Chetan SQL Learns

8/8/2019 Chetan SQL Learns

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Session: Each connected User with Oracle Instance is considered

as one Session.

Instance: All users interact with Instance to retrieve or store

information.

Database: Is like the notebook where Oracle Instance writes theinformation and is encrypted.

SQL commands are used to store and retrieveinformation.SQL is the language that

we will be using to communicate with the Instance.

PL/SQL is the

superset of SQL i.e. we use PL/SQL instead of SQL when our

target/goal requires multiple SQL commands to be executed

based on certain logic, event or condition then we take the

benefit of this language

SQL*Plus is basically the environment where we write SQL or

PL/SQL commands. In other words its a front end tool we use

to communicate with Oracle Instance.

iSQL*Plus is the exact same product as that of SQL*Plus

except that its the web version of SQL*Plus.

It can be accessed

with a URL having following pattern

http://machine_name:5560/isqlplus

Before accessing this URL, execute the following command

which will start the iSQL*Plus application server.

Isqlplusctl start

What are SQL Statements/Commands?

SQL is nothing but a set of statements (commands) and are

categorized into following five groups viz., DQL: Data Query

Language, DML: Data Manipulation Language, DDL: Data

Definition Language, TCL: Transaction Control Language,

DCL: Data Control Language.

The commands falling in each of these categories are shown in

the following table.DQL SELECT

DML DELETE, INSERT, UPDATEDDL CREATE, DROP, TRUNCATE, ALTER

TCL COMMIT, ROLLBACK, SAVEPOINT

DCL GRANT, REVOKE


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SQL Operators grouped together into four groups.

NULL is a special value and

just keep in mind that its not Zero. It can be visualized as empty

field occupying zero byte.

Single Row function

as the name states gets implemented on single records whereas

Group functions get implemented on the multiple records. These

functions will get clarified in your mind once we start discussing

one by one.


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What are SQL Joins?

Joins help in extracting data from two or more tables based on

some condition specified in the WHERE clause

There are following

different kinds of Joins and each having its own application. Outof these, two are having extremely high importance viz., Equi-

Join and Outer-Join.

Self Joins

Cartesian Products

Equijoins

Outer Joins

Self Join

A self join is a join of a table to itself.

Cartesian Product

If in the Join statement you forget to put the WHERE condition

or intentionally dont write the WHERE clause at all, the result

will be Cartesian product. Such type of output will be very rarel


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useful. If one table had five records and other one had six

records; in the output you will get thirty records (5x6=30).

Thats why we use the word product with such type of join.


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Constraints also known as Data Integrity Constraints

NOT NULL is a type of constraint once implemented on any

column; you would not be able to insert NULL values in it.

Similarly with UNIQUE, once defined on any column we wouldnot be able to insert any record in the table with a value that was

already there in UNIQUE constraint column

On the other hand, CHECK constraint is called customconstraint. You have given the liberty to create or define the

constraint according to your needs.

Example:

CREATE TABLE suppliers

( supplier_id numeric(4),

supplier_name varchar2(50),

CONSTRAINT check_supplier_id

CHECK (supplier_id BETWEEN 100 and 9999));

Similarly in this case too Oracle Instance will make sure that

before inserting or updating any information into this suppliers

table, the supplier_id lies between 100 and 9999. If it doesnt

then the user will get a constraint violation error and the

request will not get processed.


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Lets now talk about Primary Key constraint. Once you

implement this constraint on any column, you will not be able to

insert duplicate and NULL value i.e. each value in that column

68

will be a unique identifier for that whole record/row. MoreoverOracle creates an implicit index on that column

Figure 2-18: Difference between Joining and Linking. Joining is at thequery level whereas linking is a physical link between tables. Joins

statements can be executed on tables having Referential Integrity

constraint or not. The word Join should not be confused with as I if

you are joining the two tables permanently.


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------------------------------------------------------------------------------------------------

SELECT identifies the columns to bedisplayed.

FROM identifies the table containing thosecolumns.

Defining a Null Value A null is a value that is unavailable,unassigned, unknown,or inapplicable. A null is not the same as a zero or a blankspace.


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Defining a Column AliasA column alias: Renames a column heading Requires double quotation marks if itcontains spaces orspecial characters, or if it is case-sensitive

Character Strings and Dates Character strings and date values areenclosed in singlequotation marks. Character values are case sensitive, and

date values areformat sensitive. The default date format is DD-MON-RR.

Comparison Conditions

Operator Meaning

Not equal toBETWEEN Between two values (inclusive)...AND...


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IN(set) Match any of a list of valuesLIKE Match a character patternIS NULL Is a null value

< Less than= Greater than or equal to> Greater than= Equal to

.

