The Language of SQL- P18 pptx

The AND Operator The following examples will all be taken from this Orders table: OrderID CustomerName State QuantityPurchased PricePerItem 1 William Smith IL 4 2.50 2 Natalie Lopez CA 1

Boolean Logic

In the previous chapter, we introduced the concept of selection criteria, but only in its simplest form We’re now going to expand on that concept to greatly enhance our ability to specify the rows that are returned from a SELECT This is where the pure logic of SQL comes into play In this chapter, we are going to introduce

a number of operators that will allow you to create complex logical expressions With these new capabilities, if a user comes to you and says that she wants a list

of all female customers who live in ZIP codes 60601 through 62999, but excluding anyone who’s under the age of 30 or who doesn’t have an email address, that will

be something you can provide

Complex Logical Conditions

The WHERE clause introduced in the previous chapter utilized only simple selection criteria You sawWHEREclauses such as:

WHERE QuantityPurchased ¼ 5

The condition expressed in thisWHEREclause is quite simple: It returns all rows where the QuantityPurchased column has a value of 5

In the real world, the selection of data is often far from straightforward Accordingly, let’s now turn our attention to ways of specifying some more complex logical conditions in your selection criteria

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The ability to devise complex logical conditions is sometimes called Boolean logic.

This term, taken from mathematics, refers to the ability to formulate complex conditions that are evaluated as either true or false In the aforementioned example, the condition QuantityPurchased ¼ 5 is evaluated as either true or false for each row in the table Obviously, you only want to see rows where the condition is evaluated as true

The principle keywords used to create complex Boolean logic areAND, OR, and

NOT These three operators are used to add additional functionality to theWHERE

clause In the proper combination, theAND, OR, and NOT operators, along with parentheses, can specify just about any logical expression you can imagine

The AND Operator

The following examples will all be taken from this Orders table:

OrderID CustomerName State QuantityPurchased PricePerItem

1 William Smith IL 4 2.50

2 Natalie Lopez CA 10 1.25

3 Brenda Harper NY 5 4.00

Here’s an example of aWHEREclause that uses theANDoperator:

SELECT

CustomerName,

QuantityPurchased

FROM Orders

WHERE QuantityPurchased > 3

AND QuantityPurchased < 7

TheANDclause means that all conditions must evaluate to true for the row to be selected

This SELECT specifies that the only rows to be retrieved are those where the QuantityPurchased is both greater than 3 and less than 7 Therefore, only these two rows are returned:

CustomerName QuantityPurchased

William Smith 4

Brenda Harper 5

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Notice that the row for Natalie Lopez is not returned Why? Natalie purchased a

quantity of 10, which, in fact, does satisfy the first condition (QuantityPurchased

> 3) However, the second condition (QuantityPurchased < 7) is not satisfied

and therefore is not true When using theANDoperator, all conditions specified

must be true

The OR Operator

Let’s now look at the OR operator The AND clause meant that all conditions

must evaluate to true for the row to be selected TheOR clause means that the

row will be selected if any of the conditions is determined to be true.

Here’s an example, taken from the same table:

SELECT

CustomerName,

QuantityPurchased,

PricePerItem

FROM Orders

WHERE QuantityPurchased > 8

OR PricePerItem > 3

TheSELECTreturns this data:

CustomerName QuantityPurchased PricePerItem

Natalie Lopez 10 1.25

Brenda Harper 5 4.00

Why are the rows for Natalie Lopez and Brenda Harper displayed, and not the

row for William Smith? The row for Natalie Lopez is selected because it meets the

requirements of the first condition (QuantityPurchased > 8) It doesn’t matter

that the second condition (PricePerItem > 3) isn’t true, because only one

con-dition needs to be true for anORcondition

Likewise, the row for Brenda Harper is selected because the second condition

(PricePerItem > 3) is true for that row The row for William Smith isn’t selected

because it doesn’t satisfy either of the two conditions

Using Parentheses

Let’s say that you are only interested in orders from customers from either the

state of Illinois or the state of California Additionally, you only want to see

orders where the quantity purchased is greater than 8 To satisfy this request, you put together thisSELECTstatement:

SELECT

CustomerName,

State,

QuantityPurchased

FROM Orders

WHERE State ¼ 'IL'

OR State ¼ 'CA'

AND QuantityPurchased > 8

When you execute this, you are expecting to get back only one row, for Natalie Lopez This is because you have two rows for customers in Illinois or California (Smith and Lopez) But only one of those (Lopez) has a quantity purchased greater than 8

However, when you execute this statement, you see:

CustomerName State QuantityPurchased

William Smith IL 4

Natalie Lopez CA 10

What went wrong? Why did you get back two rows instead of one? The answer lies with how SQL interprets theWHEREclause, which contains bothANDandOR

operators Like other computer languages, SQL has a predetermined order of evaluation, which specifies the order in which various operators are interpreted Unless told otherwise, SQL always processes the AND operator before the OR

operator So, in the previous statement, it first looks at the AND and evaluates the condition:

State ¼ 'CA'

AND QuantityPurchased > 8

The row that satisfies that condition is for Natalie Lopez It then evaluates theOR

operator, which allows for rows where the State equals IL That adds in the row for William Smith Therefore, it determines that both the William Smith and the Natalie Lopez rows meet the condition

Obviously, this isn’t what was meant This type of problem often comes up when

ANDandORoperators are combined in a singleWHEREclause The way to resolve

Chapter 8 ■ Boolean Logic

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the ambiguity is to use parentheses to specify the exact order of evaluation that

you would like Anything in parentheses is always evaluated first

Here’s how parentheses can be added to the previous SELECT to correct the

situation:

SELECT

CustomerName,

State,

QuantityPurchased

FROM Orders

WHERE (State ¼ 'IL'

OR State ¼ 'CA')

AND QuantityPurchased > 8

When this is executed, you now see this data:

CustomerName State QuantityPurchased

Natalie Lopez CA 10

The parentheses in theSELECT statement force theORexpression (State¼ ‘IL’

OR State¼ ‘CA’) to be evaluated first This produces the intended result

Multiple Sets of Parentheses

Let’s say that you want to select two different sets of rows from the Orders table:

first, rows for customers in New York, and second, rows for customers in Illinois

who have made a purchase with a quantity between 3 and 10 The following

SELECTaccomplishes this requirement:

SELECT

CustomerName,

State,

QuantityPurchased

FROM Orders

WHERE State ¼ 'NY'

OR (State ¼ 'IL'

AND (QuantityPurchased >= 3

AND QuantityPurchased <= 10))