IST 220 – Intro to DB Lab 2 Specifying Criteria in SELECT Statements

 We have learned in Lab 1  The SELECT statement can have up to six clauses  The first two clauses SELECT and FROM are always used  The ORDER BY clause, when used, will be the last one  In this lab, we will practice the WHERE clause, which  Comes right after the FROM clause, and  Specifies criteria: only rows meet the conditions will be retrieved Basic SELECT Syntax

Clauses in a SELECT Statement SELECT … FROM … WHERE … … ORDER BY …

 Many conditions can be specified using comparison operators (see list on right)  These operators work with all 3 types of data commonly in use:  Numbers  Dates (e.g., >= means on or after a date)  Text string (based on alphabetical order) Using Comparison Operators Comparison Operators These operators take 2 operands (or expressions) in the form Expr1 Operator Expr2 For instance, VenderState = ‘IA’ InvoiceTotal <> 0

 We will use the Invoices table, and retrieve data from the invoice number, date, and total amount columns  Q1: Retrieve invoices with a total amount greater than $1,000  Q2: Retrieve invoices with a date before Sept 1, 2009 Queries about Invoices

 Sometimes a query needs to use a compound condition with multiple comparisons linked with logical operators:  AND  OR  NOT  Q3: Retrieve invoices with a total amount greater than $1,000 but less than $2,500  Q4: Retrieve invoices with a total amount less than $1,000 or greater than $2,500 Using Logical Operators

 BETWEEN … AND …  Specifying an inclusive range with continuous values  Q5: Rewrite Q3 using this pattern (and notice the diff.)  IN (…, … [, …])  Specifying a list of discrete values  It can be to used to simplify compound conditions using OR (e.g., state = ‘CA’ or state = ‘DC’ or state = ‘IA’)  Q6: Rewrite the compound condition above using the IN syntax Some Useful Operators

 NULL is defined as a special value that stands for not available or not applicable  For instance, an invoice that has not been paid will have a NULL value stored in the PaymentDate field  The IS NULL phrase (and IS NOT NULL )  Null is not a specific value so you cannot use compare it using any of the comparison operators  Q7: Find out invoice number, vendor ID, and invoice total of all invoices that have not been paid. Dealing with NULL Value

 LIKE is used for matching strings  A pattern is specified instead of exact values  The following wildcard characters are often used:  %matches any string of zero or more characters  _ matches any one single character  Q8: list name, city, and state of all vendors that are located in cities with a name starting with the letters 'san' The LIKE Keyword