Page 47 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Database Management Systems/Managing Database

Notes string representing that color’s name; allowable operations include addition and subtraction,
but not multiplication.
A data type can also be thought of as a constraint placed upon the interpretation of data in a type
system, describing representation, interpretation and structure of values or objects stored in
computer memory. The type system uses data type information to check of computer programs
that access or manipulate the data.
Here are the most common data types:

char(size) Fixed-length character string. Size is specified in parenthesis. Max 255 bytes.
Varchar(size) Variable-length character string. Max size is specified in parenthesis.
number(size) Number value with a max number of column digits specified in parenthesis.
Date Date value
number(size,d) Number value with a maximum number of digits of "size" total, with a
maximum number of "d" digits to the right of the decimal.

3.4 Schema Definition

It is the overall structure is called a database schema. Database schema is usually graphical
presentation of the whole database. Tables are connected with external keys and key colums.
When accessing data from several tables, database schema will be needed in order to find
joining data elements and in complex cases to find proper intermediate tables. Some database
products use the schema to join the tables automatically.
Database system has several schemas according to the level of abstraction. The physical schema
describes the database design at physical level. The logical schema describes the database design
at logical level. A database can also have sub-schemas (view level) that describes different views
of database.

3.5 Basic Structure of SQL Queries

The general format of any SQL query is as follows:
SELECT [DISTINCT] select-list
FROM from-list
WHERE qualification
1. Select Clause: SELECT is a command from DML language which is used to select specific
columns of the tables. Every SQL query starts with SELECT keyword and is followed by
the list of the columns that forms the resulting relation.
2. From: This clause specifies the names of the tables from which data is to be retrieved.
FROM is followed by a set of range values which uniquely identifies a table, when its
name occurs more than once.
3. Where: WHERE keyword is used to specify a condition. All the tuples which satisfies the
condition are selected.
4. Distinct: The use of this keyword is to prevent the duplication of rows, when this keyword
is used all the tuples of the table are unique. There is no redundant data. The use of this
keyword is optional.

5. Select-List: Collection of all the column names that user want in the new relation.

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