Transcript Lecture Powerpoint

Programming for
Geographical Information Analysis:
Advanced Skills
Lecture 5: Database connectivity
Dr Andy Evans
Java and Databases
SQL
Java and SQL
Databases
Flat file, relational, or object orientated stores of data records
and their relationships.
Oracle
MySQL
MS SQL Server
PostgreSQL ("Postgres")
Access (~format used by Arc Geodatabases)
Java DB: “Derby”
Comes free with Java 8, or from:
http://www.oracle.com/technetwork/java/javadb/overview/index.html
Can add the jar files to any java project.
Nice, small, DB system, which can be rolled into even small
mobile apps, and distributed.
Jar it up with your classfiles, by sticking them in the same
directory and using the following Manifest to make a
automatically running file:
Manifest-Version: 1.0
Main-Class: YourClass
Class-Path: derby.jar
Software connections
Software can be written in many different languages.
Languages can represent even basic data types in
different ways.
We therefore need something to do the translation.
Inter-process communication
To communicate we can:
Ensure everyone uses the same type of compiled
code : JVM, .Net
Ensure everyone uses the same types of objects and
give some way of invoking methods in a OS
framework: COM
Supply software that can translate the binary code of
one into the binary code of the other : CORBA, bridge
drivers
Translation drivers
For Java Database communication we have various options:
Java → database with java interface/driver.
Derby Driver
Java — standard protocol [PC or Network] → database
that accepts standard protocols.
MySQL Connector/J
Java → standardized driver → database
ODBC (Open Database Connectivity)
Database packages
Java Database Connectivity (JDBC)
Packages:
java.sql
javax.sql
Opening a connection
1)
2)
3)
4)
Load the driver
Open the connection
Issue SQL
Close the connection
Database drivers
If the manufacturer supplied a java driver, you should be able to
load it.
MySQL: http://www.mysql.com/products/connector/
DriverManager.registerDriver(new com.mysql.jdbc.Driver());
Oracle database: http://www.orafaq.com/wiki/JDBC
DriverManager.registerDriver(new
oracle.jdbc.driver.OracleDriver());
SQL Server: http://msdn.microsoft.com/en-us/sqlserver/aa937724
DriverManager.registerDriver(new
com.microsoft.sqlserver.jdbc.SQLServerDriver());
Postgres: https://jdbc.postgresql.org/
DriverManager.registerDriver(new org.postgresql.Driver());
Alternative method
As an alternative, you can load the driver into the JVM and
DriverManager will find it. The following code uses the default
classloader to do this:
Java DB:
Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
Opening a connection
Connection conn = null;
try {
conn = DriverManager.getConnection
("jdbc:xxxx:URL");
} catch (SQLException e) {}
xxxx = DB driver (“mysql”, “derby”, “oracle:thin”, “sqlserver”,
“postgresql”)
URL is a URL string or file:
"jdbc:derby: " + "//" + serverName + ":" + portNumber + "/" + dbName
Where serverName can be "localhost", or you can give a real
database directory:
"jdbc:derby:c:\\databases\\myDB"
Closing a connection
As usual with connections to things, it is polite to close
them:
try {
conn.close();
} catch (SQLException e) {}
ODBC
(Open Database Connectivity)
Use where no driver available.
Driver Manager keeps a list of data sources.
ODBC Driver Manager opens a driver that communicates
with a database when requested.
JDBC-ODBC bridge used to ask the Driver Manager to open
a driver connection.
ODBC Data Source Manager
Driver managers in Windows and Mac.
Windows: odbcad32.exe
ODBC Data Source Manager
Give the source a name
and (for Excel) select the
workbook.
Note under “Options>>” that
the default is “Read Only”.
Access
The same kinds of
options, but some
additional ones for
databases.
Under “Advanced” you
can set default
passwords to use.
Opening/closing ODBC connection
Connection conn = null;
try {
conn = DriverManager.getConnection
("jdbc:odbc:sourceName");
} catch (SQLException e) {}
try {
conn.close();
} catch (SQLException e) {}
ODBC Pros
Common to most machines and driver supplied.
As long as the ODBC data is uptodate, you can refer to the
database by name, not location.
ODBC Cons
Rather inefficient:
Program → Bridge → Driver → Database
JBDC-ODBC bridge was meant to be a stop-gap and doesn’t
implement all the JDBC methods.
