Transcript Chapter 10 - Databases

Chapter 10 – Database
Management
10.1 An Introduction to Databases
10.2 Relational Databases and SQL
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10.1 An Introduction to
Databases
• The Server Explorer
• Accessing a Database with a Data Table
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Database Terminology
• A table is a rectangular array of data.
• Each column of the table, called a field,
contains the same type of information.
• Each row, called a record, contains all the
information about one entry in the database.
• In a phone book, there are fields for name,
address, and phone number. Your entry in the
phone book is a record.
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Database Management
Software (DBMS)
• Used to create databases
• Databases can contain one or more
related tables
• Examples of DBMS include Access and
Oracle
• You should already know all of this from
your database course
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The Server Explorer
• Allows the programmer to view
information located on other computers
• Can also be used to view a database
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Using the Server Explorer
1. Place your mouse over the “Server Explorer” tab on the
left side of Visual Studio.
2. Right-click on “Data Connections”, and select “Add
Connection”.
3. In the Data Link Properties window that appears, click on
the “Provider” tab at the top. Select the item “Microsoft
Jet 4.0 OLE DB Provider” from the OLE DB Providers List
box, and then press the next button.
4. Click on the “...” button to the right of the first text box.
This will open up a file,browser that allows you to locate
any file then press Open.
5. Clear the contents of the “User name” text box.
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Server Explorer continued
6. Press the Test Connection button. The message box
stating “Test Connection Succeeded” will appear. Press
the OK button on that message box, and then press the
OK button on the Data Link Properties box.
7. An icon should appear in Server Explorer. Click on the +
sign to the left of the icon to expand this entry. Three
subentries will appear: Tables, Views, and Stored
Procedures.
8. Expand the Tables entry to reveal the subentries.
9. Expand an entry to reveal the fields of the table.
10. Double-click on a table to show the table in a grid.
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Demo – vbBooks.mdb
• Don’t really need this method for MS
Access databases as Access is on most
computers
• But remember that you can access just
about any type of database using the
Server Explorer!
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Accessing a Database with a
Data Table
• A DataTable object holds the contents of
a database table as a rectangular array.
• A data table is similar to a twodimensional array; it has rows and
columns.
• The following declares a DataTable
variable
Dim dt As New DataTable()
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Connecting with a DataTable
Dim dt As New DataTable()
Dim connStr As String = _
"Provider=Microsoft.Jet.OLEDB.4.0;" & _
"Data Source = vbBooks.mdb "
Dim sqlStr As String = "SELECT * FROM Titles"
Dim dataAdapter As New _
OleDb.OleDbDataAdapter(sqlStr, connStr)
dataAdapter.Fill(dt)
dataAdapter.Dispose()
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Connection String
Dim connStr As String = _
"Provider=Microsoft.Jet.OLEDB.4.0;"
& _
"Data Source = MEGACITIES.MDB "
• This string must be exact!
• The VB editor can help you! (if you know
what to look for (output box)
• Cut and paste if possible
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Properties of the DataTable
• After these six lines of code are
executed, the number of records in the
table is given by dt.Rows.Count
• The number of fields in the table is given
by dt.Columns.Count
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Properties of the DataTable
• The records are numbered 0 through
dt.Rows.Count – 1
• The fields are numbered 0 through
dt.Columns.Count – 1
(just like arrays – numbering starts
at 0)
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More Properties
• The name of the jth field is given by
dt.Columns(j)
• The entry in the jth field of the ith record
is
dt.Rows(i)(j)
• The string containing the entry in the
specified field of the ith record is
dt.Rows(i)(“fieldName”)
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Bound Controls
• A data table bound to a list box can transfer
information automatically into the list box.
• The following statement binds a list box to a
data table:
lstBox.DataSource = dt
• The contents of a specified table can be
displayed in the list box by:
lstBox.DisplayMember = “Author"
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Demo – DBtextbooks
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Exercises p. 502
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10.2 Relational Databases
and SQL
• Primary and Foreign Keys (quick review)
• SQL Requests
• The DataGrid Control
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Primary Keys
• A primary key is used to uniquely
identify each record.
• Databases of student enrollments in a
college usually use a field of Student
Number as the primary key.
