Monday, 6 March 2017

GETTING STARTED WITH ORACLE

*  Creating a table
*  Datatypes
*  Displaying table definition using DESCRIBE
*  Inserting rows into a table
*  Selecting rows from a table
*  Editing SQL buffer
*  Summary
*  Exercises
 
Creating a Table
A Table is a collection of rows and columns.  Data in relational model is stored in tables. Let us create a table first. Then we will understand how to store data into table and retrieve data from the table.

Before a table is created the following factors of a table are to be finalized.

*  What data table is supposed to store.
*  The name of the table. It should depict the content of the table.
*  What are the columns that table should contains
*  The name, data type and maximum length of each column of the table.
*  What are the rules to be implemented to main data integrity of the table.

The following is an example of  creation of  COURSES table. We actually have six tables in the application that we use throughout the book. We will discuss more about all the tables in the next chapter.  But for now, let us create COURSES table and understand how to use basic SQL commands.

The following CREATE TABLE command is used to create COURSES table.

SQL> create table COURSES
  2  (  ccode        varchar2(5),
  3     name         varchar2(30),
  4     duration     number(3),
  5     fee          number(5),
  6     prerequisite varchar2(100)
7    );

Table Created

The above command creates a table called COURSES. This table contains 5 columns. We will discuss about rules to be implemented in this table in the next chapter, where we will recreate this table with all the required rules.

For the time being I want to keep things simple. That is the reason why I am not taking you into constraint and remaining.

Well, we have created our first table. If command is successful, Oracle responds by displaying the message Table Created.

Rules to be followed for names
The following are the rules to be followed while naming an Oracle Object. These rules are applicable for name of the table and column.

*  The name must begin with a letter - A-Z or a-z.
*  Letters, digits and special characters – underscore (_),  $ and  # are allowed.
*  Maximum length of the name is 30 characters.
*  It must not be an SQL reserved word.
*  There should not be any other object with the same name in your account.

Note:  A table can contain up to1000 columns in Oracle8 or above, whereas in Oracle7 a table can contain only 254 columns.

Datatypes
Each column of the table contains the datatype and maximum length, if it is length is applicable. Datatype of the column specifies what type of data can be stored in the column.

The datatype VARCHAR2 is to store strings that may have different number of characters, NUMBER is used to store numbers. The maximum length, which is given in parentheses after the datatype, specifies how many characters (or digits) the column can store at the most. For example, column VARCHAR2 (20) would mean it can store up to 20 characters.

Table-1 lists out datatypes available in Oracle8i along with what type of data can be stored and maximum length allowed.


Datatype
Description
VARCHAR2( len)
Can store up to len number of characters.  Each character would occupy one byte. Maximum width is 4000 characters.
VARCHAR(len)
Same as VARCHAR2. But use VARCHAR2  as Oracle might change the usage of VARCHAR in future releases.
CHAR(len)
Fixed length character data. If len is given then it can store up to len number of characters. Default width is 1. String is padded on the right with spaces until string is of len size. Maximum width is 2000.
NUMBER
Can store numbers up to 40 digits plus decimal point and sign.
NUMBER (p ,s)
P represents the maximum significant digits allowed. S is the number of digits on the right of the decimal point.
DATE
Can store dates in the range 1-1-4712 B.C to 31-12-4712 AD.
LONG
Variable length character values up to 2 gigabytes. Only one LONG column is allowed per table. You cannot use LONG datatype in functions, WHERE clause of SELECT,  in indexing  and subqueries.
RAW and LONG RAW
Equivalent to VARCHAR2 and LONG respectively, but used for storing byte-oriented or binary data such as digital sound or graphics images.
CLOB, BLOB, NCLOB
Used to store large character and binary objects. Each can accommodate up to 4 gigabytes. We will discuss more about it later in this book.
BFILE
Stores a pointer to an external file. The content of the file resides in the file system of the operation system. Only the name of the file is stored in the column.
ROWID
Stores a unique number that is used by Oracle to uniquely identify each row of the table.
NCHAR (size)
Same as CHAR, but supports national language.
NVARCHAR2 (size)
Same as VARCHAR2, but supports national language.
Table 1: Oracle Datatypes.

Displaying table definition using DESCRIBE
You can display the structure of a table using SQL*PLUS command DESCRIBE.  It displays then name, datatype and whether the column can store null value for each column of the table.

The following is the syntax of DESCRIBE command.

1.       DESC[RIBE] objectname

Displays the column definitions for the specified object. The object may be a table, view, synonym, function or procedure.