Issues with Strings and Access. On Excel and text, see also:
http://support.microsoft.com/kb/178717
javax.sql.DataSource
What happens if the URL changes and you are using another
driver?
Nice thing about ODBC is that you call the database by name.
There is a java database registration setup called the Java
Naming and Directory Interface (JNDI).
You can use this, with a driver, to get a DataSource object, which
replaces the DriverManager. Databases are registered by name –
if the location changes, the JND can be updated.
java.util.Properties
What is there are access restrictions on the database?
To understand this, we need to understand Properties.
Properties : convenience class for storing key:value
String pairs.
Useful for loading and saving things like regionalised text
in different languages and user settings.
Properties
Properties p = new Properties();
p.setProperty(key, value); // both Strings
String a = p.getProperty(key);
store() : methods taking in various output streams.
load() : method taking in various input streams.
java.util.ResourceBundle
String a =
ResourceBundle.getBundle("setup").getString
("keyString");
Where setup.properties is a text file like this:
# This is a comment
keyString=Text
anotherKey=Text can\nhave escape characters
anotherKeyString=Some more text
in the directory the class files are in.
java.util.ResourceBundle
Good for regionalisation
# Filemenu resource bundle
saveMenu=Save
closeMenu=Close
String saveMenuText =
ResourceBundle.getBundle("en-gb")
.getString("saveMenu");
Can be set up in more complex way in families for e.g.
language regionalisation.
Connection properties
Using a password:
Properties p = new Properties();
p.setProperty("user", userName);
p.setProperty("password", password);
conn = DriverManager.getConnection
("jdbc:derby:c:\\databases\myDB", p);
Creating Databases
Connection conn = null;
String strUrl =
"jdbc:derby:c:\\databases\\myDB;create=true";
try {
conn = DriverManager.getConnection(strUrl);
} catch (SQLException sqle) {
sqle.printStackTrace();
}
Java and Databases
SQL
Java and SQL
SQL
(Structured Query Language)
ISO Standard for database management.
Allows creation, alteration, and querying of databases.
Broadly divided into:
Data Manipulation Language (DML) : data operations
Data Definition Language (DDL) : table & database operations
Often not case-sensitive, but better to assume it is.
Commands therefore usually written UPPERCASE.
Some databases require a semi-colon at the end of lines.
Creating Tables
CREATE TABLE tableName (
col1Name type,
col2Name type
)
List of datatypes at:
http://www.w3schools.com/sql/sql_datatypes.asp
CREATE TABLE Results (
Address varchar(255),
Burglaries int
)
Note the need to
define String max
size.
SELECT command
SELECT column, column
FROM tables in database
WHERE conditions
ORDER BY things to ordered on
SELECT Addresses
FROM crimeTab
WHERE crimeType = burglary
ORDER BY city
Wildcards
*
: All (objects)
%
: Zero or more characters (in a string)
_
: One character
[chars]
: Any character listed
[^charlist] or [!charlist]
: Any character not listed
E.g. Select all names with an a,b, or c somewhere in them:
SELECT * FROM Tab1 WHERE name LIKE '%[abc]%'
Case sensitivity
If you need to check without case sensitivity you can force the
datatype into a case insensitive character set first:
WHERE name = 'bob' COLLATE SQL_Latin1_General_CP1_CI_AS
Alternatively, if you want to force case sensitivity:
WHERE name = 'Bob' COLLATE SQL_Latin1_General_CP1_CS_AS
Counting
Can include count columns.
Count all records:
COUNT (*) AS colForAnswers
Count all records in a column:
COUNT (column) AS colForAnswers
Count distinct values:
COUNT(DISTINCT columnName) AS colForAnswers
SELECT crimeType, COUNT(DISTINCT Address)
AS HousesAffected FROM crimes
Joining tables
Primary key columns: Each value is unique to only one row.
