A brief review of Database
Database (DB):
an organised collection of data
Relational DBs (first published by Edgar F. Codd (IBM) in 1970) store data in:
tables A table in a database consists of:
rows & columns
Rows: records
Columns: attributes of the records
SQL:
Structured Query Language, The industry standard database query language (relational Database)
A brief review of Database (cont.)
SELECT NAME, ID FROM STUDENT WHERE GENDER=‘MALE’;
INSERT INTO STUDENT VALUES ( 6, ‘MARY’, ‘FEMALE’ );
DELETE FROM STUDENT WHERE NAME=‘JERRY’; DROP TABLE STUDENT;
Overview of JDBC cont.
Java DataBase Connectivity: java.sql
Major components in JDBC:
JDBC API gives access of programming data from the Java. By using JDBC API, applications can execute SQL statements and retrieve results and updation to the database. It standardise:
-Way to establish connection to database
-Approach to initiating queries
-The data structure of query result
-It does NOT standardise SQL syntax
-JDBC Driver Manager is the class in JDBC API.
-The objects of this class can connect Java applications to a JDBC driver
How to use JDBC
Establishing a connection to DB
Create a Statement instance to conduct SQL statements
Execute queries from JDBC
Receive, hold and process query results from DB
Check meta-information of results & databases
1)Establish a connection
Creates instance of a Connection object
Connection conn = DriverManager.getConnection( “url”, “username”, “password”); [throws SQLException]
String url= "jdbc:derby: // localhost:1527/ CarDB; create=true";
return Connection object:Creates a Statement object for sending SQL statements to the database.
2)Statement
Statement: object that can accept and execute a string that is a SQL statement
Statement statement = conn.createStatement();
Executing statements:
statement.executeXXXXX("a SQL statement");
exa:
executeUpdate: for database modifications //数据库的修改
executeQuery: for database queries //数据库的查询
Execute DB update statement:
statement.executeUpdate(“CREATE TABLE CARTABLE”);
例子：
public void createTable() {
try{
Statement statement=conn.createStatement(); //创建一个新的statement
String newTable="cartable"; //tablename
statement.executeUpdate("drop table if exists "+newTable); //更新
String sqlCreateTable="CREATE TABLE "+newTable+" (ID INT, " + "MAKE VARCHAR(50), MODEL VARCHAR(50), PRICE INT )";
statement.executeUpdate(sqlCreateTable);//更新数据库
} catch (SQLException ex) {
System.err.println("SQLException: " + ex.getMessage());
}
}
Query Statement //数据查询
Execute DB Query statement:
ResultSet: an object of type ResultSet is like a cursor（光标）
next() advances cursor to next tuple（数组）:
The first time next() returns the first tuple
If no more tuples then next() returns FALSE
Accessing components of a tuple:
getXXX(attribute name OR column index)
ResultSet rs=Statement.executeQuery(“SELECT * FROM CAR”);
while (rs.next()) {
String attribute1= rs.getString(“A1”);
int attribute2= rs.getInt (2);
}
More methods control the cursor:
previous(): moves the cursor backwards one row.Returns true if the cursor is now positioned on a row and false if the cursor is positioned before the first row.
first()/last(): moves the cursor to the first/last row
afterLast(): moves the cursor to the end of this ResultSet object, just after the last row
beforeFirst(): moves the cursor to the front of this ResultSet object, just before the first row
public ResultSet getQuery() { //数据库结果集的数据表
ResultSet rs=null;
try {
Statement statement=mysqlConn.createStatement();
String sqlQuery="select model, price from car " + "where brand=‘toyota‘“;
rs=statement.executeQuery(sqlQuery);
while( rs.next() ) {
String model= rs.getString ("model");
// String model= rs.getString (1);
int price = rs.getInt(2);
System.out.println(model+": $"+price);
}
System.out.println("Table created");
} catch (SQLException ex) {
Logger.getLogger(DBOperations.class.getName()).log(Level.SEVERE, null, ex);
}
return(rs);
}
ResultSet metadata:
ResultSetMetaData: objects that can hold ResultSet metadata
ResultSetMetaData rsmd = rs.getMetaData();
To get number of columns: rsmd.getColumnCount();
To get the table name of a given column in the result set: rsmd.getTableName(int);
To get data type of a given column: rsmd.getColumnType(int);
Closing:
Close ResultSet:
rs.close( );
Close Statement:
statement.close( );
Close Connection:
conn.close( );
A statement will be closed if the connection instance it belongs to is closed;
A resultset will be closed if the statement instance it holds results for is closed.
