... "AND stay_exercise (? не очень понятная переменная) IN " + excercise.toSqlQuery() + " other stuff "...

fun List<String>.toSqlQuery(): String {
    if(this.isEmpty()) return ""
    val query = StringBuilder
    forEachIndexed { i, ex ->
        query.append(ex)
        if (i + 1 != this.length()) query.append(",")
    }
    query.append(")")
    return query.toString()
}