To display the structure of COURSES table, enter:

SQL> DESC COURSES
 Name                                           Null?    Type
 ---------------------------------------------- -------- ----------
 CCODE                                          NOT NULL VARCHAR2(5)
 NAME                                                    VARCHAR2(30)
 DURATION                                                NUMBER(3)
 FEE                                                     NUMBER(5)
 PREREQUISITE                                            VARCHAR2(100)


DESCRIBE is an SQL*Plus command and can be abbreviated to DESC.

Inserting rows into a table
Now, let us see how to insert rows into COURSES table. SQL command INSERT is used to insert new row into the table.

While inserting rows, you may enter value for each column of the table or selected columns.

The following command inserts a row into COURSES table.

insert into courses
    values('ora','Oracle database',25,4500,'Knowledge of Windows');

Note: After inserting the required row, issues COMMIT command to make sure the changes are made permanent.  We will discuss more about COMMIT command later in this book but for the time being it is sufficient to know that COMMIT command will make changes permanent. Without COMMIT, rows that are inserted might be lost if there is any power failure.

During insertion, character values are enclosed in single quotes. Unless otherwise specified we have to supply a value for each column of the table. If the value of any column is not known or available then you can give NULL as the value of the column.
For example, the following insert will insert a new row with null value for PREREQUISITE column.

insert into courses
 values('c','C Programming',25,3000,null);


Note: INSERT command can insert only one row at a time. For multiple row, INSERT command must be issued for multiple times.

DATE type values must be in the format DD-MON-YY or DD-MON-YYYY, where MON is the first three letters of the month (Jan, Feb). If only two digits are given for year then current century is used. For example, if you give 99 for year, Oracle will take it as 2099 as the current century is 2000.  So it is important to remember this and give four digits if required.

The following is the complete syntax for INSERT command.

2.       INSERT INTO tablename [(columns list)]
3.              {VALUES (value-1,...) |  subquery }

We will see how to insert row into a table using a subquery later in this book.

Inserting a row with selected columns
It is possible to insert a new row by giving values only for a few columns instead of giving values for all the available columns.

The following INSERT command will insert a new row only two values.

insert into courses(ccode,name)
     values ('odba','Oracle Database Administration');


The above command will create a new row in COURSES table with values for only two columns – CCODE and NAME. The remaining columns will take NULL value or the default value, if the column is associated with default value. We will discuss more about default value in the next chapter.

NULL value
Null value means a value that is not available or not known. When a column’s value is not known then we store NULL value into the column. NULL value is neither 0 nor blank nor any other known value.  We have already seen how to store null value into a column and when Oracle automatically stores null value into a column. We will discuss more about how to process null value later in this chapter.

Selecting rows from a table
Let us see how to retrieve data of a table. SELECT command of SQL is used to retrieve data from one or more tables.  It implements operators of relational algebra such as projection, and selection.

The following is the syntax of SELECT command. The syntax given here is incomplete. For complete syntax, please refer to online documentation.


SELECT [DISTINCT | ALL]
     {* | table.* | expr }  [alias ]
     [ {table}.*| expr }  [alias ]  ] ... 

    FROM  [schema.]object 
       [, [schema.]object ] ...

    [WHERE condition]

    [ORDER BY {expr|position} [ASC | DESC]
      [, {expr|position} [ASC | DESC]] ...]



schema is the name of the user whose table is being accessed. Schema prefix is not required if the table is in the current account. Schema prefix is required while we are accessing a table of some other account and not ours.

The following is an example of a basic SELECT command.

select  * from courses;

CCODE NAME                  DURATION       FEE PREREQUISITE
----- -------------------- --------- --------- -------------------------
ora   Oracle database             25      4500 Windows
vbnet VB.NET                      30      5500 Windows and programming
c     C programming               20      3500 Computer Awareness
asp   ASP.NET                     25      5000 Internet and programming
java  Java Language               25      4500 C language
xml   XML Programming             15      4000 HTML,Scripting, ASP/JSP

The simplest SELECT command contains the following:

*  Columns to be displayed. If * is given, all columns are selected.
*  The name of the table from where rows are to be retrieved.

Projection
Projection is the operation where we select only a few columns out of the available columns. The following is an example of projection.

select name,fee from courses;

NAME                       FEE
-------------------- ---------
Oracle database           4500
VB.NET                    5500
C programming             3500
ASP.NET                   5000
Java Language             4500
XML Programming           4000
 
Using expressions in SELECT command
It is also possible to include expressions in the list of columns. For example, the following SELECT will display discount to be given for each course.

select name,fee, fee * 0.15 from courses;

NAME                       FEE  FEE*0.15
-------------------- --------- ---------
Oracle database           4500       675
VB.NET                    5500       825
C programming             3500       525
ASP.NET                   5000       750
Java Language             4500       675
XML Programming           4000       600

Column Alias
The column heading of an expression will be the expression itself. However, as it may not be meaningful to have expression as the result of column heading, we can give an alias to the column so that alias is displayed as the column heading.