We can join two tables if one has a primary key column which is
used in rows in the other:
Table2
Table1
P_key
columnA
P_key
columnB
id
1
A
a
HH
1
2
B
b
GG
1
3
C
c
YY
3
SELECT Table1.columnA, Table2.columnB
FROM Table1
JOIN Table2
ON Table1.P_Key=Table2.id
A
HH
A
GG
C
YY
Altering data
Two examples using UPDATE:
UPDATE Table1
SET column1 = ‘string’
WHERE column2 = 1
UPDATE Table1
SET column1 = column2
WHERE column3 = 1
SQL
Introductory tutorials:
http://www.w3schools.com/sql/default.asp
Java and Databases
SQL
Java and SQL
JDBC SQL
Three methods:
Statements: Standard, simple, SQL.
PreparedStatements: Compiled SQL statements that are altered
to new data through input parameters. Useful, for example, in
looped structures.
CallableStatements: for SQL procedures stored in the database.
Produce ResultSet objects.
Example: Select
Statement st = conn.createStatement();
ResultSet rs = st.executeQuery
("SELECT a,b,c FROM Table1");
Three different execution methods:
executeQuery : simple SQL queries.
executeUpdate : anything that changes or creates a Table,
e.g. UPDATE. Returns number of rows effected.
execute: Complex, multi-return queries.
st.close(); efficient, but will happen at conn.close()
Example: Creating tables
String createTable =
"CREATE TABLE Results (" +
"Address varchar(255)," +
"Burglaries int" +
")";
Statement st = null;
try {
st = conn.createStatement();
st.execute (createTable);
} catch (SQLException ex) {
ex.printStackTrace();
}
Auto-commit
You can run multiple queries on the same statement.
By default Connection objects are set to auto-commit – all
changes are solidified after single statements are run.
conn.setAutoCommit(booleanAutoCommit);
If set to false (off) the changes will only be solidified when
commit called:
conn.commit();
Until then you can rollback all changes since the last commit,
by calling:
conn.rollback();
Also options to setup and rollback to savepoints.
Escape characters
Remember that some characters, like “_”, are wildcards.
If we want to use these literally, they need a backslash infront of
them “\_”.
However, backslash is a String escape character in Java, so you
can define what is a special escape character to ignore in
statement execution:
stmt.executeQuery(
"SELECT name FROM Table1 WHERE Id
LIKE '\_%' {escape '\'}");
In some cases the escape occurs in a table name, in which case
treat literally by enclosing in [] e.g. [Sheet1$]
ResultSets
Links to a cursor on the database and converts data to Java
types.
ResultSet rs = st.executeQuery
("SELECT a, b, c FROM Table1");
while (rs.next()) {
Can also use
int i = rs.getInt("a");
column index
number
or
String s = rs.getString("b"); (starting 1).
or
Object o = rs.getObject("c");
}
Use object if unsure, and cast.
A new SQL query will close the current ResultsSet.
ResultSets
Standard results sets only let you read from beginning to end
a line at a time.
If you want to write to the data (without using SQL UPDATE) or
go back and forwards, you need a scrollable statement.
Statement st = conn.createStatement(
ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_UPDATABLE,
ResultSet.HOLD_CURSORS_OVER_COMMIT);
Only need to worry about
this if you’re not autocommitting.
Scrollable ResultSet
rs.beforeFirst();
rs.afterLast();
rs.next()
rs.previous()
Looping:
while (!rs.isAfterLast()) {
Finding number of rows:
rs.last();
int numberOfRows = rs.getRow();
Scrollable ResultSet
st.setFetchSize(25);
Fetch 25 rows at a time.
rs.absolute(2);
Move to row 2
rs.updateInt(3, 10);
Update col 3 of current row.
rs.updateInt("Name",10);
Update by col name.
also updateObject, updateString, updateFloat and
others.
rs.updateRow(); Must call this after updating data in all
the columns you want to change for that row. You also need to
call rs.close() to commit all changes.
Inserting a row
rs.moveToInsertRow();
rs.updateString(1, "Bob");
rs.updateObject(2, someObject);
rs.insertRow();
rs.first();
(In general the cursor points at the row used prior to insert.)
conn.commit();
If needed.
rs.close();
Database metadata
Especially useful in debugging is getting metadata about the
database:
DatabaseMetaData md = conn.getMetaData();
ResultSet rs = md.getTables(
null, null, "%", null);
while (rs.next()) {
System.out.println(rs.getString(3));
}
Further info
Online guide:
http://docs.oracle.com/javase/tutorial/jdbc/basics/
Next Lecture
XML
Practical
JDBC