists. A `NULL` at an existing target is returned as `NULL`; it does not select the fallback.

**Examples**
**Comparing daily sales**

```sql
CREATE TABLE sales (day INTEGER, amount INTEGER);
INSERT INTO sales VALUES (1, 120), (2, 150), (3, 90);
```

```sql
SELECT day, amount,
       lag(amount) OVER (ORDER BY day) AS adjacent_amount
FROM sales
ORDER BY day
FORMAT csv;
```

```text
1,120,\N
2,150,120
3,90,150
```

**Providing a fallback**

Use the current amount when there is no adjacent row:

```sql
SELECT day,
       lag(amount, 1, amount) OVER (ORDER BY day) AS adjacent_amount
FROM sales
ORDER BY day
FORMAT csv;
```

```text
1,120
2,120
3,150
```

See [window functions](https://coordinatordb.com/docs/reference/functions/window-functions) for partitioning, named windows and output ordering.