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Primary Key Fields
• VB.NET will insist that every record have
an entry in the primary-key field and that
the same entry does not appear in two
different records.
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Related Tables
• Two or more tables are usually related.
• vbBooks.mdb – the two tables Titles and
Publishers are related by their PubID
field.
• Publishers.PubID is a primary key.
• We say that Titles.PubID is a foreign
key of Publishers. PubID
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Foreign Keys
• Foreign keys can be specified when a
table is first created. VB.NET will insist
on the Rule of Referential Integrity.
• This Rule says that each value in the
foreign key must also appear in the
primary key of the other table.
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Join
• A foreign key allows VB.NET to link (or
join) together two tables from a relational
database
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SQL
• Structured Query Language developed
for use with relational databases
• Very powerful language
• Allows for the request and display of
specified information from a database
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Four SQL Requests
• Show the records of a table in a specified
order
SELECT * FROM Table1 ORDER BY field1 ASC
• or
SELECT * FROM Table1 ORDER BY field1 DESC
Specifies
ASCending
Or
DESCending
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Show just the records that
meet certain criteria
* means
"all the fields"
Specified
Criteria
SELECT * FROM Table1 WHERE criteria
Name of the
Table where the
Records may be found
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Join the tables together
• connected by a foreign key, and present
the records as in Requests I and II.
SELECT * FROM Table1 INNER JOIN Table2 ON foreign
field = primary field WHERE criteria
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Make available just some of
the fields
• of either the basic tables or the joined
table.
SELECT field1, field2, . . ., fieldN FROM
Table1 WHERE criteria
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Criteria Clause
• A string containing a condition of the type used
with If blocks.
• Uses the standard operators <, >, and =
• Also can use the operator Like.
• Like uses the wildcard characters “_” and “%”
to compare a string to a pattern.
• Note: Similar to windows wildcards ? and *
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Like Examples
• An underscore character stands for a single
character in the same position as the
underscore character.
• The pattern “B_d” is matched by “Bid”, “Bud”,
and “Bad”.
• A percent sign stands for any number of
characters in the same position as the asterisk.
• The pattern “C%r” is matched by “Computer”,
“Chair”, and “Car”.
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SELECT clause
• SELECT fields FROM clause
• fields is either * (to indicate all fields) or a
sequence of the fields to be available
(separated by commas)
• clause is either a single table or a join of
two tables
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Join clause
• A join of two tables is indicated by a clause of
the form
table1 INNER JOIN table2 ON foreign key of
table1=primary key of table2
• Appending WHERE criteria
to the end of the sentence restricts the records
to those satisfying criteria.
• Appending ORDER BY field(s) ASC (or
DESC) presents the records ordered by the
specified field or fields.
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General SQL statements
• SELECT www FROM xxx WHERE yyy
ORDER BY zzz
• where SELECT www FROM xxx is
always present
• May be accompanied by one or both of
WHERE yyy and ORDER BY zzz.
• The xxx portion might contain an INNER
JOIN phrase.
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More on SQL statements
• The single quote, rather than the normal
double quote, is used to surround
strings.
• Fields may be specified with the table
they come from by
tableName.FieldName
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Virtual Tables
• SQL statements create a new “virtual” table from
existing tables.
• “Virtual” tables don’t exist physically.
• For all practical purposes, VB.NET acts as if
they did.
• You may also see a “virtual” table called a
view.
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The DataGrid Control
• Displays the values for an entire view in
table format (demo toolbox)
• The prefix for the name of a DataGrid
control is dg.
dgDisplay.DataSource = dt
• displays the contents of the data table dt
in the data grid.
Demo – 10-2-1
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Changing the Contents of a
Database
• Data grids can also be used to add, modify, and
delete records from a database.
• After a DataAdapter has been created, the
statement
Dim commandBuilder As New
OleDbCommandBuilder(dataAdapter)
will automatically generate the commands used
for the Insert, Update, and Delete operations.
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Using the DataAdapter to
Change a Database
• If changes is an Integer variable, then
the statement
changes = dataAdapter.Update(dt)
will store all of the insertions, updates,
and deletions made in the data table to
the database and assign the number of
records changed to the variable
changes.
Demo – 10-2-3
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Comments
1. SQL statements are case insensitive.
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Exercises p. 516
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