How can I create a row filter using a mapping table to restrict access to specific users in a Databricks table?
To create a row filter using a mapping table to restrict access to specific users in a Databricks table, you can follow these steps:

1. Create a mapping table with the usernames that should have access to the data. For example:

```sql
USE CATALOG main;
DROP TABLE IF EXISTS valid_users;
CREATE TABLE valid_users(username string);
INSERT INTO valid_users VALUES ('fred@databricks.com'), ('barney@databricks.com');
```

2. Create a new filter function that checks if the current user is in the valid_users table. If the user is found, the function returns true.

```sql
DROP FUNCTION IF EXISTS row_filter;
CREATE FUNCTION row_filter() RETURN BOOLEAN
BEGIN
  DECLARE @found BOOLEAN;
  SET @found = EXISTS(SELECT 1 FROM valid_users v WHERE v.username = CURRENT_USER());
  RETURN @found;
END;
```

3. Apply the row filter during table creation or add it later using an ALTER TABLE statement. When applying to a whole table, use the ON () syntax. For a specific row, use ON (row).

```sql
DROP TABLE IF EXISTS data_table;
CREATE TABLE data_table (x INT, y INT, z INT) WITH ROW FILTER row_filter ON ();
INSERT INTO data_table VALUES (1, 2, 3), (4, 5, 6), (7, 8, 9);
```

4. Now, users who are not in the valid_users table will not be able to access the data in the data_table.

Please note that this example is based on the information provided in the documentation, and you should adapt it to your specific use case. Also, keep in mind the limitations and supportability mentioned in the documentation.