Beginning SQL

Chapter 9
282
MemberDetails.FirstName,
MemberDetails.LastName,
COUNT(*) - 1
FROM (Orders INNER JOIN MemberDetails
ON Orders.MemberId = MemberDetails.MemberId)
INNER JOIN SalesPerson
ON Orders.SalesPersonId = SalesPerson.SalesPersonId
GROUP BY SalesPerson.SalesPersonId,
SalesPerson.FirstName,
SalesPerson.LastName,
MemberDetails.MemberId,
MemberDetails.FirstName,
MemberDetails.LastName
HAVING COUNT(Orders.MemberId) > 1
ORDER BY SalesPerson.SalesPersonId
There’s very often more than one way to write a query. This section shows you one alternative way of
writing the same query you wrote in the previous Try It Out, though there may be even more ways. This
alternative way uses a self-join, with the Orders table joined to itself. Consider the following statement:
SELECT SalesPerson.SalesPersonId,
SalesPerson.FirstName,
SalesPerson.LastName,
MemberDetails.MemberId,
MemberDetails.FirstName,
MemberDetails.LastName,
COUNT(*) AS “Repeat Orders”
FROM ((Orders O1 INNER JOIN Orders O2
ON O1.MemberId = O2.MemberId AND O1.SalesPersonId = O2.SalesPersonId
AND O1.OrderId > O2.OrderId)
INNER JOIN MemberDetails
ON O1.MemberId = MemberDetails.MemberId)
INNER JOIN SalesPerson
ON O1.SalesPersonId = SalesPerson.SalesPersonId
GROUP BY SalesPerson.SalesPersonId,
SalesPerson.FirstName,
SalesPerson.LastName,
MemberDetails.MemberId,
MemberDetails.FirstName,
MemberDetails.LastName
ORDER BY SalesPerson.SalesPersonId;
The SELECT column list is virtually identical, except that repeat order values are now obtained using
COUNT(*), whereas before they were obtained with COUNT(*) – 1. The reason for this change is that
this query, you see shortly, returns only the repeat orders and not the first order placed by a member.
Also, because of this change, you don’t need a HAVING clause that limits records where each group has
more than one member.
The difference between this query and the query you previously wrote is in the FROM clause and its use
of a self-join. The Orders table is joined to itself, the first table being given an alias of O1 and the second
an alias of O2. The MemberId and SalesPersonId columns are joined on the basis of being the same, and
the OrderDate column is joined on the basis of O1’s OrderDate being after O2’s, which ensures that only
the repeat orders are included. If there’s only one order, then clearly O1.OrderDate can never be higher
than O2.OrderDate, as the order in O2 is the same record with identical values.