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Category: java Mysql
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Khóa học miễn phí Java & MySQL – Result Set nhận dự án làm có lương
Java & MySQL – ResultSet
The SQL statements that read data from a database query, return the data in a result set. The SELECT statement is the standard way to select rows from a database and view them in a result set. The java.sql.ResultSet interface represents the result set of a database query.
A ResultSet object maintains a cursor that points to the current row in the result set. The term “result set” refers to the row and column data contained in a ResultSet object.
The methods of the ResultSet interface can be broken down into three categories −
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Navigational methods − Used to move the cursor around.
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Get methods − Used to view the data in the columns of the current row being pointed by the cursor.
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Update methods − Used to update the data in the columns of the current row. The updates can then be updated in the underlying database as well.
The cursor is movable based on the properties of the ResultSet. These properties are designated when the corresponding Statement that generates the ResultSet is created.
JDBC provides the following connection methods to create statements with desired ResultSet −
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createStatement(int RSType, int RSConcurrency);
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prepareStatement(String SQL, int RSType, int RSConcurrency);
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prepareCall(String sql, int RSType, int RSConcurrency);
The first argument indicates the type of a ResultSet object and the second argument is one of two ResultSet constants for specifying whether a result set is read-only or updatable.
Type of ResultSet
The possible RSType are given below. If you do not specify any ResultSet type, you will automatically get one that is TYPE_FORWARD_ONLY.
Type Description ResultSet.TYPE_FORWARD_ONLY The cursor can only move forward in the result set. ResultSet.TYPE_SCROLL_INSENSITIVE The cursor can scroll forward and backward, and the result set is not sensitive to changes made by others to the database that occur after the result set was created. ResultSet.TYPE_SCROLL_SENSITIVE. The cursor can scroll forward and backward, and the result set is sensitive to changes made by others to the database that occur after the result set was created. Concurrency of ResultSet
The possible RSConcurrency are given below. If you do not specify any Concurrency type, you will automatically get one that is CONCUR_READ_ONLY.
Concurrency Description ResultSet.CONCUR_READ_ONLY Creates a read-only result set. This is the default ResultSet.CONCUR_UPDATABLE Creates an updateable result set. All our examples written so far can be written as follows, which initializes a Statement object to create a forward-only, read only ResultSet object −
try( Statement stmt = conn.createStatement( ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);) } catch(Exception ex) { .... } finally { .... }
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Khóa học miễn phí Java & MySQL – CallableStatement nhận dự án làm có lương
Java & MySQL – CallableStatement
The CallableStatement interface is used to execute a call to a database stored procedure.
Suppose, you need to execute the following stored procedure in TUTORIALSPOINT database −
DELIMITER $$ DROP PROCEDURE IF EXISTS `TUTORIALSPOINT`.`getEmpName` $$ CREATE PROCEDURE `TUTORIALSPOINT`.`getEmpName` (IN EMP_ID INT, OUT EMP_FIRST VARCHAR(255)) BEGIN SELECT first INTO EMP_FIRST FROM Employees WHERE ID = EMP_ID; END $$ DELIMITER ;
Three types of parameters exist: IN, OUT, and INOUT. The PreparedStatement object only uses the IN parameter. The CallableStatement object can use all the three.
Here are the definitions of each −
| Parameter | Description |
|---|---|
| IN | A parameter whose value is unknown when the SQL statement is created. You bind values to IN parameters with the setXXX() methods. |
| OUT | A parameter whose value is supplied by the SQL statement it returns. You retrieve values from theOUT parameters with the getXXX() methods. |
| INOUT | A parameter that provides both input and output values. You bind variables with the setXXX() methods and retrieve values with the getXXX() methods. |
The following code snippet shows how to employ the Connection.prepareCall() method to instantiate a CallableStatement object based on the preceding stored procedure −
CallableStatement cstmt = null;
try {
String SQL = "{call getEmpName (?, ?)}";
cstmt = conn.prepareCall (SQL);
. . .
}
catch (SQLException e) {
. . .
}
finally {
. . .
}
The String variable SQL, represents the stored procedure, with parameter placeholders.
Using the CallableStatement objects is much like using the PreparedStatement objects. You must bind values to all the parameters before executing the statement, or you will receive an SQLException.
If you have IN parameters, just follow the same rules and techniques that apply to a PreparedStatement object; use the setXXX() method that corresponds to the Java data type you are binding.
When you use OUT and INOUT parameters you must employ an additional CallableStatement method, registerOutParameter(). The registerOutParameter() method binds the JDBC data type, to the data type that the stored procedure is expected to return.
