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SQL Introduction

About SQL

SQL stands for Structured Query Language.

SQL is a language that enables you to work with a database. Using SQL, you can insert records,

update records, and delete records. You can also create new database objects such as databases and

tables. And you can drop (delete) them. More advanced features include creating stored procedures

(self contained scripts), views (pre-made queries), and setting permissions on database objects (such

as tables, stored procedures, and views).Although SQL is an ANSI (American National Standards Institute) standard, there are many

different versions of SQL. Different database vendors have their own variations of the language.

Having said this, to be in compliance with the ANSI standard, they need to at least support the

major commands such as DELETE, INSERT, UPDATE, WHERE etc. Also, you will find that

many vendors have their own extensions to the language - features that are only supported in their

database system. Furthermore, transact-SQL is an extension to the ANSI standard and provides

extra functionality.

Using SQL

If you need to build a website with a database providing the content, you will generally need the

following:A server side scripting language (i.e. ColdFusion, PHP, ASP/.NET)

A database query language (i.e. SQL)

A client side markup language and style sheets (i.e. HTML/CSS)

Although SQL can be quite involved, you can achieve a lot with a handful of SQL statements.

When using SQL on a website, you will often find yourself either selecting a record, inserting a

record, updating a record, or deleting a record. Fortunately, SQL has commands for performing

each of these actions.

The next lesson covers the SQL syntax basics.

SQL Inner Join

As discussed in the previous lesson, you should use the SQL INNER JOIN when you only want toreturn records where there is at least one row in both tables that match the join condition.

Example SQL statement

SELECT * FROM Individual

INNER JOIN Publisher

ON Individual.IndividualId = Publisher.IndividualId

WHERE Individual.IndividualId = '2'

Source Tables

Left Table

Id FirstName LastName UserName

1 Fred Flinstone freddo

2 Homer Simpson homey

3 Homer Brown notsofamous

4 Ozzy Ozzbourne sabbath

5 Homer Gain noplacelike

Right Table

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IndividualId AccessLevel

1 Administrator

2 Contributor

3 Contributor

4 Contributor

10 Administrator

ResultIndividualId FirstName LastName UserName IndividualId AccessLevel

2 Homer Simpson homey 2 Contributor

Next lesson covers the SQL OUTER JOIN.

SQL Outer Join

This lesson covers both the left outer join, the right outer join, and the full outer join, and explains

the differences between them. There are some occasions where you would need to use a left outer

join or a right outer join, and others where you would need a full outer join. The join type you use

will depend on the situation and what data you need to return.

Left Outer Join

Use this when you only want to return rows that have matching data in the left table, even if there'sno matching rows in the right table.


SELECT * FROM Individual AS IndLEFT JOIN Publisher AS Pub

ON Ind.IndividualId = Pub.IndividualId

Source Tables

Left Table


1 Fred Flinstone freddo2 Homer Simpson homey




Right Table


1 Administrator

2 Contributor

3 Contributor

4 Contributor

10 Administrator

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Result

IndividualId FirstName LastName UserName IndividualId AccessLevel

1 Fred Flinstone freddo 1 Administrator


3 Homer Brown notsofamous 3 Contributor

4 Ozzy Osbourne sabbath 4 Contributor

5 Homer Gain noplacelike NULL NULL

Right Outer JoinUse this when you only want to return rows that have matching data in the right table, even if there's no

matching rows in the left table.Example SQL statement

SELECT * FROM Individual AS Ind

RIGHT JOIN Publisher AS PubON Ind.IndividualId = Pub.IndividualId

Source TablesLeft Table







Right Table


1 Administrator

2 Contributor

3 Contributor

4 Contributor

10 Administrator

Result






NULL NULL NULL NULL 10 Administrator Full Outer Join

Use this when you want to all rows, even if there's no matching rows in the right table.

Example SQL statementSELECT * FROM Individual AS Ind

FULL JOIN Publisher AS Pub

ON Ind.IndividualId = Pub.IndividualIdSource Tables

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Left Table






5 Homer Gain noplacelikeRight Table


1 Administrator

2 Contributor

3 Contributor

4 Contributor

10 Administrator

Result







NULL NULL NULL NULL 10 Administrator

SQL Insert

Up until now, this tutorial has covered the SELECT statement and variations on it. We are now about tolearn a new statement - the INSERT statement.