The following example will use alias DISCOUNT for the expression FEE * 0.15.

select name, fee, fee * 0.15 DISCOUNT from courses

NAME                       FEE  DISCOUNT
-------------------- --------- ---------
Oracle database           4500       675
VB.NET                    5500       825
C programming             3500       525
ASP.NET                   5000       750
Java Language             4500       675
XML Programming           4000       600

The following are the arithmetic operators that can be used in expressions.

Operator
Description
+
Add
-
Subtract
*
Multiply
/
Divide

ORDER BY clause
It is possible to display the rows of a table in the required order using ORDER BY clause.  It is used to sort rows on the given column(s) and in the given order at the time of retrieving rows. Remember, sorting takes place on the row that are retrieved and in no way affects the rows in the table. That means the order of the rows will remain unchanged.

Note: ORDER BY must always be the last of all clauses used in the SELECT command.

The following SELECT command displays the rows after sorting rows  on  course fee.

select name, fee from  courses order by fee;

NAME                       FEE
-------------------- ---------
C programming             3500
XML Programming           4000
Oracle database           4500
Java Language             4500
ASP.NET                   5000
VB.NET                    5500


Note:  Null values are placed at the end in ascending order and at the beginning in descending order.

The default order for sorting is ascending. Use option DESC to sort in the descending order. It is also possible to sort on more than one column.

To sort rows of COURSES table in the ascending order of DURATION and descending order of FEE, enter:

select  name, duration, fee from courses
order by  duration , fee desc;

NAME                  DURATION       FEE
-------------------- --------- ---------
XML Programming             15      4000
C programming               20      3500
ASP.NET                     25      5000
Oracle database             25      4500
Java Language               25      4500
VB.NET                      30      5500


First, all rows are sorted in the ascending order of DURATION column. Then the rows that have same value in DURATION column will be further sorted in the descending order of FEE column.

  Using column position

Instead of giving the name of the column, you can also give the position of the column on which you want to sort rows.

For example, the following SELECT sorts rows based on discount to be given to each course.

select  name, fee, fee * 0.15
from courses
order by 3;

NAME                       FEE  FEE*0.15
-------------------- --------- ---------
C programming             3500       525
XML Programming           4000       600
Oracle database           4500       675
Java Language             4500       675
ASP.NET                   5000       750
VB.NET                    5500       825


Note: Column position refers to position of the column in the selected columns and not the position of the column in the table.

The above command uses column position in ORDER BY clause. Alternatively you can use column alias in ORDER BY clause as follows:

select  name, fee, fee * 0.15 discount
from courses
order by discount;

NAME                       FEE  DISCOUNT
-------------------- --------- ---------
C programming             3500       525
XML Programming           4000       600
Oracle database           4500       675
Java Language             4500       675
ASP.NET                   5000       750
VB.NET                    5500       825

Selection
It is possible to select only the required rows using WHERE clause of SELECT command. It implements selection operator of relational algebra.

WHERE clause specifies the condition that rows must satisfy in order to be selected.  The following example select rows where FEE is more than or equal to 5000.

select name, fee from courses
where fee >= 5000

NAME                       FEE
-------------------- ---------
VB.NET                    5500
ASP.NET                   5000

The following relational and logical operators are used to form condition of WHERE clause. Logical operators – AND, OR – are used to combine conditions.   NOT operator reverses the result of the condition. If condition returns true,  NOT will make the overall condition false.



Operator
Meaning
=
Equal to
!= or  <>
Not equal to
>=
Greater than or equal to
<=
Less than  or equal to
Greater than
Less than
AND
Logical ANDing
OR
Logical Oring
NOT
Negates result of condition.

The following SELECT command displays the courses where duration is more than 15 days and course fee is less than 4000.

select  * from courses
where duration > 15 and fee < 4000;

CCODE NAME                  DURATION       FEE PREREQUISITE
----- -------------------- --------- --------- -------------------
c     C programming               20      3500 Computer Awareness

The following SELECT command retrieves the details of course with code ORA.

select * from courses
where ccode = 'ora';

CCODE NAME                  DURATION       FEE PREREQUISITE
----- -------------------- --------- --------- ----------------
ora   Oracle database             25      4500 Windows


Note: When comparing   strings, the case of the string must match. Lowercase letters are not equivalent to uppercase letters.

SQL Operators
Apart from standard relational operators (= and  >), SQL has some other operators that can be used in conditions.

Operator
What it does?
BETWEEN value-1 AND value-2
Checks whether the value is in the given range. The range is inclusive of the given values.
IN(list)
Checks whether the value is matching with any one of the values given in the list. List contains values separated by comma(,).
LIKE  pattern
Checks whether the given string is matching with the given pattern.  More on this later.
IS NULL   and IS NOT NULL
Checks whether the value is null or not null.
Table 2: SQL Operators.

