Extract the date of the first entry — first

A helper function for extracting the first date of entry of (of one or all participant) of one sensor. Note that this function is specific to the first date of a sensor. After all, it wouldn't make sense to extract the first date for a participant of the accelerometer, while the first device measurement occurred a day later.

Usage

first_date(db, sensor, participant_id = NULL)

Arguments

db: A database connection to an m-Path Sense database.
sensor: The name of a sensor. See sensors for a list of available sensors.
participant_id: A character string identifying a single participant. Use get_participants to retrieve all participants from the database. Leave empty to get data for all participants.

Value

A string in the format 'YYYY-mm-dd' of the first entry date.

Examples

if (FALSE) { # \dontrun{
db <- open_db()
first_date(db, "Accelerometer", "12345")
} # }