How to create alphabetic Sticky Header dynamically in flutter - header

Question

I am very junior in Flutter. I have a Sqflite database that dynamically creates a list of Customer company names, using Cards and Listiles. I can get the database to list the customer names in alphabetic order, but I have been struggling for days to try and create Sticky Headers that sorts the customer names under alphabetic headings. I have imported the sticky header packages, but this sort by each and every entry of a customer name. It should instead, as example, group all the customer names starting with letter 'A' under sticky header 'A', then all customer starting with letter 'B' under sticky header 'B', so on and so forth. It will be really appreciated is someone can perhaps help me resolve this issue.
Here is the code:
Widget build(BuildContext context) {
return Expanded(
child: FutureBuilder(
future: db.getTodo(),
initialData: const [],
builder: (BuildContext context, AsyncSnapshot<List> snapshot) {
var data = snapshot
.data; // this is the data we have to show. (list of todo)
var datalength = data!.length;
return datalength == 0
? const Center(
child: Text('no data found'),
)
: ListView.builder(
itemCount: datalength,
itemBuilder: (context, i) => StickyHeader(
header: Container(
width: double.infinity,
color: Colors.red,
padding: const EdgeInsets.all(16),
child: Text('Header $i'),
),
content: CustomerCard(
id: data[i].id,
title: data[i].title,
name: data[i].name,
phone: data[i].phone,
fax: data[i].fax,
email: data[i].email,
street: data[i].street,
city: data[i].city,
town: data[i].town,
code: data[i].code,
isExpanded: data[i].isExpanded,
insertFunction: insertFunction,
deleteFunction: deleteFunction,
),
),
);
}),
In the above picture, you can see the headings in red. These headings should be able to be a letter A or B or C, all the way down to Z all depending on the customer names. Secondly, the header should group all customer names starting with the same letter, hence, group all the A's together.
Remember my list/map is dynamic and part of a SQFlite database.
Thank you so much. I have read extensively and can't seem to find a solution.