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    Tuesday 21 April 2020

    STRUCTURE OF DBMS or Components of DBMS

    STRUCTURE OF DBMS or Components of DBMS

    DBMS (Database Management System) acts as an interface between the user and the database. The user requests the DBMS to perform various operations (insert, delete, update and retrieval) on the database. The components of DBMS perform these requested operations on the database and provide necessary data to the users.

    The various components of DBMS are shown below: -


    1.      DDL Compiler - Data Description Language compiler processes schema definitions specified in the DDL. It includes metadata information such as the name of the files, data items, storage details of each file, mapping information and constraints etc.


    2.      DML Compiler and Query optimizer - The DML commands such as insert, update, delete, retrieve from the application program are sent to the DML compiler for compilation into object code for database access. The object code is then optimized in the best way to execute a query by the query optimizer and then send to the data manager.

    3.      Data Manager - The Data Manager is the central software component of the DBMS also knows as Database Control System.
    The Main Functions Of Data Manager Are: –
    • Convert operations in user's Queries coming from the application programs or combination of DML Compiler and Query optimizer which is known as Query Processor from user's logical view to physical file system.
    • Controls DBMS information access that is stored on disk.
    • It also controls handling buffers in main memory.
    • It also enforces constraints to maintain consistency and integrity of the data.
    • It also synchronizes the simultaneous operations performed by the concurrent users.
    • It also controls the backup and recovery operations.

    4. Data Dictionary - Data Dictionary is a repository of description of data in the database. It contains information about
    • Data - names of the tables, names of attributes of each table, length of attributes, and number of rows in each table.
    • Relationships between database transactions and data items referenced by them which is useful in determining which transactions are affected when certain data definitions are changed.
    • Constraints on data i.e. range of values permitted.
    • Detailed information on physical database design such as storage structure, access paths, files and record sizes.
    • Access Authorization - is the Description of database users their responsibilities and their access rights.
    • Usage statistics such as frequency of query and transactions.
    Data dictionary is used to actually control the data integrity, database operation and accuracy. It may be used as a important part of the DBMS.

    Importance of Data Dictionary 
    Data Dictionary is necessary in the databases due to following reasons:
    • It improves the control of DBA over the information system and user's understanding of use of the system.
    • It helps in documenting the database design process by storing documentation of the result of every design phase and design decisions.
    • It helps in searching the views on the database definitions of those views.
    • It provides great assistance in producing a report of which data elements (i.e. data values) are used in all the programs.
    • It promotes data independence i.e. by addition or modifications of structures in the database application program are not effected.

    4.      Data Files - It contains the data portion of the database.

    5.      Compiled DML - The DML complier converts the high level Queries into low level file access commands known as compiled DML.

    6.      End Users - End users are the people whose jobs require access to the database for querying, updating, and generating reports;

        There are several categories of end users:

    • Casual end users occasionally access the database, but they may need different information each time. They use a sophisticated database query interface
    • Naive or parametric end users make up a sizable portion of database end users. Their main job function revolves around constantly querying and updating the database, using standard types of queries and updates
    • Sophisticated end users include engineers, scientists, business analysts, and others who thoroughly familiarize themselves with the facilities of the DBMS in order to implement their own applications to meet their complex requirements.
    • Standalone users maintain personal databases by using ready-made pro-gram packages that provide easy-to-use menu-based or graphics-based interfaces. An example is the user of a financial software package that stores a variety of personal financial data.


    Monday 13 April 2020

    SQL Data Types


    SQL Data Types



    The data type of a column defines what value the column can hold: integer, character, money, date and time, binary, and so on.

    Each column in a database table is required to have a name and a data type.

    In MySQL there are three main data types: string, numeric, and date and time.

