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Structured Query Language 1

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Visual Basic Table Access

Visual Basic provides 2 mechanisms to access data in tables:– Record-at-a-time access

MoveFirst, MoveLast,MoveNext, MovePrevious– Set-at-a-time access

Select, Insert, Update, Delete

Both methods can accomplish the same thing, but set-at-a-time is usually more efficient.

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Visual Basic Table Access

Set-at-a-time access means that you do not have to explicitly manipulate the record pointer or perform a loop.

This creates a non-procedural environment were you describe what the solution looks like, not how to do it.

The most popular set-at-a-time language is called Structured Query Language.

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Structured Query Language

Structured Query Language (SQL) is a 4th generation language designed to work with relational sets.

Commands exist to create and load tables, select subsets of tables, and modify existing tables.

SQL is not a full programming language. It is intended to be embedded in another, more traditional, language.

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Structured Query Language

SQL commands are divided into 3 primary groups:– Data Definition Language commands– Data Manipulation Language commands– Data Control Language commands

Visual Basic supports the first 2 categories very well. It does not have complete support for the 3rd group.

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Data Manipulation Language

Visual Basic DML commands include:– SELECT

Retrieve data from a database, create copies of tables, specify tuples for updating

– INSERTAdd data to tables

– UPDATEChange existing data in tables

– DELETERemove data from tables

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Generic Select Statement

By far, the most commonly used DML statement is the SELECT. It combines a range of functionality into one complex command. This is the generic format.

SELECT {field-list | * | DISTINCTROW field} FROM table-list WHERE expression GROUP BY group-fields

HAVING group-expressionORDER BY field-list;

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Select Clauses

SELECT – The list of “columns” to select from the table. Also allows you to select all (with *), or to remove duplicate rows (with DISTINCTROW).

FROM - A required clause that lists the tables that the select works on. You can define "alias" names with this clause to speed up query input and to allow recursive "self-joins".

WHERE - An optional clause that selects rows that meet the stated condition. A "sub-select" can appear as the expression of a where clause. This is called a "nested select".

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Select Clauses

GROUP BY - An optional clause that groups rows according to the values in one or more columns and sorts the results in ascending order (unless otherwise specified). The duplicate rows are not eliminated, rather they are consolidated into one row. This is similar to a control break in traditional

programming.

HAVING - An optional clause that is used with GROUP BY. It selects from the rows that result from applying the GROUP BY clause. This works the same as the WHERE clause, except that it only applies to the output of GROUP BY.

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Select Clauses

ORDER BY - An optional clause that sorts the final result of the SELECT into either ascending or descending order on one or more named columns.

There can be complex interaction between the WHERE, GROUP BY, and HAVING clauses. When all three are present the WHERE is done first, the GROUP BY is done second, and the HAVING is done last.

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Single Table Select Examples

Example 1: Retrieve all information about students

SELECT *

FROM STUDENT;

Example 2: Find the last name, Bear ID, and credits of all students

SELECT LAST_NAME, BEAR_NUM, CREDITS

FROM STUDENT;

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Select Examples

Example 3: Select all employees from the 'ACCT' department.

SELECT *

FROM EMPLOYEES

WHERE EMP-DEPT = 'ACCT';

Example 4: Show what salary would be if each employee received a 10% raise.

SELECT LAST_NAME, SALARY AS CURRENT, SALARY * 1.1 AS PROPOSED

FROM EMPLOYEES;

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Single Table Select Examples

Example 5: Find all information about students who are math majors

SELECT * FROM STUDENTWHERE MAJOR = 'Math';

Example 6: Find the Bear ID of all History majors

SELECT BEAR_NUMFROM STUDENTWHERE MAJOR = 'History';

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Enhanced Where Clauses

The WHERE clause can be enhanced to be more selective. Operators that can appear in WHERE conditions include:

=, <> ,< ,> ,>= ,<=INBETWEEN...AND...LIKEIS NULLAND, OR, NOT

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Single Table Select Examples

Example 1: Find the student ID of all math majors with more than 30 credit hours.

