To get the maximum of two values in MySQL, we can use the predefined function “greatest”. The syntax of greatest() function is as follows −SELECT greatest(value1, value2);Applying the above query, To get the maximum value from two values. The query is as follows −Case 1We are giving both the values int.mysql> SELECT greatest(100, -300); After executing the above query, we will get the following output+--------------------+ | greatest(100, -300) | +--------------------+ | 100 | +--------------------+ 1 row in set (0.00 sec)Case 2We are giving both the ... Read More

We can modify a column size with the help of ALTER command. Let us see how to modify column size. Suppose we are defining any column with some size. At the time of inserting if we are giving more size in comparison to the one we defined, then an error will generate.The above problem can be reduced while modifying the size. For more understanding, we can create a table with the help of CREATE command −mysql> CREATE table ModifyColumnNameDemo -> ( -> id int, -> StudentName varchar(10) -> ); Query OK, 0 rows affected (0.45 sec)After creating a table successfully, ... Read More

To test whether a row exists in a MySQL table or not, use exists condition. The exists condition can be used with subquery. It returns true when row exists in the table, otherwise false is returned. True is represented in the form of 1 and false is represented as 0.For better understanding, firstly we will create a table with the help of CREATE command. The following is the query to create a table −mysql> CREATE table ExistsRowDemo -> ( -> ExistId int, -> Name varchar(100) -> ); Query OK, 0 rows affected (0.53 sec)After creating the table successfully, we will ... Read More

The error-#1046 can occur when we are creating a table, but forget to select the database. Let us say we have started MySQL as shown below −After giving the correct password, the above window will open. Now create a table without choosing any database. This will show an error −mysql> CREATE table TblUni -> ( -> id int, -> Name varchar(100) -> );ERROR 1046 (3D000): No database selectedThe following screenshot is showing the same error −Now, choose any database to get rid of the above error. Firstly, let us check how many databases are present in MySQL with the help ... Read More

The backup of a table can be made with the help of backup table as well as mysqldump utility. The backup table concept was used in MySQL version 5.0 and its earlier version. Here, I am performing backup with the help of mysqldump. Firstly, we will open cmd with the help of shortcut key. The mysqldump will run at the cmd. Therefore, firstly open cmd with the help of shortcut key − windowskey+R; Here is the snapshot − Now, cmd will open − In this, the MySQL bin folder is present at the following location − ... Read More

A relationship is an important part of any Entity relationship diagram as it shows the relation between two different entities. In an n - ary relationship, the n shows the number of entities in the relationship. It can be anything but the most popular relationships are unary, binary and ternary where the number of entities respectively are one, two and three.More information about Unary, Binary and Ternary relationships is as follows −Unary RelationshipWhen there is a relationship between two entities of the same type, it is known as a unary or recursive relationship. This means that the relationship is between ... Read More

A centralized database is stored at a single location such as a mainframe computer. It is maintained and modified from that location only and usually accessed using an internet connection such as a LAN or WAN. The centralized database is used by organisations such as colleges, companies, banks etc.As can be seen from the above diagram, all the information for the organisation is stored in a single database. This database is known as the centralized database.AdvantagesSome advantages of Centralized Database Management System are −The data integrity is maximised as the whole database is stored at a single physical location. This ... Read More

Database SecurityDatabase security has many different layers, but the key aspects are:AuthenticationUser authentication is to make sure that the person accessing the database is who he claims to be. Authentication can be done at the operating system level or even the database level itself. Many authentication systems such as retina scanners or bio-metrics are used to make sure unauthorized people cannot access the database.AuthorizationAuthorization is a privilege provided by the Database Administer. Users of the database can only view the contents they are authorized to view. The rest of the database is out of bounds to them.The different permissions for ... Read More

Locks are an integral part to maintain concurrency control in DBMS. A transaction in any system implementing lock based concurrency control cannot read or write a statement until it has obtained the required locks.There are two types of locks in Lock based protocols. These are:Binary Locks - These can only be in one of two states, locked or unlocked.Shared/Exclusive Locks - Shared locks are acquired when only read operation is to be performed. Shared locks can be shared between multiple transactions as there is no data being altered. Exclusive locks are used when write operation is performed. Only the transaction holding the ... Read More

In a distributed database management system, the database is not stored at a single location. Rather, it may be stored in multiple computers at the same place or geographically spread far away. Despite all this, the distributed database appears as a single database to the user. A diagram to better explain this is as follows:As seen in the figure, the components of the distributed database can be in multiple locations such as India, Canada, Australia, etc. However, this is transparent to the user i.e the database appears as a single entity.Types of Distributed Database Management SystemThe following are the types ... Read More