The SQL INSERT command allows you to insert a record into a table in your database. As with the

SELECT syntax, the INSERT syntax is quite straight forward.SQL statement

INSERT INTO Individual

VALUES ( '6', 'Benny', 'Hill', 'hillbenny' )

Source Table

IndividualId FirstName LastName UserName






Result

Now if we do a SELECT on the Individual table, we can see the new record added to the bottom of the

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result set.







6 Benny Hill hillbennySee - nothing to it!

Now, it's important to note that the INSERT statement is used only when you want to add a new record tothe table. If you want to update an existing record, use an UPDATE statement. The UPDATE command is

described in the next lesson.

SQL Update

The SQL UPDATE statement allows you to update an existing record in the database.

The UPDATE command uses a WHERE clause. If you don't use a WHERE clause, all rows will be updated.

In fact, the syntax for a basic UPDATE statement is very similar to a SELECT statement.SQL statement

UPDATE Individual

SET UserName = 'funnyman'WHERE IndividualId = '6'

Source Table







6 Benny Hill funnyman

Result

Now if we select this record, we can see the updated value.



Updating Multiple Fields

To update multiple fields, separate each field assignment with a comma.SQL statement

UPDATE Individual

SET UserName = 'getserious', FirstName = 'Onetree'

WHERE IndividualId = '6'

Result


6 Onetree Hill getserious

Next lesson covers the DELETE statement.

SQL Delete

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On top of these functions, different database vendors have their own built-in functions for their products.

Also, most products enable programmers to program their own User Defined Functions. For information onproprietary functions, you should consult the vendor's documentation.

In the next lesson you will learn about one of the more commonly used SQL functions - the COUNT

function.

SQL Count

A commonly used aggregate function in SQL is COUNT. COUNT returns the number of rows that match

the given criteria.

COUNT(*)If we only want to see how many records are in a table (but not actually view those records), we could use

COUNT(*). COUNT(*) returns everything - including null values and duplicates.

SQL statementSELECT COUNT(*) FROM Individual

Source Table







6 Bono u2

Result

6

COUNT(column name)

If we want to see how many non-null values are in a given column, we use COUNT(column name) where

column name is the name of the column we want to test.SQL statement

SELECT COUNT(LastName) FROM Individual

Source Table







6 Bono u2

Result

5

Combining COUNT & DISTINCT

If we only want to see how many unique names are in the table, we could nest the DISTINCT inside a

COUNT function.SQL statement

SELECT COUNT(DISTINCT(FirstName)) FROM Individual

Result

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4

SQL Create

Most database systems provide a way for you to create database objects via a WYSIWYG interface. For

example, Microsoft's SQL Server has Enterprise Manager. The Enterprise Manager gives you a kind ofgraphical representation of your database system. You can browse through your databases, view the tables

and their contents etc.

Despite having a tool like Enterprise Manager to make these tasks easier, there can be good reasons forwanting to perform some of these tasks programatically. Possible reasons could include:

Your application allows users to create objects on the fly.

You have multiple environments (for example development, staging, production). It's much easier, and lesserror prone, to run a script against 3 environments than to open up Enterprise Manager and repeat the steps

in 3 environments - especially if you have lots of changes.

You don't have access to Enterprise Manager (or your database system's equivalent).

Fortunately, SQL has a number of CREATE commands that enable you to programatically create databaseobjects including the database, its tables and more.

Here are the CREATE commands supported by SQL Server:

CREATE ACTION

CREATE CACHECREATE CELL CALCULATION

CREATE CUBECREATE DATABASE

CREATE DEFAULT

CREATE FUNCTION

CREATE INDEXCREATE MEMBER

CREATE MINING MODEL

CREATE PROCEDURECREATE RULE

CREATE SCHEMACREATE SETCREATE STATISTICS

CREATE TABLE

CREATE TRIGGER

CREATE UNIQUE CLUSTERED INDEXCREATE VIEW

We're not going to cover all these here but, over the next few lessons, we'll cover some of the most common

CREATE commands.SQL Create Database

You can create a database using the CREATE DATABASE command.

