Read data WHERE condition with "NOT IN"

Chronoform v.6.1.4
I need a chronoform dropdown field which populates data from a DB table with read_data2 action.
I tried to put a condition like

username  NOT IN  {var:read_data1.Data1.username}  

in WHERE condition fields of second read_data ( read_data2) where read_data1 is another array with already booked usernames which is place above read_data2. But it throws an error 1064.
What is the correct syntax to put a WHERE NOT In condition . ?

{var:read_data1.Data1.username} will output a string, and NOT IN looks for an array.

Then how can we pass an 1D array of result from another read data action to NOT IN?

EDITED
​
{var:read_data1.[n].Data1.username}

ALSO: if you're only returning one entry to filter against, why are you using NOT IN instead of != ?

Thank you healyhatman
But this is also not working. My read_data debug is given below. I want to use this result in the next read_data action as
username NOT IN read_data4.Data4.empid array
​

Array
(
    [read_data4] => Array
        (
            [log] => Array
                (
                    [0] => SELECT `Data4`.`empid` AS `Data4.empid` FROM `elibrary_leaves` AS `Data4` LIMIT 100;
                )

            [var] => Array
                (
                    [0] => Array
                        (
                            [Data4] => Array
                                (
                                    [empid] => 10000877
                                )

                        )

                    [1] => Array
                        (
                            [Data4] => Array
                                (
                                    [empid] => 10007774
                                )

                        )

                    [2] => Array
                        (
                            [Data4] => Array
                                (
                                    [empid] => 10002748
                                )

                        )

                    [3] => Array
                        (
                            [Data4] => Array
                                (
                                    [empid] => 10003383
                                )

                        )

                    [4] => Array
                        (
                            [Data4] => Array
                                (
                                    [empid] => 10003395
                                )

                        )

                )

        )

)

​
both {var:read_data4.0.Data4.empid} and {var:read_data4.[0].Data4.empid} in WHERE NOT IN clause shows error.
​
I was able to use php code action with array_coloun() function to return single column array and use it as {var:php1} in NOT IN field. can't we produce same result through read_data action?

Not [a number] but literally [n] as in the letter n

Thank you healyhatman,
It worked. Sorry for a late reply.
It works in my case but I noticed that it behaves differently when DISTINCT phrase is used to retrieve fields for the NOT IN array.