How do you use the lag function in SQL?

The `LAG` function in SQL is a window function that provides access to a row at a specified physical offset prior to the current row within the result set. This is useful for comparing values with their previous rows. The basic syntax of the `LAG` function is as follows:

```sql
LAG(column_expression [, offset [, default_value]]) OVER (PARTITION BY partition_expression ORDER BY sort_expression)
```

- `column_expression`: The column whose previous value you want to retrieve.
- `offset` (optional): The number of rows back from the current row to retrieve the value. If not specified, it defaults to 1.
- `default_value` (optional): The value to return when the offset goes beyond the first row. If not specified, it defaults to `NULL`.
- `PARTITION BY`: Divides the result set into partitions to which the `LAG` function is applied independently.
- `ORDER BY`: Specifies the order of rows within each partition.

Here's a simple example to illustrate how to use the `LAG` function:

```sql
SELECT
    column_name,
    column_value,
    LAG(column_value) OVER (ORDER BY column_name) AS previous_value
FROM
    your_table;
```

In this example, `column_value` is the column for which you want to retrieve the previous value, and `column_name` is used for ordering the rows.

Keep in mind that the `LAG` function is available in databases that support window functions, such as PostgreSQL, SQL Server, MySQL 8.0+, and others. If you are using an older version of MySQL that does not support window functions, you might need to use other techniques, such as self-joins or subqueries, to achieve similar results.