SQL syntaxCREATE DATABASE database_name

Example CodeThis statement creates a database called "Payroll". Because no arguments have been specified, the database

data files and transaction logs will be created automatically in the default location.

CREATE DATABASE Payroll

Adding Arguments

There are a number of optional arguments that you can supply with the CREATE DATABASE command.

You should check your database system's documentation for the specific arguments supported and their

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usage, but here's an example of supplying arguments when creating a database using Microsoft's SQL

Server.Example Code

In this example, we are supplying the name and location of the database's data file and transaction log. We

are also specifying the initial size of these files (with the SIZE argument), the maximum size it can grow to(with the MAXSIZE argument) and the growth increment of each file (using the FILEGROWTH) argument.USE masterGO

CREATE DATABASE PayrollON( NAME = Payroll_dat,

FILENAME = 'c:\program files\microsoft sqlserver\mssql\data\payrolldat.mdf',

SIZE = 20MB,MAXSIZE = 70MB,FILEGROWTH = 5MB )

LOG ON( NAME = 'Payroll_log',

FILENAME = 'c:\program files\microsoft sqlserver\mssql\data\payroll.ldf',

SIZE = 10MB,MAXSIZE = 40MB,FILEGROWTH = 5MB )

GO

Next up is the CREATE TABLE command.

SQL Create TableYou create a table using the CREATE TABLE command.

SQL syntax

CREATE TABLE table_name(column_name_1 datatype,column_name_2 datatype,...)

Example

CREATE TABLE Individual(IndividualId int,FirstName Varchar(255),LastName Varchar(255),UserName Char(10))

Result

This results in an empty table. You can now use an INSERT statement to add data to the table.


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Data Types

You'll notice we explicitly stated the data type in our CREATE TABLE statement. This is because, whenyou create a column, you need to tell the database what type of data it can hold.

The exact data types and how their expressed differs with each database system and vendor, but you'll find

that generally, there will be support for fixed length strings (eg char), variable length strings (eg varchar),date/time values (eg datetime), numbers and integers (eg, bigint, int, smallint, tinyint, numeric).

The following base data types are available in SQL Server 2000.bigint Binary bit char cursor

datetime Decimal float image int

money Nchar ntext nvarchar real

smalldatetime Smallint smallmoney text timestamp

tinyint Varbinary Varchar uniqueidentifier

You may need to consult your database system's documentation if you're unsure of which data type to use or

how it is expressed in that system.

Next, we learn how to create an index for our table with the CREATE INDEXcommand.

SQL Create IndexIndexes can be created against a table to make searches more efficient. A database index is similar to an

index of a book - a book index allows you to find information without having to read through the wholebook. A database index enables the database application to find data quickly without having to scan the

whole table.

Indexes can have a slight impact on performance so you should only create indexes against tables and

columns that will be frequently searched against. For example, if users of your application often searchagainst the LastName field then that field is a great candidate for an index.

You create an index using the CREATE INDEX command.

SQL syntax

CREATE INDEX index_nameON table_name (column_name)

Example

CREATE INDEX IndividualIndexON Individual (LastName)

SQL Alter TableIn an earlier lesson, we created a table with the CREATE TABLE command. In this lesson, we will modify

the table using the ALTER TABLE command.Add a Column

SQL syntax

ALTER TABLE table_nameADD column_name datatype

Example SQL Statement

ALTER TABLE IndividualADD age int

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Change the DatatypeSQL syntax

ALTER TABLE table_name

ALTER COLUMN column_name datatypeExample SQL Statement

ALTER TABLE Individual

ALTER COLUMN age numeric

Drop a Column'Dropping' a column means removing or deleting that column.

SQL syntax

ALTER TABLE table_nameDROP COLUMN column_name


ALTER TABLE Individual

DROP COLUMN age

SQL Syntax

The SQL syntax is quite an easy one to grasp. Most of the actions you need to perform are done with a SQL

statement.Generally, a SQL statement begins by stating what to do (for example, "SELECT"), then states which object

to do it to (for example, using the "FROM" clause).


It may also have a condition added to the end (for example, with a WHERE clause).SELECT * FROM IndividualWHERE FirstName = 'Homer'

SQL is not case sensitive - the above examples could just have easily used all lowercase or all uppercase.Different programmers have their own preferences. For readability purposes, many SQL programmers

prefer to use uppercase for SQL commands and lowercase for everything else.