Once you call your stored procedure, you retrieve the value from the OUT parameter with the appropriate getXXX() method. This method casts the retrieved value of SQL type to a Java data type.
Closing CallableStatement Object
Just as you close other Statement object, for the same reason you should also close the CallableStatement object.
A simple call to the close() method will do the job. If you close the Connection object first, it will close the CallableStatement object as well. However, you should always explicitly close the CallableStatement object to ensure proper cleanup.
CallableStatement cstmt = null;
try {
String SQL = "{call getEmpName (?, ?)}";
cstmt = conn.prepareCall (SQL);
. . .
}
catch (SQLException e) {
. . .
}
finally {
cstmt.close();
}
We”re using try with resources which handles the resource closure automatically. Following example demonstrates all of the above said concepts.
This code has been written based on the environment and database setup done in the previous chapter.
Copy and paste the following example in TestApplication.java, compile and run as follows −
import java.sql.CallableStatement;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class TestApplication {
static final String DB_URL = "jdbc:mysql://localhost/TUTORIALSPOINT";
static final String USER = "guest";
static final String PASS = "guest123";
static final String QUERY = "{call getEmpName (?, ?)}";
public static void main(String[] args) {
// Open a connection
try(Connection conn = DriverManager.getConnection(DB_URL, USER, PASS);
CallableStatement stmt = conn.prepareCall(QUERY);
) {
// Bind values into the parameters.
stmt.setInt(1, 102); // This would set ID
// Because second parameter is OUT so register it
stmt.registerOutParameter(2, java.sql.Types.VARCHAR);
//Use execute method to run stored procedure.
System.out.println("Executing stored procedure..." );
stmt.execute();
//Retrieve employee name with getXXX method
String empName = stmt.getString(2);
System.out.println("Emp Name with ID: 102 is " + empName);
} catch (SQLException e) {
e.printStackTrace();
}
}
}
Now let us compile the above example as follows −
C:>javac TestApplication.java C:>
When you run TestApplication, it produces the following result −
C:>java TestApplication Executing stored procedure... Emp Name with ID: 102 is Zaid C:>
JDBC SQL Escape Syntax
The escape syntax gives you the flexibility to use database specific features unavailable to you by using standard JDBC methods and properties.
The general SQL escape syntax format is as follows −
{keyword ''parameters''}
Here are the following escape sequences, which you would find very useful while performing the JDBC programming −
d, t, ts Keywords
They help identify date, time, and timestamp literals. As you know, no two DBMSs represent time and date the same way. This escape syntax tells the driver to render the date or time in the target database”s format. For Example −
{d ''yyyy-mm-dd''}
Where yyyy = year, mm = month; dd = date. Using this syntax {d ”2009-09-03”} is March 9, 2009.
Here is a simple example showing how to INSERT date in a table −
//Create a Statement object
stmt = conn.createStatement();
//Insert data ==> ID, First Name, Last Name, DOB
String sql="INSERT INTO STUDENTS VALUES" +
"(100,''Zara'',''Ali'', {d ''2001-12-16''})";
stmt.executeUpdate(sql);
Similarly, you can use one of the following two syntaxes, either t or ts −
{t ''hh:mm:ss''}
Where hh = hour; mm = minute; ss = second. Using this syntax {t ”13:30:29”} is 1:30:29 PM.
{ts ''yyyy-mm-dd hh:mm:ss''}
This is combined syntax of the above two syntax for ”d” and ”t” to represent timestamp.
escape Keyword
This keyword identifies the escape character used in LIKE clauses. Useful when using the SQL wildcard %, which matches zero or more characters. For example −
String sql = "SELECT symbol FROM MathSymbols WHERE symbol LIKE ''%'' {escape ''''}";
stmt.execute(sql);
If you use the backslash character () as the escape character, you also have to use two backslash characters in your Java String literal, because the backslash is also a Java escape character.
fn Keyword
This keyword represents scalar functions used in a DBMS. For example, you can use SQL function length to get the length of a string −
{fn length(''Hello World'')}
This returns 11, the length of the character string ”Hello World”.
call Keyword
This keyword is used to call the stored procedures. For example, for a stored procedure requiring an IN parameter, use the following syntax −
{call my_procedure(?)};
For a stored procedure requiring an IN parameter and returning an OUT parameter, use the following syntax −
{? = call my_procedure(?)};
oj Keyword
This keyword is used to signify outer joins. The syntax is as follows −
{oj outer-join}
Where outer-join = table {LEFT|RIGHT|FULL} OUTERJOIN {table | outer-join} on search-condition. For example −
String sql = "SELECT Employees FROM {oj ThisTable RIGHT OUTER JOIN ThatTable on id = ''100''}";
stmt.execute(sql);
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