SELECT BEAR_NUMFROM STUDENTWHERE MAJOR = 'Math' AND CREDITS > 30;

Example 2: Find the student ID and last name of students with between 30 and 60 hours (inclusive).

SELECT BEAR_NUM, LAST_NAMEFROM STUDENTWHERE CREDITS BETWEEN 30 AND 60;

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Single Table Select Examples

Example 3: Retrieve the ID of all students who are either a math or an art major.

SELECT BEAR_NUMFROM STUDENTWHERE MAJOR IN ('Math','Art');

Example 4: Retrieve the ID and course number of all students without a grade in a class.

SELECT BEAR_NUM, COURSENUMFROM ENROLLWHERE GRADE IS NULL;

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Single Table Select Examples

Example 5: List the course number and faculty ID for all math courses.

SELECT COURSENUM, FACULTY_ID

FROM CLASS

WHERE COURSENUM LIKE 'MTH*';

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Aggregate Function Select

SQL also allows several aggregate functions to appear in the SELECT line of the SELECT statement. These include:

Max, Min, Avg, Sum, Count, StdDev, StdDevP, Var, VarP. Example 1: How many students are there?

SELECT COUNT(*)FROM STUDENT;

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Aggregate Function Select

Example 2: Find the number of departments that have faculty in them.SELECT COUNT(DISTINCTROW DEPT)

FROM FACULTY;

Example 3: Find the average number of credits for students who major in math.

SELECT AVG(CREDITS)

FROM STUDENT

WHERE MAJOR = 'Math';

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Ordering the Select Result

Example 1: List the names and IDs of all faculty members arranged in alphabetical order.

SELECT FACULTY_ID, FACULTY_NAMEFROM FACULTYORDER BY FACULTY_NAME;

Example 2: List names and IDs of faculty members.

SELECT FACULTY_ID, FACULTY_NAMEFROM FACULTYORDER BY FACULTY_NAME, FACULTY_ID DESC;

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SQL Join Operation

A JOIN operation is performed when more than one table is specified in the FROM clause. You join two tables if you need information from both.

You must specify the JOIN condition explicitly in SQL. This includes naming the columns the two tables have in common and the comparison operator.

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SQL Join Examples

Example 1: Find the name and courses that each faculty member teaches.

SELECT FACULTY.FACNAME, COURSENUMFROM FACULTY, CLASSWHERE FACULTY.FACID = CLASS.FACID;

Note how the table name is appended to the FACNAME field of the SELECT clause. This is called qualification. It is required if the same name is used in 2 tables.

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SQL Join Examples

Example 2: Find the course number and the major of all students taught by the faculty member with ID number 'F110'. (3 table JOIN)

SELECT ENROLL.COURSENUM, LAST_NAME, MAJOR

FROM CLASS , ENROLL, STUDENT

WHERE FACULTY_ID = 'F110'

AND CLASS.COURSENUM = ENROLL.COURSENUM

AND ENROLL.BEAR_NUM = STUDENT.BEAR_NUM;

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Views

Views are a way to save your select queries so that you do not have to build them each time you need them.

The view saves the procedure (not the result) for he query.

Views are a “free” form of security

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Views

Views are used to simplify queries and to provide security.

They are often called "virtual tables" because the table is not stored in the database. Instead, the procedure to derive the view is stored.

The view is generated whenever it is requested, thus it is always up-to-date and does not take up any disk space.

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Views

You build views by first creating a valid select and then adding one line of code before the select.

Any valid select can fill in the select portion. In all cases (except for update) views can be

used in the same was as select statements.

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Views

Example: Build a view called CLASS_LIST that contains the student IDs and last name for all students in the class 'ART103A'.

CREATE VIEW CLASS_LIST AS

SELECT BEAR_NUM, LAST_NAME

FROM ENROLL, STUDENT

WHERE COURSENUM = 'ART103A'

AND ENROLL.BEAR_NUM = STUDENT.BEAR_NUM;