Instead of executing 3 different queries you can use like shown below,
DB::table('delivery_sap')
->whereNotIn('cust', function ($query) {
$query->select('cust_name')->from('customer');
})
->whereNotIn('cust_no', function ($query) {
$query->select('cust_code')->from('customer');
})
->select('cust', 'cust_no')
->distinct('cust')
->get();
This code will give the exact same query which is asked in the question,
to check the query, use following code
DB::table('delivery_sap')
->whereNotIn('cust', function ($query) {
$query->select('cust_name')->from('customer');
})
->whereNotIn('cust_no', function ($query) {
$query->select('cust_code')->from('customer');
})
->select('cust', 'cust_no')
->distinct('cust')
->toSql();
Output will be,
select distinct `cust`, `cust_no` from `delivery_sap`
where `cust` not in (select `cust_name` from `customer`)
and `cust_no` not in (select `cust_code` from `customer`)