The SQL syntax allows you to include line breaks at logical points without it breaking the statement. Forexample, the above example could have been written all on one line - or across 4 lines.

Also, some database systems require that you use a semicolon at the end of each SQL statement (although

this tutorial doesn't use the semicolon).DML & DDL

SQL is divided into two main categories; Data Manipulation Language (DML), and Data Definition

Language (DDL). An explanation follows.

Data Manipulation Language (DML)DML enables you to work with the data that goes into the database. DML is used to insert, select, update,

and delete records in the database. Many of your SQL statements will begin with one of the following

commands:

SELECT - Retrieves data from the database

INSERT - Inserts new data into the database

UPDATE - Updates existing data in the database

DELETE - Deletes existing data from the database

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Data Definition Language (DDL)

You may also occasionally need to create or drop a table or other datbase

object. SQL enables you to do this programatically using DDL.

Examples of DDL commands:

CREATE DATABASE - Creates a new databaseALTER DATABASE - Modifies the database

DROP DATABASE - Drops (deletes) a database

CREATE TABLE - Creates a new table

ALTER TABLE - Modifies the table

DROP TABLE - Drops (deletes) a table

These are just some of the object classes that can be defined using DDL. As you can probably guess, the

syntax is generally the same for any object, although, each object will have properties specific to that objectclass.

As you can see, the SQL syntax is quite simple. It is also very powerful syntax - you can do a lot of damage

with one line of code!

SQL SelectThe SELECT statement is probably the most commonly used in SQL. It simply retrieves data from the

database.

Lets have a look at a simple SELECT statement:SELECT * FROM Individual

This SQL SELECT statement is attempting to retrieve all columns from a table called individuals.How do we know it is trying to select all columns? Because it is using an asterisk (*). This is a quick way of

selecting all columns - it's much easier than writing out the names of all columns (especially if there are a lot

of columns).

Of course, this SQL SELECT statement assumes that there is a table called individuals. If there wasn't, an

error would be generated.Lets have a look at the table the statement is trying to select data from:







Because our select statement asks to display all columns and all records, we would see the following:IndividualI

dFirstNameLastNameUserName






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AND LastName = 'Brown'

Result

IndividualI


3 Homer Brown notsofamousOR Operator

The OR operator filters the query to only those records that satisfy either one or the other condition.SELECT * FROM IndividualWHERE FirstName = 'Homer'OR LastName = 'Ozzbourne'

Result

IndividualI






SQL Order By

Using a SQL SELECT statement can retreive many hundreds or even thousands of records. In some cases

you might find it useful to sort the records by a given column. For example, when selecting records from theIndividual table, you might like to sort them by the LastName column.SQL statement


ORDER BY LastName

Source Table







Result

IndividualI



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Descending Order

By default, ORDER BY sorts the column in ascending order - that is, from lowest values to highest values.You could also explicitly state this using the ASC keyword, but it's not necessary.If you want highest values to appear first, you can use the DESC keyword.

SQL statement


ORDER BY LastName DESC

Result

IndividualI


2 Homer Simpson homey4 Ozzy Ozzbourne sabbath



3 Homer Brown notsofamousSorting By Multiple Columns

You can sort by multiple columns by stating each column in the ORDER BY clause, separating each

column name with a comma. SQL will first order the results by the first column, then the second, and so onfor as many columns that are included in the ORDER BY clause.

SQL statement

SELECT * FROM IndividualORDER BY FirstName, LastName

Result

IndividualI






SQL Top

In the preceeding lessons on the SELECT statement, the examples have returned all records that havematched our SELECT criteria. This is great if you want to look at every record, but, what if you only want

to look at the first few records?

Sounds like you need the SQL TOP clause.

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The TOP clause allows us to specify how many rows to return. This can be useful on very large tables when

there are thousands of records. Returning thousands of records can impact on performance, and if you areworking with a production database, this could have an adverse impact on the users.

Note: The SQL TOP clause is Transact-SQL, and not part of ANSI SQL. Therefore, depending on your

database system, you may not be able to use this clause.