Logical Conditions

NOT

Returns TRUE if the following condition isfalse

ORReturns TRUE ifeithercomponent conditionis true

AND

Returns TRUE ifboth component conditionsare true

Using the ORDER BYClause Sort retrieved rows with the ORDER BYclause:


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ASC: ascending order, default DESC: descending order

The ORDER BY clause comes last in theSELECT

statement:

3 - 11 Copyright 2009, Oracle. All rights reserved.

Character-Manipulation FunctionsThese functions manipulate characterstrings:REPLACE BLACK and BLUE

('JACK and JUE','J','BL')

LENGTH('HelloWorld') 10

INSTR('HelloWorld', 'W') 6

LPAD(salary,10,'*') *****24000RPAD(salary, 10, '*') 24000*****

CONCAT('Hello', 'World') HelloWorld

TRIM('H' FROM 'HelloWorld') elloWorld

SUBSTR('HelloWorld',1,5) Hello

Function Result

Pageno 107

NVL FunctionConverts a null value to an actual value:


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Types of Group Functions AVG

COUNT MAX MIN STDDEV SUM VARIANCE

The UNION operator returns resultsfrom bothqueries after eliminating duplications.

The UNION ALL operator returnsresults from bothqueries, including all duplications.

The INTERSECT operator returns rowsthat are

common to both queries.

The MINUS operator returns rows in thefirst query


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that are not present in the second query.

Pageno 236

DELETE Statement


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You can remove existing rows from a tableby using theDELETE statement:Specific rows are deleted if you specify theWHERE clause All rows in the table are deleted if you omitthe WHEREclause:

TRUNCATE Statement Removes all rows from a table, leaving thetable emptyand the table structure intact Is a data definition language (DDL)

statement rather than aDML statement; cannot easily be undone

State of the Data AfterCOMMIT Data changes are made permanent in thedatabase.

The previous state of the data ispermanently lost. All users can view the results. Locks on the affected rows are released;those rows are


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available for other users to manipulate. All savepoints are erased.

Database ObjectsTable:- Basic unit of storage; composed of rows

View:- Logically represents subsets of data from one ormore tables

Sequence:- Generates numeric values

Index:- Improves the performance of some queries

Synonym:- Gives alternative names to objects

Naming RulesTable names and column names: Must begin with a letter Must be 130 characters long Must contain only AZ, az, 09, _, $, and#

Must not duplicate the name of anotherobject owned bythe same user


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Must not be an Oracle serverreservedword

FOREIGN KEYConstraint:Keywords FOREIGN KEY: Defines the column in thechild table at thetable-constraint level

REFERENCES: Identifies the table andcolumn in the parenttable ON DELETE CASCADE: Deletes thedependent rows in thechild table when a row in the parent table is

deleted ON DELETE SET NULL: Convertsdependent foreign keyvalues to null

Dropping a Table


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All data and structure in the table aredeleted. Any pending transactions are committed. All indexes are dropped. All constraints are dropped. You cannotroll back the DROP TABLEstatement.

Advantages of ViewsTo restrict data accessTo make complex queries easyTo provide data independenceTo provide data independence

IndexesAn index: Is a schema object Can be used by the Oracle server to

speed up the retrievalof rows by using a pointer Can reduce disk I/O by using a rapid pathaccess method


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to locate data quickly Is independent of the table that it indexes Is used and maintained automatically bythe Oracle server

How Are Indexes Created? Automatically: A unique index is createdautomaticallywhen you define a PRIMARY KEY orUNIQUE constraint ina table definition. Manually: Users can create nonuniqueindexes oncolumns to speed up access to the rows.

Pageno 375

Cartesian Products A Cartesian product is formed when:


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A join condition is omitted A join condition is invalid All rows in the first table are joined to all rows

in the secondtable

To avoid a Cartesian product, alwaysinclude a valid joincondition in a WHERE clause.

Pageno 52(2)

Cascading Constraints The CASCADE CONSTRAINTS clause isused along withthe DROP COLUMN clause. The CASCADE CONSTRAINTS clausedrops all referential

integrity constraints that refer to the primaryand uniquekeys defined on the dropped columns.


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The CASCADE CONSTRAINTS clause alsodrops allmulticolumn constraints defined on thedropped columns.

Pageno 44(2)

{Copying Rows from AnotherTable Write yourINSERT statement with asubquery.

INSERT INTO sales_reps(id, name, salary,commission_pct)

SELECT employee_id, last_name, salary,commission_pctFROM employeesWHERE job_id LIKE '%REP%';33 rows inserted.

Do not use the VALUES clause.

Match the number of columns in theINSERT clause withthat in the subquery.}


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Overview of Multitable INSERTStatements

INSERT ALLINTO table_a VALUES(,,)INTO table_b VALUES(,,)INTO table_c VALUES(,,)

{MERGE Statement Syntax

You can conditionally insert or update rowsin a table by usingthe MERGE statement.

^Pageno 85(2) ^}


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