What MDX can and SQL Can't perform on the same datatbase Can anyone explain or show me example(s) what MDX can do and SQL can not do on same database? please...
It's not a matter of what one can do vs the other, as you can always, in theory, write SQL to replace your MDX. It's a matter of how hard it is.

Example:

If you want the sales per month of your top 5 clients from last year in EMEA, your MDX query is something like

SELECT TopCount( [Clients].Members, 5, ( [Measures].[Sales], [Time].[2018] ) ) on Rows,
[Time].[2018].Children on Columns
from [Sales]
where [Geography].[EMEA]


and your cube definition will take care of what columns to query from which tables, getting the Territory name from the dim_territory table, the customer name from the customer table, etc.

The equivalent in SQL would be something like

Select c.customer_name, t.month_name, sum(f.sales)
from fact_sales f, dim_customer c, dim_time t, dim_geography g
where f.time_id = t.time_id 
  and f.customer_id = c.customer_id 
  and f.geography_id = g.geography_id
  and g.territory = 'EMEA'
  and t.year= 2018
  and c.customer_name in (
    select c.customer_name, sum(f.sales)
    from fact_sales f, dim_customer c, dim_time t
    where f.customer_id = c.customer_id 
      and f.time_id = t.time_id
      and t.year = 2018
    group by t.year
    limit 5
  )
group by t.year, t.month_name
order by t.month_number


Not only the SQL query is much more complex to write (I'm not even sure what I wrote is correct, tbh), but a lot of variations that in MDX just require a small rewrite, in SQL they will require several nested queries, and lots of filtering, grouping etc.

Plus, MDX allows you a cross-tab result, where you get one column for each month (or whatever dimension level you choose), whereas in SQL it will return a normalised dataset and you'll have to pivot it on the client side if you want it in tabular format.