The Language of SQL- P32 pptx

Here’s the generalSELECTstatement that we have seen: SELECT columnlist FROM tablelist WHERE condition GROUP BY columnlist HAVING condition ORDER BY columnlist Subqueries can be inserted

In Chapter 4, we talked about composite functions These were functions that contained other functions In a similar manner, it is possible for SQL queries to contain other queries The queries that are contained within other queries are

called subqueries.

The topic of subqueries is somewhat complex, primarily because there are many different ways in which they can be used Subqueries can be found in many dif-ferent parts of theSELECTstatement, each with different nuances and require-ments As a query contained within another query, subqueries can be related to and dependent on the main query, or they can be completely independent of the main query Again, this distinction results in different usage requirements

No matter how subqueries are used, they add a great deal of flexibility to the ways

in which you can write SQL queries It many cases, subqueries provide func-tionality that could be accomplished by other means In such instances, personal preferences will come into play as you decide whether or not you want to utilize the subquery solution However, as you will see, there are certain situations where subqueries are absolutely essential for the task at hand

With that, let’s begin our discussion with an outline of the basic types of subqueries

Types of Subqueries

Subqueries can be utilized not only with SELECT statements, but also with the INSERT, UPDATE, and DELETE statements, which will be covered in

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Chapter 17 In this chapter, however, we’re going to restrict our discussion of subqueries to theSELECT

Here’s the generalSELECTstatement that we have seen:

SELECT columnlist

FROM tablelist

WHERE condition

GROUP BY columnlist

HAVING condition

ORDER BY columnlist

Subqueries can be inserted into virtually any of the clauses in the SELECT

statement However, the way in which the subquery is stated and used

varies slightly, depending on whether it is used in a columnlist, a tablelist, or a condition.

But what exactly is a subquery? A subquery is merely aSELECTstatement that is inserted inside another SELECT statement Additionally, a SELECT statement can have more than one subquery

To sum up, there are three main ways in which subqueries can be specified:

■ When a subquery is part of a tablelist, it specifies a data source.

■ When a subquery is part of a condition, it becomes part of the selection

criteria

■ When a subquery is part of a columnlist, it creates a single calculated

column

The remainder of this chapter will explain each of these three scenarios in detail

Using a Subquery as a Data Source

When a subquery is specified as part of theFROMclause, it instantly creates a new data source This is similar to the concept of creating a view and then referencing that view in aSELECT The only difference is that a view is permanently saved in

a database A subquery used as a data source isn’t saved It exists only tempora-rily, as part of theSELECTstatement

Chapter 14 ■ Subqueries

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We will first consider an example that illustrates how subqueries can be used as

data sources Let’s say we have this Customers table:

CustomerID CustomerName

Also, this Orders table:

OrderID CustomerID OrderAmount OrderType

We would like to see a list of customers, along with a total sum of the cash orders

they have placed The followingSELECTaccomplishes that task:

SELECT

CustomerName AS 'Customer Name',

ISNULL (CashOrders.SumOfOrders, 0) AS 'Total Cash Orders'

FROM Customers

LEFT JOIN

(SELECT

CustomerID,

SUM (OrderAmount) as 'SumOfOrders'

FROM Orders

WHERE OrderType ¼ 'Cash'

GROUP BY CustomerID) AS CashOrders

ON Customers.CustomerID ¼ CashOrders.CustomerID

ORDER BY Customers.CustomerID

Two blank lines were inserted to clearly separate the subquery from the rest of

the statement The subquery is the middle section of the statement

The results were:

Customer Name Total Cash Orders

Adam Petrie shows no cash orders, since he didn’t place any orders Even though Brenda Harper placed two orders, they were both credit orders, so she also shows

no cash orders Note that theISNULL function converts the NULL values that would normally appear for Adam and Brenda to a 0

Let’s now analyze how the subquery works The subquery in the previous state-ment is:

SELECT

CustomerID,

SUM (OrderAmount) as 'Total Cash Orders'

FROM Orders

WHERE OrderType ¼ 'Cash'

GROUP BY CustomerID

In general form, the mainSELECTstatement in the above is:

SELECT

CustomerName AS 'Customer Name',

ISNULL (OrderCounts.SumOfOrders, 0) AS 'Total Cash Orders'

FROM

Customers

INNER JOIN

(subquery) AS CashOrders

ON Customers.CustomerID ¼ CashOrders.CustomerID

ORDER BY Customers.CustomerID

If the subquery were executed by itself, the results would be:

CustomerID SumOfOrders

Chapter 14 ■ Subqueries

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We only see data for customers 1 and 2 The WHERE clause in the subquery

enforces the restriction that we only look at cash orders

In this example, the entire subquery is referenced as if it were a separate table or

view Notice that the subquery is given a table alias of CashOrders, which allows

the columns in the subquery to be referenced in the mainSELECT As such, the

following line in the mainSELECTreferences data in the subquery:

ISNULL (CashOrders.SumOfOrders, 0) AS 'Total Cash Orders'

CashOrders.SumOfOrders is a column that is taken from the subquery

Is it truly necessary to use a subquery to obtain the desired data? In this case, the

answer is yes We might have attempted to simply join the Customers and

Orders tables via a left join, as in the following:

SELECT

CustomerName AS 'Customer Name',

Sum (OrderAmount) AS 'Total Cash Orders'

FROM Customers

LEFT JOIN Orders

ON Customers.CustomerID ¼ Orders.CustomerID

WHERE OrderType ¼ 'Cash'

GROUP BY Customers.CustomerID, CustomerName

ORDER BY Customers.CustomerID

However, this statement yields the following data:

Customer Name Total Cash Orders

We no longer see any rows for Brenda Harper or Adam Petrie, because theWHERE

clause exclusion for cash orders is now in the main query rather than in a subquery

As a result, we no longer see any rows for customers who didn’t place cash orders

Using a Subquery in Selection Criteria

In Chapter 8, we introduced the first format of theINoperator The example we

used was:

WHERE State IN ('IL', 'NY' )