Comparison Operators. Selecting Rows with Conditional Restrictions

Size: px

Start display at page:

Download "Comparison Operators. Selecting Rows with Conditional Restrictions"

Marianna Bailey
6 years ago
Views:

1 Selecting Rows with Conditional Restrictions You have learnt, so far, how to list all the rows from a table using the SELECT command with the asterisk character as a wildcard. Of course, you might need content facing specific criteria. You can select partial table content by placing restrictions on the rows to be included in the output. Use the WHERE clause to add conditional restrictions to the SELECT statement. Syntax: SELECT FROM [WHERE columnlist tablelist conditionlist]; List all information for the student with faculty number WHERE STUD_FACNUMBER = Numerous conditional restrictions can be placed on the selected table contents. For example, the comparison operators shown in Table 123 can be used to restrict output. Table 123 Comparison Operators SYMBOL MEANING = Equal to < Less than <= Less than or equal to > Greater than >= Greater than or equal to < > or!= Not equal to Using Comparison Operators on Character Attributes Computers identify all characters by their numeric ASCII (American Standard Code for Information Interchange) codes. Therefore, characters can be compared, according to their ASCII code order. For example B is greater than A, and C is greater than B. String comparisons are made from left to right. FROM WHERE STUDENT STUD_LNAME > D

2 This query s output table will contain all info about students with last name starting with D, E, F and etc. It will omit last names starting with B. How to fine tune such searches will be discussed later. Using Comparison Operators on Date Attributes List all students born after 01. Jan WHERE STUD_DOB > # # Arithmetic Operators: The Rule of Precedence The Arithmetic Operators ARITHMETIC OPERATOR DESCRIPTION + Add - Subtract * Multiply / Divide ^ Rise the power of Table 231 Rules of precedence 1. Perform operators within parentheses 2. Perform power operations 3. Perform multiplications and divisions 4. Perform additions and subtractions Logical operators: AND, OR and NOT Logical operators allow you to include multiple search conditions. 1. List all students whose last name are Smith and are born after 20. Jan S WHERE STUD_LNAME = Smith AND STUD_DOB > #01/20/0988# NOTE Date format: mm/dd/yy # delimiters are used for dates in MS Access 2. List all students from Detroit or New York. S WHERE STUD_CITY = Detroit OR STUD_CITY = New York 3. List all who are not from New York. S WHERE NOT( STUD_CITY = New York )

4 WHERE STUD_NAME LIKE Johns?n IN: Used to check whether an attribute value within a value list The following query can be handled more efficiently with IN operator WHERE STUD_FACNUMBER = OR STUD_FACNUMBER = Revised query: WHERE STUD_FACNUMBER IN (123456, ) NOTE: All of the values in the list must be of the same data type EXIST: Used to check whether a subqery returns any rows The EXIST special operator can be used whenever there is a requirement to execute a command based on the result of another query. That is, a subquery returns any rows, run the main query; otherwise, do not. For example, the following query will list all vendors, but only if there are products to order: FROM VENDOR WHERE EXIST ( FROM PRODUCT WHERE P_QOH < P_MIN) Problems: Use figure P1 to solve the following problems: 1. Using table EMP_2 created in previous lab, write the SQL code that will list all attributes for a job code of Write the SQL code to delete the row for the person named William Smithfield, who was hired on June 22, 2004, and whose job code classification is 500. (Hint: Use logical operators to include all of the information given in this problem.) 3. Using a single command sequence, write the SQL code that will change the project number (PROJ_NUM) to 25 for all employees whose job classification (JOB_CODE) is 502 or higher. 4. Write the SQL code that will change the PROJ_NUM to 18 for those employees who were hired before January 1, 1994 and whose job code is at least Write the SQL code required to list all employees whose last names start with Smith. In other words, the rows for both Smith and Smithfield should be included in the listing.

6. List all database designers and programmers hired after 1990.

Use table EMPLOYEE write a query to list all employees whose last name starts with A. 8.

5 6. List all database designers and programmers hired after (Hint: Check the job codes from table JOB) Figure P1 ConstructCo database 7. Use table EMPLOYEE write a query to list all employees whose last name starts with A. 8. Use table JOB, write a query to list all jobs containing the words Designer or Engineer in their description. 9. List all employees whose job code is 501,502 or 503. (Hint: Use IN command) 10. List all projects with balance greater than

Some Basic Aggregate Functions FUNCTION OUTPUT The number of rows containing non-null values The maximum attribute value encountered in a given column

SQL Functions Aggregate Functions Some Basic Aggregate Functions OUTPUT COUNT() The number of rows containing non-null values MIN() The minimum attribute value encountered in a given column MAX() The maximum