    String data types:
    Data type
    Description
    CHAR(size)
    A FIXED length string (can contain letters, numbers, and special characters). The size parameter specifies the column length in characters - can be from 0 to 255. Default is 1
    VARCHAR(size)
    A VARIABLE length string (can contain letters, numbers, and special characters). The size parameter specifies the maximum column length in characters - can be from 0 to 65535
    BINARY(size)
    Equal to CHAR(), but stores binary byte strings. The size parameter specifies the column length in bytes. Default is 1
    VARBINARY(size)
    Equal to VARCHAR(), but stores binary byte strings. The size parameter specifies the maximum column length in bytes.
    TINYBLOB
    For BLOBs (Binary Large OBjects). Max length: 255 bytes
    TINYTEXT
    Holds a string with a maximum length of 255 characters
    TEXT(size)
    Holds a string with a maximum length of 65,535 bytes
    BLOB(size)
    For BLOBs (Binary Large OBjects). Holds up to 65,535 bytes of data
    MEDIUMTEXT
    Holds a string with a maximum length of 16,777,215 characters
    MEDIUMBLOB
    For BLOBs (Binary Large OBjects). Holds up to 16,777,215 bytes of data
    LONGTEXT
    Holds a string with a maximum length of 4,294,967,295 characters
    LONGBLOB
    For BLOBs (Binary Large OBjects). Holds up to 4,294,967,295 bytes of data
    ENUM(val1, val2, val3, ...)
    A string object that can have only one value, chosen from a list of possible values. You can list up to 65535 values in an ENUM list. If a value is inserted that is not in the list, a blank value will be inserted. The values are sorted in the order you enter them
    SET(val1, val2, val3, ...)
    A string object that can have 0 or more values, chosen from a list of possible values. You can list up to 64 values in a SET list

    Numeric data types:
    Data type
    Description
    BIT(size)
    A bit-value type. The number of bits per value is specified in size. The size parameter can hold a value from 1 to 64. The default value for size is 1.
    TINYINT(size)
    A very small integer. Signed range is from -128 to 127. Unsigned range is from 0 to 255. The size parameter specifies the maximum display width (which is 255)
    BOOL
    Zero is considered as false, nonzero values are considered as true.
    BOOLEAN
    Equal to BOOL
    SMALLINT(size)
    A small integer. Signed range is from -32768 to 32767. Unsigned range is from 0 to 65535. The size parameter specifies the maximum display width (which is 255)
    MEDIUMINT(size)
    A medium integer. Signed range is from -8388608 to 8388607. Unsigned range is from 0 to 16777215. The size parameter specifies the maximum display width (which is 255)
    INT(size)
    A medium integer. Signed range is from -2147483648 to 2147483647. Unsigned range is from 0 to 4294967295. The size parameter specifies the maximum display width (which is 255)
    INTEGER(size)
    Equal to INT(size)
    BIGINT(size)
    A large integer. Signed range is from -9223372036854775808 to 9223372036854775807. Unsigned range is from 0 to 18446744073709551615. The size parameter specifies the maximum display width (which is 255)
    FLOAT(sized)
    A floating point number. The total number of digits is specified in size. The number of digits after the decimal point is specified in the d parameter. This syntax is deprecated in MySQL 8.0.17, and it will be removed in future MySQL versions
    FLOAT(p)
    A floating point number. MySQL uses the p value to determine whether to use FLOAT or DOUBLE for the resulting data type. If p is from 0 to 24, the data type becomes FLOAT(). If p is from 25 to 53, the data type becomes DOUBLE()
    DOUBLE(sized)
    A normal-size floating point number. The total number of digits is specified in size. The number of digits after the decimal point is specified in the d parameter
    DOUBLE PRECISION(sized)

    DECIMAL(sized)
    An exact fixed-point number. The total number of digits is specified in size. The number of digits after the decimal point is specified in the d parameter. The maximum number for size is 65. The maximum number for d is 30. The default value for size is 10. The default value for d is 0.
    DEC(sized)
    Equal to DECIMAL(size,d)

    Date and Time data types:

    Data type
    Description
    DATE
    A date. Format: YYYY-MM-DD. The supported range is from '1000-01-01' to '9999-12-31'
    DATETIME(fsp)
    A date and time combination. Format: YYYY-MM-DD hh:mm:ss. The supported range is from '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. Adding DEFAULT and ON UPDATE in the column definition to get automatic initialization and updating to the current date and time
    TIMESTAMP(fsp)
    A timestamp. TIMESTAMP values are stored as the number of seconds since the Unix epoch ('1970-01-01 00:00:00' UTC). Format: YYYY-MM-DD hh:mm:ss. The supported range is from '1970-01-01 00:00:01' UTC to '2038-01-09 03:14:07' UTC. Automatic initialization and updating to the current date and time can be specified using DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP in the column definition
    TIME(fsp)
    A time. Format: hh:mm:ss. The supported range is from '-838:59:59' to '838:59:59'
    YEAR
    A year in four-digit format. Values allowed in four-digit format: 1901 to 2155, and 0000.
    MySQL 8.0 does not support year in two-digit format.