SQL statement

SELECT TOP 3 * FROM Individual

Source Table





4 Ozzy Ozzbourne sabbath5 Homer Gain noplacelike

Result

IndividualI





Specifying a Percentage

You have the option of specifying a percentage of the result set instead of an absolute value. You do this

with the PERCENT keyword.

SQL statement

SELECT TOP 40 PERCENT * FROM Individual

Result

IndividualI



2 Homer Simpson homeySQL TOP and the ORDER BY clauseIf you are using the TOP clause along with the ORDER BY clause, the TOP clause is applied to the ordered

result set.

Therefore, if we add an ORDER BY to the above query, we end up with something like this:SQL statement

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SELECT TOP 40 PERCENT * FROM Individual

ORDER BY LastName DESC

Result

IndividualI




SQL Distinct

Once a table starts getting a lot of data in it, some columns will contain duplicate values. For example, many

Individuals share first names and surnames. Most of the time this isn't a problem. But sometimes you will

want to find out how many unique values there are in a table. To do this you can use the DISTINCTkeyword.

SQL statement

SELECT DISTINCT(FirstName) FROM Individual

Source Table







Result

Using the DISTINCT keyword, all customers with a name of "Homer" are counted as one.

FirstName

Fred

Homer

Ozzy

SQL In

The SQL IN operator assists you in providing multiple values in your WHERE clause. This can provide

very useful when you need to compare your value to a list of values. Often this list could be the result of aquery from another table.

SQL Syntax

SELECT * FROM table_nameWHERE column_name IN (value1,value2,...)

Example

SQL statementSELECT * FROM Individual

WHERE LastName IN ('Simpson','Ozzbourne','Flinstone')

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Source Table






Result

IndividualI





You might have noticed that this returns the same result as the following SQL statement:

SELECT * FROM IndividualWHERE LastName = 'Simpson'OR LastName = 'Ozzbourne'OR LastName = 'Flinstone'

Yes, we could just have easily used that statement but the SQL IN statement is more concise.

SQL IN and SubqueriesNow, where the SQL IN operator becomes really useful is when you need to compare a value against theresult of another query.

For example, lets say we have another table called "Publisher". This table contains users who are allowed to

contribute to the website via an administration console. All users in the Publisher table are also in theIndividual table, but not all users in the Individual table are in the Publisher table.

Source Tables

Individual Table







Publisher Table


1 Administrator

2 Contributor

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3 Contributor

4 Contributor

Our task is to return a list of usernames from all publishers who have an access level of "Contributor".

You may notice that the usernames are in the Individual table but the access level is stored in the Publisher

table. Also, there could potentially be many contributors. This is a good candidate for the SQL IN operator!SQL statement

SELECT UserName FROM Individual

WHERE IndividualId IN(SELECT IndividualIdFROM Publisher

WHERE AccessLevel = 'Contributor')

Result

UserName

homey

notsofamous

sabbath

In this example there aren't many records in the Publisher table, but imagine if there were thousands - the IN

statement is great for this sort of thing.SQL Alias

In SQL, an alias is a name that you give a table. This can make it easier to work with table names -

especially when they are long. You could name the alias anything, but usually you'd make it short.

You may be thinking "a table already has a name, why give it another one?". Well, there are some goodreasons for creating an alias. The main reasons are:

Queries can sometimes get very long. Aliases can make your query easier to read.

You may find yourself referencing the same table name over and over again - this will occur if you'reworking with multiple tables and you need to refer to columns from those tables. It can be annoying to have

to write the whole name all the time - especially if it's a long one.

You may need to work with multiple instances of the same table, for example, a self join. If you're notfamiliar with joins, they are covered later in this tutorial.

As mentioned, an alias could be anything. For example, if you have a table called Individual you could give

it an alias of i. Another table called IndividualProductPurchase could have an alias of, say, ippAlias SyntaxSELECT * FROM table_name AS alias_name


SELECT o.JobTitle FROM Individual AS i, Occupation AS oWHERE i.FirstName = 'Homer'ORDER BY o.JobTitle

SQL Join

The SQL JOIN refers to using the JOIN keyword in a SQL statement in order to query data from two tables.

When you perform a SQL join, you specify one column from each table to join on. These two columnscontain data that is shared across both tables.

