This blog is to provide details for java interview preparation. Everybody is busy in day to day project work and most of times we are not able to spend time for interview preparation due to workload. I am trying to provide the consolidated details of java topics and sharing my java interview experiences.
Here are more DB interview question for java developer. Mostly I have been asked question preliminary on DB Index, Query optimisation, Standard query and DB functions. DB Interview Questions : Part -1.
Q 1- How to Enable/Disable Indexes?
A - Disable Index
ALTER INDEX [IndexName] ON TableName DISABLE GO
Enable Index
ALTER INDEX [IndexName] ON TableName REBUILD
Q 2- What is max number of columns in index?
A- 32
Q 3- How many maximum columns in partitions can be used? A - 16
Q 4- How to replace specific values from query result? i.e. how query return 'M' and 'F' can be converted into 'Male' and Female'.
A - This can be done using Oracle Decode function the syntax will be like
SELECT employee_name,
DECODE(type,'M', 'Male',
'F', 'Female')type
FROM employee;
Q 5 - If the column of the table is having case sensitive data like 'Abc' and 'ABC' and we want to do case sensitive search on table. How we can improve performance of case sensitive search?
A- In this case we can define case sensitive index on table so that each search is restricted to particular value compare to full table scan. You can force all your data to be the same case by using UPPER() or LOWER():
select * from my_table where upper(column_1) = upper('my_string');
or
select * from my_table where lower(column_1) = lower('my_string');
If column_1 is not indexed on upper(column_1) or lower(column_1), as appropriate, this may force a full table scan. In order to avoid this you can create a function-based index.
create index my_index on my_table ( lower(column_1) );
If you're using LIKE then you have to concatenate a % around the string you're searching for.
select * from my_table where lower(column_1) = lower('my_string') || '%';
Q 5 - What are Synonyms?
A Synonyms give you the ability to provide alternate names for database objects. You can alias object names; for example, using the Employee table as Emp. You can also shorten names. This is especially useful when dealing with three and four part names; for example, shortening server.database.owner.object to object.
Q 6 - What is the Difference between Update Lock and Exclusive Lock?
A -When Exclusive Lock is on any process, no other lock can be placed on that row or table. Every other process have to wait till Exclusive Lock completes its tasks.
Update Lock is a type of Exclusive Lock, except that it can be placed on the row which already has Shared Lock on it. Update Lock reads the data of the row which has the Shared Lock as soon as the Update Lock is ready to change the data it converts itself to the Exclusive Lock.
Q 7 -What is a Surrogate Key?
A - A surrogate key is a substitution for the natural primary key. It is just a unique identifier or number for each row that can be used for the primary key to the table. The only requirement for a surrogate primary key is that it should be unique for each row in the table. It is useful because the natural primary key can change and this makes updates more difficult. Surrogated keys are always integer or numeric.
A 8 - Design the Database process which can handle 1 million requests at same time and What should be DB consideration?
A - All the major databases and operating systems will work in 1 million requests at same time; provided sufficient hardware with sufficient bandwidth to memory, disk, and network are provided.
All databases are built for this kind of scenario -- that is, where you need to update lots of rows from different clients all the time. This is what indexes and primary keys are used for, and the database is optimized for updating in this manner. (i.e., UPDATE your_table SET your_column=your_value where your_key=12)
The hardware requirements are going to be your biggest issue, and I suspect you will need to think about quite a lot here, including:
Failover (what happens when your main server fails?)
Clustering (You may simply need to have more than one database server to handle your load)
Processors (2? 4? 8? 16? Multi-core? Does the db provide multi-core support that is well optimized?)
Memory (The faster the better, but your chipset must be able to handle the bandwidth as well)
Disk (Faster I/O the better. eSATA/SATA or Fiber, etc.)
Network (You'll need lots of bandwidth to handle this kind of traffic)
Virtualization (Does it make sense to build this as real hardware? Why not as virtual servers in the cloud? Amazon / Rackspace / etc.?)
