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PT2520 Unit 7: SQL Queries II
SQL
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SQL Overview
SQL is the language of relational databases.
It is used for every aspect of database development and management.
Anyone who works with relational databases is expected to have knowledge of SQL.
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History
SQL is the programming language used for accessing and manipulating data and objects in relational databases.
The first versions of SQL were developed by IBM in the 1970s.
SQL first became an ANSI standard in 1986 and an ISO standard in 1987.
There was a major revision to the standard in 1992.
Additional modifications were made in 1999, 2003 and 2006.
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Nature of SQL
SQL is a declarative language.
Procedural languages like C# or Java describe how to accomplish a task step by step.
In a declarative language you say what you want to do, not how.
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SQL Functionality
SQL is not case sensitive.
In some environments, SQL statements must end with a semicolon.
SQL is usually divided into two broad areas of functionality:
DDL (Data Definition Language)
DML (Data Manipulation Language)
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DDL
Data Definition language is the set of SQL keywords and commands used to create, alter, and remove database objects.
A example is the CREATE TABLE command:
CREATE TABLE TestTable (
TestID INT IDENTITY (1,1),
TestDescription NVARCHAR(255) )
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DML
Data Manipulation Language is the set of keywords and commands used to retrieve and modify data.
SELECT, UPDATE, INSERT and DELETE are the primary actions of DML.
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8. Starting a New Query Window
One way to start a new query window in SQL Server is to right-click the database folder in the object explorer window and select New Query from the context menu. It will open up a new query window.
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Select Statement
The SELECT statement is used to retrieve data from the database.
The basic syntax is:
SELECT <columnName>, <columnName>
FROM <TableName>
SELECT StudentFirstName, StudentLastName, StudentPhone
FROM Student
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Group By
When a SELECT clause includes an aggregate function and columns that are not a part of that function, you must use the GROUP BY keywords to group by each the non-included columns.
This is necessary because you are mixing functions that operate on multiple rows with columns that refer to values in individual rows only.
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Group By Example
SELECT TutorKey, COUNT(SessionTimeKey) AS [Total Sessions] FROM Session GROUP BY TutorKey
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Having
The HAVING keyword is used when there is an aggregate function in the criteria of a query.
SELECT TutorKey, COUNT(SessionTimeKey) AS [Total Sessions]
FROM Session
GROUP BY TutorKey
HAVING COUNT(SessionTimeKey)<4
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Joins
In database design and normalization, the data are broken into several discrete tables.
Joins are the mechanism for recombining the data into one result set.
We will look at three kinds of joins:
Inner joins
Equi joins
Outer joins
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14. Basic Inner Join Syntax
SELECT <column1, column2> FROM <table1> INNER JOIN <table2> ON <table1>.<column>=<table2>.<column
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Inner Joins
Inner joins return related records from each of the tables joined.
SELECT TutorLastName,
TutorFirstName,
SessionDateKey,
SessionTimeKey,
StudentKey
SessionStatus
FROM Tutor
INNER JOIN Session
ON Tutor.TutorKey = Session.TutorKey
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Equi Joins
Equi joins present an alternative way to perform inner joins. Some older RDMSs only support this alternative form. The example below also uses an alias for the table name.
SELECT t.TutorKey,
TutorLastName,
TutorFirstName,
SessionDateKey,
SessionTimeKey,
StudentKey
FROM Tutor t,
Session s
WHERE t.TutorKey = s.TutorKey
AND TutorLastName = ʻBrownʼ
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Outer Join Syntax
Outer joins return records that are not matched. The following query returns tutors that have no sessions scheduled.
SELECT <column1>, <column2>
FROM <table1>
LEFT OUTER JOIN <table2> ON <table1>.<column>=<table2>.<column>
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Outer Join Example
SELECT t.TutorKey, TutorLastName, SessionDateKey FROM Tutor t LEFT OUTER JOIN Session s ON t.TutorKey = s.TutorKey WHERE SessionDateKey IS Null
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Inserts
To insert a record into a table, you use the following syntax:
INSERT INTO <tablename>(<ColumnName>, <columnName>, ...)
VALUES(<value1>, <value2>, ...)
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Updates
Updates allow you to change existing records. The syntax is:
UPDATE <TableName>
SET <ColumnName> = <New Value>,
<ColumNmae>=<new value>
WHERE <ColumnName> = <criteria>
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Deletes
Deletes allow you to remove a record from a table:
DELETE FROM <TableName>
WHERE <columnName> = <criteria>
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22. Notes on Deletes and Updates
Deletes and updates are dangerous. If you do not specify a criterion, the update or delete will be applied to all the rows in a table.
Also, referential integrity may prevent a deletion. You cannot delete a parent that has children in another table.
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Creating a Trigger
Triggers are programs that are triggered by an event, typically INSERT, UPDATE or DELETE.
They can be used to enforce business rules that referential integrity and constraints alone cannot enforce.
The basic syntax for creating a trigger is:
CREATE TRIGGER <trigger_name> ON <table_name>
[FOR, AFTER, INSTEAD OF] [INSERT, UPDATE, DELETE] AS
{SQL Code}
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24. Documentation: Testing Plans
When testing the database, you should document all your SQL queries and their results.
On the next slide is a sample of a test table, showing the test and results.
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Sample Test Table
Rule to Test
Means of testing
Expected Result
Result
Return all students by Gender
SELECT StudentLastName, StudentFirstName, StudentGender
FROM Student
WHERE StudentGender='M'
Return all male students
Returned all male students
Return unduplicated count of students from tutoring sessions
SELECT Count(StudentID) FROM Session
SELECT Count(DISTINCT StudentID) FROM
Session
Return unduplicated students from session
Returns duplicated students
Returns unduplicated student Count
Return hours for student per month
SELECT Tutorkey,
MONTH(SessionDateKey) AS [Month],
YEAR(SessionDateKey) AS [Year],
((COUNT (SessionTimeKey))* 30.0)/60.0 AS [Hours]
FROM Session
GROUP BY TutorKey, MONTH(SessionDateKey), YEAR(SessionDateKey)
ORDER BY YEAR(SessionDateKey),MONTH(SessionDateKey)
Hours grouped by student and month
Returns hours grouped by student and month
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