Is it possible to fetch unique dedicated Business Code for my clients registration?

If i would be you, i would probably do something like this....

I would add a additional data-field into my db, which would detect all already used codes, when doing a DB-Query. --> I hope i did not misunderstand your wished functionality.

Of course everything can be generated by code, that means...

-everytime when you a new registration, you also write corresponding values into your DB (update, save or insert).

@Velo-Ninja Sir, Kindly elaborate on this. How to DB-Query already used code by additional data-field? Kindly update my code on this. It will be very helpful if you could update.

@SANAWAD ONLINE

For me it is a little bit difficult understand all the flow of your project, because i do not see all of your DBs and single steps. It would turn into a SERVICE if i would write code for you.

But what you can do yourself? You can use --> CONSOLE to investigate your own code.

Take a look onto this expanded example of your code....

import wixData from 'wix-data';
 //--------------- User-Interface -------------------
const COLLECTION1 = "collection";
const COLLECTION2 = "BusinessRegister1";
const LIMIT = 1000;

$w.onReady(function() {uniqueDropDown1();
    $w('#dropdown1').onchange((event)=>{console.log(event.target.id+"-clicked!");
        create_uniqueDropDown2();
        $w("#dropdown2").enable(); console.log("DropDow-2 enabled!");
    });
    //-----------------
    $w('#dropdown2').onchange((event)=>{console.log(event.target.id+"-clicked!");
        create_uniqueDropDown3();
        $w("#dropdown3").enable(); console.log("DropDow-3 enabled!");
    });
    //-----------------
    $w('#dropdown3').onchange((event)=>{console.log(event.target.id+"-clicked!");
        create_uniqueDropDown4();
        $w("#dropdown4").enable(); console.log("DropDow-4 enabled!");
    });
});

//---------------------- [ FUNCTIONS ] ----------------------------------------------------
function uniqueDropDown1 (){ console.log("Generating Unique-DropDown-1 running...");
    wixData.query(COLLECTION1)
    .limit(LIMIT)
    .find()
    .then(results=> {console.log("RESULTS: ", results.items);
        const uniqueTitles = getUniqueTitles(results.items);
        $w("#dropdown1").options = buildOptions(uniqueTitles);
    });

    function getUniqueTitles(items) {
        const titlesOnly = items.map(item => item.userType);
        return [...new Set(titlesOnly)];
    }
    function buildOptions(uniqueList) {
        return uniqueList.map(curr => {
            return {label:curr, value:curr};
        });
    }
}
 
function create_uniqueDropDown2() {
    wixData.query(COLLECTION1)
    .contains("userType", $w("#dropdown1").value)
    .limit(LIMIT)
    .find()
    .then(results=> {console.log("RESULTS: ", results.items);
        const uniqueTitles = getUniqueTitles(results.items);
        $w("#dropdown2").options = buildOptions(uniqueTitles);
    });
    function getUniqueTitles(items) {
        const titlesOnly = items.map(item => item.category);
        return [...new Set(titlesOnly)];
    }
    function buildOptions(uniqueList) {
        return uniqueList.map(curr => {
            return {label:curr, value:curr};
        });
    }
}
     
function create_uniqueDropDown3() {
    wixData.query(COLLECTION1)
    .contains("category", $w("#dropdown2").value)
    .limit(LIMIT)
    .find()
    .then(results=> {console.log("RESULTS: ", results.items);
        const uniqueTitles = getUniqueTitles(results.items);
        $w("#dropdown3").options = buildOptions(uniqueTitles);
    });
    function getUniqueTitles(items) {
        const titlesOnly = items.map(item => item.subCategory);
     return [...new Set(titlesOnly)];
    }
    function buildOptions(uniqueList) {
        return uniqueList.map(curr => {
            return {label:curr, value:curr};
        });
    }
}
 
function create_uniqueDropDown4() {
    wixData.query(COLLECTION2)
    .contains("subCategory", $w("#dropdown3").value)
    .limit(LIMIT)
    .find()
    .then(results => {console.log("RESULTS: ", results.items);
        const uniqueTitles = getUniqueTitles(results.items);
        $w("#dropdown4").options = buildOptions(uniqueTitles);
    });
    function getUniqueTitles(items) {
        const titlesOnly = items.map(item => item.businessCode);
        return [...new Set(titlesOnly)];
    }
    function buildOptions(uniqueList) {
        return uniqueList.map(curr => {
            return {label:curr, value:curr};
        });
    }
}

You should always work with CONSOLE, especialy if you have issues!

You just showing the code for your unique generated dropdowns, which will fill your dropdowns automaticaly from the corresponded DATABASES/COLLECTIONS.

I see no if-else-queries inside your code and also no saving-processes. All the given code right now, is just for the filling of your dropdowns.