Thankfully a good majority of the scaling issues are handled either at the hardware or db level. That is, you can start your database now on a slow server and as traffic increases you can adjust accordingly with minimal changes to your code or your db structure. What will change is your hardware and your db configuration. Most database servers will support failover/clustering with little to no change to your existing code or structures. (But be sure to research possible costs and the efficiency thereof first)
DELETEFROM TableName
WHERE ID NOTIN(SELECT MAX(ID)FROM TableName
GROUPBY Column1,
Column2,
Column3HAVING MAX(ID)ISNOTNULL)
Q 10 : Finding N'th Maximum salary SQL query?
// Using Sub query SELECT*FROM Employee Emp1
WHERE(N-1)=(SELECT COUNT(DISTINCT(Emp2.Salary))FROM Employee Emp2
WHERE Emp2.Salary > Emp1.Salary
)
//Another way to get 2'nd maximum salary Select max(Salary) From Employee e where e.sal < ( select max(sal) from employee );
Q 11 - Database Query Optimisation tips with JDBC:
A : Use prepared statement pooling
Database supports prepared statement pooling for pooled connections, as discussed in the JDBC 3.0 specification, through the TimesTenObservableConnectionDS class. Note that statement pooling is transparent to an application. Use of the PreparedStatement object, including preparing and closing the statement, is no different.
Use arrays of parameters for batch execution
You can improve performance by using groups, referred to as batches, of statement executions, calling the addBatch() and executeBatch() methods forStatement or PreparedStatement objects.
A batch can consist of a set of INSERT, UPDATE, DELETE, or MERGE statements. Statements that return result sets, such as SELECT statements, are not allowed in a batch. A SQL statement is added to a batch by calling addBatch() on the statement object. The set of SQL statements associated with a batch are executed through the executeBatch() method.
For PreparedStatement objects, a batch consists of repeated executions of a statement using different input parameter values. For each set of input values, create the batch by using appropriate setXXX() calls followed by the addBatch() call. The batch is executed by the executeBatch() method.
// turn off autocommit
conn.setAutoCommit(false);
Statement stmt = conn.createStatement();
stmt.addBatch("INSERT INTO employees VALUES (1000, 'Joe Jones')");
stmt.addBatch("INSERT INTO departments VALUES (260, 'Shoe')");
stmt.addBatch("INSERT INTO emp_dept VALUES (1000, 260)");
// submit a batch of update commands for execution
int[] updateCounts = stmt.executeBatch();
conn.commit ();
Bulk fetch rows of data
Oracle provides an extension that allows an application to fetch multiple rows of data. For applications that retrieve large amounts of data, fetching multiple rows can increase performance greatly. However, when using Read Committed isolation level, locks are held on all rows being retrieved until the application has received all the data, decreasing concurrency. For more information on this feature, see "Fetching multiple rows of data".
Use the ResultSet method getString() sparingly
Because Java strings are immutable, the ResultSet method getString() must allocate space for a new string in addition to translating the underlying C string to a Unicode string, making it a costly call.
In addition, you should not call getString() on primitive numeric types, like byte or int, unless it is absolutely necessary. It is much faster to call getInt()on an integer column, for example.
Avoid data type conversions
TimesTen instruction paths are so short that even small delays due to data conversion can cause a relatively large percentage increase in transaction time.
Use the appropriate getXXX() method on a ResultSet object for the data type of the data in the underlying database. For example, if the data type of the data is DOUBLE, to avoid data conversion in the JDBC driver you should call getDouble(). Similarly, use the appropriate setXXX() method on thePreparedStatement object for the input parameter in an SQL statement. For example, if you are inserting data into a CHAR column using aPreparedStatement, you should use setString().
Q 12: What should be strategy to minimise the impact on external system which is using our table's data for processing?
Answer : Best strategy should be using database views to minimize the impact on external system, If we are changing our table structure or columns and if we are able to populate same data using View then there should not be any impact on external system and they will not require any change.
This Topic is always asked by old timer java developer as few years back Java was focusing on how to best communicate this Database and almost most of the application were facing performance issue with database query execution. Focus on few code snipet and basic understanding of jdbc driver should help you to clear this topic -------------------------------------------------------------------------------------------------------------------------------------- Java Database Connectivity API contains commonly asked Java interview questions. A good understanding of JDBC API is required to understand and leverage many powerful features of Java technology. Here are few important practical questions and answers which can be asked in a Core Java JDBC interview. These are the most commonly asked questions:- 1) what is Index and difference with cluster index?
