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Can I reuse a calculated field in a SELECT query?

Posted by: admin November 1, 2017 Leave a comment

Questions:

Is there a way to reuse a calculated field within a mysql statement. I get the error “unknown column total_sale” for:

SELECT 
    s.f1 + s.f2 as total_sale, 
    s.f1 / total_sale as f1_percent
FROM sales s

or do I have to repeat the calculation, which would make for a very long SQL statement if I added all the calculations I need.

SELECT 
    s.f1 + s.f2 as total_sale, 
    s.f1 / (s.f1 + s.f2) as f1_percent
FROM sales s

of course I can do all the calculations in my php program.

Answers:

Yes, you can reuse variables. This is how you do it:

SELECT 
    @total_sale := s.f1 + s.f2 as total_sale, 
    s.f1 / @total_sale as f1_percent
FROM sales s

Read more about it here: http://dev.mysql.com/doc/refman/5.0/en/user-variables.html

[Note: This behavior is undefined. According to the MySQL docs:]

As a general rule, you should never assign a value to a user variable and read the value within the same statement. You might get the results you expect, but this is not guaranteed.

Questions:
Answers:

The following seems to work well in my testing on MySQL 5.5:

SELECT 
    s.f1 + s.f2 as total_sale, 
    s.f1 / (SELECT total_sale) as f1_percent
FROM sales s

Questions:
Answers:

Only cross-platform supported means is by using a derived table/inline view:

SELECT x.total_sale,
       x.f1 / x.total_sale as f1_percent
  FROM (SELECT s.f1,
               s.f1 + s.f2 as total_sale, 
          FROM sales s) x

Questions:
Answers:

You can use a sub-select:

select tbl1.total_sale,
       tbl1.f1/tbl1.total_sale as f1_percent 
  from (select s.f1+s.f2 AS total_sale, 
               s.f1 
          from sales s) as tbl1;

Questions:
Answers:

You can use subqueries, like this:

SELECT 
    h.total_sale, 
    s.f1 / h.total_sale AS f1_percent
FROM sales s,
    (SELECT id, f1 + f2 AS total_sale FROM sales) h
WHERE
    s.id = h.id

Edit:
fixed cartesian product, assuming the primary key is id.
This should be equivalent to OMG Ponies’ solution after optimizing, but I think it will become harder to read if you need more subqueries.

Questions:
Answers:

I have been testing the following and it seems to work all the time, maybe there is a reason for this, is it because I have predefined the variable @total_sales as being a value not a string, and have not redefined its type during the select statement ??

If I do the following

    set @total_sales = 0;

    SELECT 
       @total_sale := s.f1 + s.f2 as total_sale, 
      s.f1 / @total_sale as f1_percent
   FROM sales s

Questions:
Answers:

I had a look at various answers here and did a few experiments.

Specifically I am using MariaDB 10.1.

For a “simple” thing you can do what Robert D suggested in his comment:

SELECT Price_Per_SqFt, (Price_Per_SqFt/2) AS col1, (SELECT col1 + 1) AS col2 FROM Items 

If you are using some sort of aggregate function with an inner join you can’t use this, but you can combine this approach with the inner join approach as follows (NB VAT = “sales tax”… and NB in financial data currency fields typically have 4 decimal places, I think it’s historic…)

SELECT 
    invoices.invoiceNo, invoices.clientID, invoices.Date, invoices.Paid, 
  invoicesWithSubtotal.Subtotal,
  ROUND( CAST( Subtotal * invoices.VATRate AS DECIMAL( 10, 4 )), 2 ) AS VAT,
  (SELECT VAT + Subtotal) AS Total
FROM invoices 
    INNER JOIN 
    ( SELECT Sum( invoiceitems.Charge ) AS Subtotal, invoices.InvoiceNo FROM invoices 
        INNER JOIN invoiceitems ON invoices.InvoiceNo = invoiceitems.InvoiceNo
            GROUP BY invoices.InvoiceNo ) invoicesWithSubtotal
    ON invoices.InvoiceNo = invoicesWithSubtotal.InvoiceNo

I wanted to use the above to create a View to list invoices with their subtotals, VAT and totals… it turned out that MariaDB (and almost certainly MySQL) don’t allow nesting in the FROM clause. However this is easily solved by making a first View which lists the InvoiceNo and Subtotal, and then making a second View which references the first. Performance-wise I have no idea at all about this sort of double-View arrangement.