The SQL ORDER BY clause

A short video covering the ORDER BY clause within a SQL SELECT statement.

For beginners. How to use the ORDER BY clause in the SELECT statement. Covers multiple sort keys, ascending, descending, how NULLs are handled, and best practice.

Here is the SQL that goes with the demo.

-- Database by Doug
-- Douglas Kline
-- 9/16/2015
-- all about ORDER BY

USE Northwind

-- refresher

-- the result of a SELECT statement is a table
-- it has columns and rows, defined by the SELECT statement


SELECT ProductName,
       ProductID, 
       supplierID
FROM   Products
WHERE  unitprice > 10

-- the SELECT statement above creates a two-column, many-row table

-- the SELECT *clause* defines the which columns are in the resulting table, and their order

-- the WHERE clause defines which rows are in the resulting table

-- refresher over...

-- but what about the the *order* of the rows?

-- is there a default ordering?

SELECT ProductName
FROM   Products

SELECT ProductName,
       ProductID
FROM   Products

-- looking at the results, it seems like they are ordered by productID
 -- let's try the above in different orders of operation?

 -- changes in ordering of the rows, even though the actual records included do not change

 -- in short, the database engine will return the records in the most convenient order (for itself)
 -- it will NOT spend any extra time ordering rows for YOUR convenience, unless you ask

 -- so here's how you do it - the ORDER BY clause

 SELECT   ProductName
 FROM     Products
 WHERE    unitprice > 10
 ORDER BY ProductName ASC

  -- the ORDER BY clause always appears at the end of the SELECT statement

 -- the query above orders by the productname in ascending order

 -- the ordering is alphabetical, because ProductName is not numeric - it is character-based

 SELECT   ProductName
 FROM     Products
 WHERE    unitprice > 10
 ORDER BY ProductName DESC

 -- orders descending

-- in these examples, productName is the sort key, used to order either ascending or descending

-- by default, the ordering of a sort key is ascending
 
 SELECT   ProductName
 FROM     Products
 WHERE    unitprice > 10
 ORDER BY ProductName 

 -- so the above and the below are equivalent

 SELECT   ProductName
 FROM     Products
 WHERE    unitprice > 10
 ORDER BY ProductName ASC


 -- another example, with numeric data

SELECT   ProductName,
         unitprice
FROM     Products
ORDER BY unitprice DESC

-- " show the products with the most expensive at the top"

-- notice that the sort key does not need to be the first column

-- furthermore, the sort key need not be in any other clause of the SELECT statement


SELECT   ProductName,
         unitprice
FROM     Products
WHERE    reorderlevel > 10
ORDER BY productID ASC

-- note that each clause (SELECT, WHERE, ORDER BY) has different fields in it

-- However, the above query can be diconcerting to users
-- it is more common to have the sort keys in the SELECT clause, and in the same order as in the SELECT clause

-- like this
SELECT   ProductID,
         ProductName,
         unitprice
FROM     Products
WHERE    reorderlevel > 10
ORDER BY productID ASC

-- now the ordering is clear to the user

-- it is also possible to use multiple sort keys

SELECT   SupplierID,
         ProductID,
         ProductName,
         unitprice
FROM     Products
ORDER BY SupplierID ASC,
         ProductID  ASC

-- this means...
-- sort first by the SupplierID, ascending
-- then, when multiple records have the same SupplierID, sort by ProductID, ascending

-- note also how NULLs are handled for numerics
-- NULLs are considered to be smaller than numeric values
SELECT   SupplierID,
         ProductID,
         ProductName,
         unitprice
FROM     Products
ORDER BY SupplierID DESC,
         ProductID  ASC

-- how about characters?

SELECT   QuantityPerUnit,
         ProductName
FROM     Products
ORDER BY QuantityPerUnit

-- again, NULLs are considered to be smaller than characters

-- for SQL embedded in systems, you should always include an order by
-- consumers of the SELECT results may assume an ordering and actually *depend* on it
-- furthermore, it is best to have a *unique* ordering
-- again, downstream systems may assume and *depend* on a unique ordering

-- a non-unique ordering:

SELECT    ProductName
FROM      Products
ORDER BY  ProductName

-- improvement to make the ordering unique:

SELECT    ProductName
FROM      Products
ORDER BY  ProductName,
          ProductID

-- all about ORDER BY, the end :-)