Non-clustered
The data is present in arbitrary order, but the logical ordering
is specified by the index. The data rows may be spread throughout the
table regardless of the value of the indexed column or expression. The
non-clustered index tree contains the index keys in sorted order, with
the leaf level of the index containing the pointer to the record (page
and the row number in the data page in page-organized engines; row
offset in file-organized engines). In a non-clustered index:
The physical order of the rows is not the same as the index order.
Typically created on non-primary key columns used in JOIN, WHERE, and ORDER BY clauses.
There can be more than one non-clustered index on a database table.
Clustered
Clustering alters the data block into a certain distinct order to
match the index, resulting in the row data being stored in order.
Therefore, only one clustered index can be created on a given database
table. Clustered indices can greatly increase overall speed of
retrieval, but usually only where the data is accessed sequentially in
the same or reverse order of the clustered index, or when a range of
items is selected. Since the physical records are in this sort order on disk, the next
row item in the sequence is immediately before or after the last one,
and so fewer data block reads are required. The primary feature of a
clustered index is therefore the ordering of the physical data rows in
accordance with the index blocks that point to them. Some databases
separate the data and index blocks into separate files, others put two
completely different data blocks within the same physical file(s).
Create an object where the physical order of rows is the same as the
index order of the rows and the bottom (leaf) level of clustered index
contains the actual data rows. Good read on index : http://www.orafaq.com/node/1403
2) DB query performance and optimization? Among the various ways to tune the performance of a query are:
Rewriting the query. There is no one way to write a
query. If you find that a query with an OUTER JOIN is taking a long
time, rewrite it using an INNER JOIN. Or perhaps the WHERE clause is
written in such a way that it is causing excessive database read
operations.
Normalizing or de-normalizing tables. While
normalizing tables results in ideal database design, it often leads to
poor querying. If you find that frequently-used queries have to span too
many normalized tables, consider some duplication of data. For example,
in the Northwind database, the Unit Cost column is present in both the
Order Details and Products table.
Adding indexes. Lack of well-defined indexes is a
frequent cause of queries taking a long time to execute. Creating
indexes will cause a query to lookup the index rather than the table.
Removing indexes. In some cases, it is a poorly
defined index that is the cause of slow query execution. For example, an
index which does not include the frequently looked up columns in a
table. In such a case, it is better to drop the index and recreate it.
3) Benefits of prepared statement? DB execution happens in this steps 1.Parse the incoming SQL query 2. Compile the SQL query 3. Plan/optimize the data acquisition path 4. Execute the optimized query / acquire and return data - statement perform step 1-4 to do the each task and prepared statement will pre execute the 1-3 and perform only 4 for each task. - prepared statements are safe as they safeguard against sql injection on using direct query. - We can bind non standard java objects like blob etc in prepared statement, but not in statement. 4) DB execute plan and how to use it? 5) Foreign key and primary key difference? 6) Materialized view in database?
7) Truncate and delete difference? 8) How to design table? 9) Database transaction management? 10) what is cursor and trigger in dB?
These are very standard API question in interview:
What are available drivers in JDBC
JDBC technology drivers fit into one of four categories:
A JDBC-ODBC bridge provides JDBC API access via one or more ODBC drivers. Note that some ODBC native code and in many cases native database client code must be loaded on each client machine that uses this type of driver. Hence, this kind of driver is generally most appropriate when automatic installation and downloading of a Java technology application is not important. For information on the JDBC-ODBC bridge driver provided by Sun, see JDBC-ODBC Bridge Driver.
A native-API partly Java technology-enabled driver converts JDBC calls into calls on the client API for Oracle, Sybase, Informix, DB2, or other DBMS. Note that, like the bridge driver, this style of driver requires that some binary code be loaded on each client machine.
