statement call in java

8/9/2019 Statement call in java

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Statements


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PreparedStatement

A prepared statement is an SQL statement with parameters

that can change at any time.

Instances of PreparedStatement contain an SQL statement thathas already been compiled. This is what makes a statement

"prepared."

Used for complex queries or repeated queries i.e. for

convenience or efficiency.


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Being a subclass ofStatement,PreparedStatement inherits all thefunctionality ofStatement.

Also, the methods executeQuery, and executeUpdate are modified sothat they take no argument. The Statement forms of these methods (the

forms that take an SQL statement parameter) should neverbe used with

a PreparedStatement object.

prepareStatement method

public PreparedStatement prepareStatement(String sql) throwsSQLException

Creates a PreparedStatement object forsending parameterized

SQL statements to the database.


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ForConvenience

The advantage ofusing SQL statements that take parameters is that

we can use the same statement and supply it with different values

each time it is executed The SQL statement contained in a PreparedStatement object may

have one ormore IN parameters.

An IN parameteris a parameterwhose value is not specified when the

SQL statement is created. Instead, the statement has a question mark

("?") as a placeholderforeach IN parameter.

The "?" is also known as a parametermarker. An application must set

a value foreach question mark in a prepared statement before

executing the prepared statement


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An example of JDBC prepared statement

PreparedStatement ps = con.prepareStatement(SELECT Name,Salary

FROM EMP WHERE Dept_No=?); This creates an statement which is ready to be executed

To execute the query, the values of the positional parameterplacemarked by the ? symbols are specified

Parameter numbers start from 1 and not 0.

We need to use set() method to do this, where the type

depends on the data type of the columnps.setString(1, D1);

After supplying values for all positional parameters, we canexecute query:

ResultSet rs=ps.executeQuery( );


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You can often make coding easier by using a for loop or a whileloop to set values for input parameters.

PreparedStatement ps;

String pstmt = SELECT Name,Salary FROM EMPWHERE

Dept_No=?;

ps= con.prepareStatement(pstmt);

String [ ] dept = {D1", D2", D3"};

int len =dept.length;

for(int i = 0; i < len; i++) {

ps.setString(1, dept[i]);

ps.executeQuery();

}


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To set an integer value to NULL, use call:

ps.setNull(1, Types.INTEGER);

The Types class contains constants that represent the variousSQL data types supported by JDBC

Once a parameter has been set with a value, it will retain thatvalue until it is reset to another value or the method

clearParameters is called.ps.clearParameters( );

ps.setString(1, D4");

ps.executeQuery


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ForEfficiency

If the same SQL statement is executed many times with differentparameters, it is more efficient to use a PreparedStatementobject. It will normally reduce execution time.

Unlike a Statement object, it is given an SQL statement when it iscreated.

Statement Object

Statement stat = con.createStatement();

ResultSet rs = stat.executeQuery(SELECT * FROM EMP

WHERE Dept_No=D1); PreparedStatement Object

PreparedStatement ps = con.prepareStatement ( SELECT *

FROM EMP WHERE DeptNo=?);

ResultSet rs = ps.executeQuery();


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The advantage to this is that in most cases, this SQL statement

will be sent to the DBMS right away, where it will be compiled.

As a result, the PreparedStatement object contains not just an SQLstatement, but an SQL statement that has been precompiled. This

means that when the PreparedStatement is executed, the DBMS

can just run the PreparedStatement's SQL statement without havingto compile it first.


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Stored Procedures

Stored procedures are essentially programs that we can write and are

internally executed by the database engine itself

These can be stored in database itself Stored procedures in Java:

These are implemented using Java static methods

A Java class is written which contain one ormore static methods

This class is wrapped in a JARfile and installed on the target database

This stored procedure is callable by Java


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public class MyStoredProcedures{

public int getPrice(String carType) throws SQLException{

Connection con=DriverManager.getConnection(jdbc:default:con);

Statement stat = con.createStatement();

ResultSet rs= stat.executeQuery(SELECT Price FROMCARDATA

WHERE CarType = +carType+);

return rs.getInt(Price);

}

//otherstatic methods

}


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CallableStatement

A CallableStatement object provides a way to call stored procedures in

a standard way forall RDBMSs.

CallableStatement cstmt = con.prepareCall(String sql);prepareCall method

public CallableStatement prepareCall(String sql) throws

SQLException

Creates a CallableStatement object forcalling database stored

procedures A stored procedure is stored in a database; the call to the stored

procedure is what a CallableStatement object contains.


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This call may take one of twoforms:

with a result parameter,

without result parameter.

Aresult parameter, a kind ofOUT parameter, is the return

value forthe stored procedure.

Both forms may have a variable numberofparameters used forinput (IN

parameters),output (OUT parameters),orboth (INOUT parameters). Aquestion mark serves as a placeholderfora parameter.


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Syntax

The syntax forinvoking a stored procedure

{call procedure_name[(?, ?, ...)]}

the square brackets indicate optional part The syntax for a procedure that returns a result parameter is:

{? = call procedure_name[(?, ?, ...)]}

The syntax for a stored procedure with no parameters would look

like this:

{call procedure_name}


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The value of each IN parameter is set by calling a set()

method, while each OUT parameter or INOUT parameter should

be registered by calling a registerOutParametermethod. Method for registering output parameters:

public void registerOutParameter(int whichParameter, int sqlType)


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Example

String callString = "{?=call getPrice(?)}" ;

CallableStatement cs = connection.prepareCall(callString);

cs.setString(1, Scorpio");cs.execute();

System.out.println(cs.getString(2));


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Before creating a CallableStatement object one must know the DBMS

being used supports stored procedures ornot ifyes then what those

procedures are. To check this, the DatabaseMetaData method

supportsStoredProcedures() will return true if the DBMS supportsstored procedure calls, and the method getProcedures() will return a

description of the stored procedures available.

statement call in java

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