Now, let us see how to use these special operators of SQL.

BETWEEN ... AND Operator
Checks whether value is in the given range. The range includes all the values in the range including the min and max values. This supports DATE type data also.

To display the list of course where DURATION is in the range 20 to 25 days, enter:

select  name
from courses
where  duration between 20 and 25;

NAME
--------------------
Oracle database
C programming
ASP.NET
Java Language

Note: BETWEEN.. AND is alternative to using  >= and  <= operators.
     
    IN Operator
Compares a single value with a list of values. If the value is matching with any of the values given in the list then condition is taken as true.

The following command will retrieve all courses where duration is either 20 or 30 days.

select name
from courses
where  duration in (20,30);

NAME
--------------------
VB.NET
C programming

The same condition can be formed even without IN operator using logical operator OR as follows:

Select name from courses where duration = 20 or duration = 30;

However, it will be more convenient to user IN operator compared with multiple conditions compared with OR operator.

LIKE operator
This operator is used to search for values when the exact value is not known.  It selects rows that match the given pattern.  The pattern can contain the following special characters.

Symbol
Meaning
%
Zero or more characters can take the place of %.
_ (underscore)
Any single character can take the place of underscore. But there must be one letter.

To select the courses where the course name contains pattern .NET, enter:

select name,duration, fee from courses
where  name like '%.NET%'

NAME                  DURATION       FEE
-------------------- --------- ---------
VB.NET                      30      5500
ASP.NET                     25      5000

The following example selects courses where second letter in the course code is “b” and column PREREQUISITE contains word  “programming”.
 
select  * from courses
where  ccode like '_b%' and prerequisite like '%programming%';

CCODE NAME                  DURATION       FEE PREREQUISITE
----- -------------------- --------- --------- ------------------------
vbnet VB.NET                      30      5500 Windows and programming

Remember LIKE operator is case sensitive.  In the above example, if CCODE contains value in uppercase (VB), then it won’t be a match to the pattern.

IS NULL and IS NOT NULL operators
These two operators test for null value. If we have to select rows where a column is containing null value or not null value then we have to use these operators.

For example the following SELECT command will select all the courses where the column FEE is null.

select * from courses
where fee is null;

Though Oracle provides NULL keyword, it cannot be used to check whether the value of a column is null. For example, the following condition will always be false as Oracle treats two null values as two different values.

select * from courses
where fee = null;

The above command does NOT work as fee though contains null value will not be equal to NULL. SO, we must use IS NULL operator.

Selecting distinct values
DISTINCT clause of SELECT command specifies only distinct values of the specified column must be selected.

The following SELECT command will display only distinct course fee values from COURSES table.

select distinct fee from courses;

      FEE
---------
     3500
     4000
     4500
     5000
     5500

Whereas the same query without DISTINCT clause will select the following.

select fee from courses;

      FEE
---------
     4500
     5500
     3500
     5000
     4500
     4000

Editing   SQL Buffer
Whenever you enter an SQL command in SQL*Plus, it is stored in an area in the memory called as SQL Buffer.  It is possible to edit the command that is stored in SQL Buffer using a set of commands provided by SQL*Plus. All these commands are SQL*Plus commands.

Note: SQL*PLUS commands like DESCRIBE are not stored in the buffer. Only SQL commands are stored in the buffer.

The following list of SQL*PLUS commands are used for editing and other operations related to SQL buffer.


Command
Purpose
A[PPEND]  text
Adds text to the end of the current line in the buffer.
DEL
Deletes current line in the buffer.
I[NPUT]  [text]
If text is given, then text is placed after the current line, otherwise it allows you to enter a series of lines and places them after the current line.
L[IST]
Displays the contents of buffer.
R[UN]
Runs the current command in the buffer.
/
Same as RUN, but doesn’t display command being executed.
SAVE filename
Saves the contents of buffer into filename, which is  a file in the host system.
GET   filename
Places the contents of filename into buffer.
START filename
Executes commands that are in the given file. The file is also called as start file and command file.
EDIT  [filename]
Invokes an editor in the host and places the contents of either given filename or SQL buffer if filename is not given.
HOST   command
Executes the command, which is a valid command in the host system.
EXIT
Quits SQL*PLUS.
Table 3: Editing Commands.

Summary
In this chapter we have seen some fundamental SQL commands such as CREATE TABLE, INSERT and SELECT. 

SELECT command is the most frequently used command. It is very important to understand how to retrieve required data using SELECT command. We have seen some special operators that are available in SQL.   We have seen  WHERE and ORDER BY clauses of SELECT command.

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