A net-protocol fully Java technology-enabled driver translates JDBC API calls into a DBMS-independent net protocol which is then translated to a DBMS protocol by a server. This net server middleware is able to connect all of its Java technology-based clients to many different databases. The specific protocol used depends on the vendor. In general, this is the most flexible JDBC API alternative. It is likely that all vendors of this solution will provide products suitable for Intranet use. In order for these products to also support Internet access they must handle the additional requirements for security, access through firewalls, etc., that the Web imposes. Several vendors are adding JDBC technology-based drivers to their existing database middleware products.
A native-protocol fully Java technology-enabled driver converts JDBC technology calls into the network protocol used by DBMSs directly. This allows a direct call from the client machine to the DBMS server and is a practical solution for Intranet access. Since many of these protocols are proprietary the database vendors themselves will be the primary source for this style of driver. Several database vendors have these in progress.
What are the types of statements in JDBC?
The JDBC API has 3 Interfaces, (1. Statement, 2. PreparedStatement, 3. CallableStatement ). The key features of these are as follows: Statement
·This interface is used for executing a static SQL statement and returning the results it produces.
·The object of Statement class can be created using Connection.createStatement() method.
PreparedStatement
oA SQL statement is pre-compiled and stored in a PreparedStatement object.
oThis object can then be used to efficiently execute this statement multiple times.
oThe object of PreparedStatement class can be created using Connection.prepareStatement() method. This extends Statement interface.
CallableStatement
oThis interface is used to execute SQL stored procedures.
oThis extends PreparedStatement interface.
oThe object of CallableStatement class can be created using Connection.prepareCall() method.
What is a stored procedure? How to call stored procedure using JDBC API?
Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters. Stored procedures can be called using CallableStatement class in JDBC API. Below code snippet shows how this can be achieved.
CallableStatement cs = con.prepareCaWhat is Connection pooling? What are the advantages of using a connection pool?
Connection Pooling is a technique used for sharing the server resources among requested clients. It was pioneered by database vendors to allow multiple clients to share a cached set of connection objects that provides access to a database.
Getting connection and disconnecting are costly operation, which affects the application performance, so we should avoid creating multiple connection during multiple database interactions. A pool contains set of Database connections which are already connected, and any client who wants to use it can take it from pool and when done with using it can be returned back to the pool.
Apart from performance this also saves you resources as there may be limited database connections available for your application. How to do database connection using JDBC thin driver ?
This is one of the most commonly asked questions from JDBC fundamentals, and knowing all the steps of JDBC connection is important.
0.import java.sql.*;
1.class JDBCTest {
2.public static void main (String args []) throws Exception
12.ResultSet rs = stmt.executeQuery("select 'Hi' from dual");
13.while (rs.next())
14.System.out.println (rs.getString(1)); // Print col 1 => Hi
15.stmt.close();
16.}
17.}
What does Class.forName() method do? Method forName() is a static method of java.lang.Class. This can be used to dynamically load a class at run-time. Class.forName() loads the class if its not already loaded. It also executes the static block of loaded class. Then this method returns an instance of the loaded class. So a call to Class.forName('MyClass') is going to do following
- Load the class MyClass.
- Execute any static block code of MyClass.
- Return an instance of MyClass.
JDBC Driver loading using Class.forName is a good example of best use of this method. The driver loading is done
0.Class.forName("org.mysql.Driver");
All JDBC Drivers have a static block that registers itself with DriverManager and DriverManager has static initializer method registerDriver() which can be called in a static blocks of Driver class. A MySQL JDBC Driver has a static initializer which looks like this: 1.static {
5.throw new RuntimeException("Can't register driver!");
6.}
7.
Class.forName() loads driver class and executes the static block and the Driver registers itself with the DriverManager.
Which one will you use Statement or PreparedStatement? Or Which one to use when (Statement/PreparedStatement)? Compare PreparedStatement vs Statement.
By Java API definitions: Statement is a object used for executing a static SQL statement and returning the results it produces. PreparedStatement is a SQL statement which is precompiled and stored in a PreparedStatement object. This object can then be used to efficiently execute this statement multiple times.
There are few advantages of using PreparedStatements over Statements
1.Since its pre-compiled, Executing the same query multiple times in loop, binding different parameter values each time is faster. (What does pre-compiled statement means? The prepared statement(pre-compiled) concept is not specific to Java, it is a database concept. Statement precompiling means: when you execute a SQL query, database server will prepare a execution plan before executing the actual query, this execution plan will be cached at database server for further execution.)
2.In PreparedStatement the setDate()/setString() methods can be used to escape dates and strings properly, in a database-independent way.
3.SQL injection attacks on a system are virtually impossible when using PreparedStatements
What does setAutoCommit(false) do?
A JDBC connection is created in auto-commit mode by default. This means that each individual SQL statement is treated as a transaction and will be automatically committed as soon as it is executed. If you require two or more statements to be grouped into a transaction then you need to disable auto-commit mode using below command
0.con.setAutoCommit(false);
Once auto-commit mode is disabled, no SQL statements will be committed until you explicitly call the commit method. A Simple transaction with use of autocommit flag is demonstrated below.
1.con.setAutoCommit(false);
2.PreparedStatement updateStmt =
3.con.prepareStatement( "UPDATE EMPLOYEE SET SALARY = ? WHERE EMP_NAME LIKE ?");
ccon.setAWhat are database warnings and How can I handle database warnings in JDBC?
Warnings are issued by database to notify user of a problem which may not be very severe. Database warnings do not stop the execution of SQL statements. In JDBC SQLWarning is an exception that provides information on database access warnings. Warnings are silently chained to the object whose method caused it to be reported.
Warnings may be retrieved from Connection, Statement, and ResultSet objects.
Handling SQLWarning from connection object
0.//Retrieving warning from connection object
1.SQLWarning warning = conn.getWarnings();
2.
3.//Retrieving next warning from warning object itself
22.//Clear all warnings reported for this resultset object.
23.rs.clearWarnings();
The call to getWarnings() method in any of above way retrieves the first warning reported by calls on this object. If there is more than one warning, subsequent warnings will be chained to the first one and can be retrieved by calling the method SQLWarning.getNextWarning on the warning that was retrieved previously.
A call to clearWarnings() method clears all warnings reported for this object. After a call to this method, the method getWarnings returns null until a new warning is reported for this object.
Trying to call getWarning() on a connection after it has been closed will cause an SQLException to be thrown. Similarly, trying to retrieve a warning on a statement after it has been closed or on a result set after it has been closed will cause an SQLException to be thrown. Note that closing a statement also closes a result set that it might have produced.
What is Metadata and why should I use it?
JDBC API has 2 Metadata interfaces DatabaseMetaData & ResultSetMetaData.
The DatabaseMetaData provides Comprehensive information about the database as a whole. This interface is implemented by driver vendors to let users know the capabilities of a Database Management System (DBMS) in combination with the driver based on JDBC technology ("JDBC driver") that is used with it. Below is a sample code which demonstrates how we can use the DatabaseMetaData
The ResultSetMetaData is an object that can be used to get information about the types and properties of the columns in a ResultSet object. Use DatabaseMetaData to find information about your database, such as its capabilities and structure. Use ResultSetMetaData to find information about the results of an SQL query, such as size and types of columns. Below a sample code which demonstrates how we can use the ResultSetMetaData
5.ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM TABLE2");
6.ResultSetMetaData rsmd = rs.getMetaData();
7.int numberOfColumns = rsmd.getColumnCount();
8.boolean b = rsmd.isSearchable(1);
What is RowSet? or What is the difference between RowSet and ResultSet? or Why do we need RowSet? or What are the advantages of using RowSet over ResultSet?
RowSet is a interface that adds support to the JDBC API for the JavaBeans component model. A rowset, which can be used as a JavaBeans component in a visual Bean development environment, can be created and configured at design time and executed at run time. The RowSet interface provides a set of JavaBeans properties that allow a RowSet instance to be configured to connect to a JDBC data source and read some data from the data source. A group of setter methods (setInt, setBytes, setString, and so on) provide a way to pass input parameters to a rowset's command property. This command is the SQL query the rowset uses when it gets its data from a relational database, which is generally the case.
Rowsets are easy to use since the RowSet interface extends the standard java.sql.ResultSet interface so it has all the methods of ResultSet. There are two clear advantages of using RowSet over ResultSet
oRowSet makes it possible to use the ResultSet object as a JavaBeans component. As a consequence, a result set can, for example, be a component in a Swing application.
oRowSet be used to make a ResultSet object scrollable and updatable. All RowSet objects are by default scrollable and updatable. If the driver and database being used do not support scrolling and/or updating of result sets, an application can populate a RowSet object implementation (e.g. JdbcRowSet) with the data of a ResultSet object and then operate on the RowSet object as if it were the ResultSet object
What is a connected RowSet? or What is the difference between connected RowSet and disconnected RowSet? or Connected vs Disconnected RowSet, which one should I use and when?
Connected RowSet
A RowSet object may make a connection with a data source and maintain that connection throughout its life cycle, in which case it is called a connected rowset. A rowset may also make a connection with a data source, get data from it, and then close the connection. Such a rowset is called a disconnected rowset. A disconnected rowset may make changes to its data while it is disconnected and then send the changes back to the original source of the data, but it must reestablish a connection to do so. Example of Connected RowSet:
A JdbcRowSet object is a example of connected RowSet, which means it continually maintains its connection to a database using a JDBC technology-enabled driver.
Disconnected RowSet
A disconnected rowset may have a reader (a RowSetReader object) and a writer (a RowSetWriter object) associated with it. The reader may be implemented in many different ways to populate a rowset with data, including getting data from a non-relational data source. The writer can also be implemented in many different ways to propagate changes made to the rowset's data back to the underlying data source. Example of Disconnected RowSet:
A CachedRowSet object is a example of disconnected rowset, which means that it makes use of a connection to its data source only briefly. It connects to its data source while it is reading data to populate itself with rows and again while it is propagating changes back to its underlying data source. The rest of the time, a CachedRowSet object is disconnected, including while its data is being modified. Being disconnected makes a RowSet object much leaner and therefore much easier to pass to another component. For example, a disconnected RowSet object can be serialized and passed over the wire to a thin client such as a personal digital assistant (PDA).
What is the benefit of having JdbcRowSet implementation? Why do we need a JdbcRowSet like wrapper around ResultSet?
The JdbcRowSet implementation is a wrapper around a ResultSet object that has following advantages over ResultSet
oThis implementation makes it possible to use the ResultSet object as a JavaBeans component. A JdbcRowSet can be used as a JavaBeans component in a visual Bean development environment, can be created and configured at design time and executed at run time.
oIt can be used to make a ResultSet object scrollable and updatable. All RowSet objects are by default scrollable and updatable. If the driver and database being used do not support scrolling and/or updating of result sets, an application can populate a JdbcRowSet object with the data of a ResultSet object and then operate on the JdbcRowSet object as if it were the ResultSet object.
Adding sample programing which will help us to easily understand the jdbc connectivity with any database. Here Adding sample code for calling function and store procedure from plain java class, which required only oracle odbc 14.jar in classpath;
Table creation script:
CREATE TABLE EMPLOYEE ( ID NUMBER(10, 0) NOT NULL, FIRST_NAME VARCHAR2(20) NOT NULL, LAST_NAME VARCHAR2(20), SALARY NUMBER NOT NULL, DEPTNO NUMBER NOT NULL , CONSTRAINT EMPLOYEE_PK PRIMARY KEY (ID) ENABLE ) ;
CREATE TABLE DEPARTMENT ( DEPT_ID NUMBER NOT NULL, DEPT_NAME VARCHAR2(20), MANAGER NUMBER, LOCATION VARCHAR2(20) , CONSTRAINT DEPARTMENT_PK PRIMARY KEY ( DEPT_ID ) ENABLE );
Function and store procedure code
--- Procedure CREATE OR REPLACE PROCEDURE raise_sal(i_empno NUMBER, i_newsal NUMBER) AS BEGIN UPDATE employee SET salary = i_newsal WHERE id = i_empno; END;
--- Function
CREATE OR REPLACE FUNCTION FUNCTION_Sal (inID NUMBER) RETURN NUMBER IS sal NUMBER; BEGIN SELECT salary INTO sal FROM employee WHERE id = inID; RETURN sal; END;
Can you think of a questions which is not part of this post? Please don't forget to share it with me in comments section & I will try to include it in the list.