You can use multiple joins in the same SQL statement to query data from as many tables as you like.

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Join Types

Depending on your requirements, you can do an "inner" join or an "outer" join. These are different in asubtle way

INNER JOIN: This will only return rows when there is at least one row in both tables that match the join

condition.LEFT OUTER JOIN (or LEFT JOIN): This will return rows that have data in the left table (left of the JOIN

keyword), even if there's no matching rows in the right table.

RIGHT OUTER JOIN (or RIGHT JOIN): This will return rows that have data in the right table (right of theJOIN keyword), even if there's no matching rows in the left table.FULL OUTER JOIN (or FULL JOIN): This will return all rows, as long as there's matching data in one of

the tables.

Join Syntax

Inner Join:

SELECT * FROM table_name1INNERJOIN table_name2

ON table_name1.column_name = table_name2.column_name

Left Join:

SELECT * FROM table_name1LEFT JOIN table_name2ON table_name1.column_name = table_name2.column_name

Right Join:

SELECT * FROM table_name1RIGHT JOIN table_name2ON table_name1.column_name = table_name2.column_name

Full Join:

SELECT * FROM table_name1FULL JOIN table_name2ON table_name1.column_name = table_name2.column_name

Example Inner Join Statement

SELECT * FROM IndividualINNER JOIN PublisherON Individual.IndividualId = Publisher.IndividualId

Note: We could use table aliases instead of the full table name. This will keep our statement shorter. Forexample:


INNER JOIN Publisher AS PubON Ind.IndividualId = Pub.IndividualId

The next few lessons cover each type of join and show examples of usage.21


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SQL Inner Join

As discussed in the previous lesson, you should use the SQL INNER JOIN when you only want to returnrecords where there is at least one row in both tables that match the join condition.


SELECT * FROM IndividualINNER JOIN Publisher

ON Individual.IndividualId = Publisher.IndividualIdWHERE Individual.IndividualId = '2'

Source Tables

Left Table







Next lesson covers the SQL OUTER JOIN.

SQL Outer Join

This lesson covers both the left outer join, the right outer join, and the full outer join, and explains the

differences between them. There are some occasions where you would need to use a left outer join or a right

outer join, and others where you would need a full outer join. The join type you use will depend on thesituation and what data you need to return.

Left Outer Join

Use this when you only want to return rows that have matching data in the left table, even if there's nomatching rows in the right table.



LEFT JOIN Publisher AS PubON Ind.IndividualId = Pub.IndividualId

Source TablesLeft Table





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Right Table


1 Administrator

2 Contributor

3 Contributor

4 Contributor

10 Administrator

Result

IndividualId

FirstNameLastNameUserName IndividualId

AccessLevel



3 Homer Brown notsofamous3 Contributor



Right Outer Join

Use this when you only want to return rows that have matching data in the

right table, even if there's no matching rows in the left table.


SELECT * FROM Individual AS IndRIGHT JOIN Publisher AS PubON Ind.IndividualId = Pub.IndividualId

Source Tables

Left Table




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Right Table


1 Administrator

2 Contributor

3 Contributor

4 Contributor

10 Administrator

Result

IndividualI


IndividualI

dAccessLevel



3 Homer Brown notsofamous3 Contributor


NULL NULL NULL NULL 10 AdministratorFull Outer Join

Use this when you want to all rows, even if there's no matching rows in the right table.Example SQL statement


FULL JOIN Publisher AS Pub

ON Ind.IndividualId = Pub.IndividualId

SQL Insert

Up until now, this tutorial has covered the SELECT statement and variations on it. We are now about tolearn a new statement - the INSERT statement.

The SQL INSERT command allows you to insert a record into a table in your database. As with theSELECT syntax, the INSERT syntax is quite straight forward.SQL statement

INSERT INTO Individual

VALUES ( '6', 'Benny', 'Hill', 'hillbenny' )

Source Table




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Result

Now if we do a SELECT on the Individual table, we can see the new record added to the bottom of the

result set.

IndividualI







6 Benny Hill hillbennySee - nothing to it!Now, it's important to note that the INSERT statement is used only when you want to add a new record to

the table. If you want to update an existing record, use an UPDATE statement. The UPDATE command isdescribed in the next lesson.

SQL Update

The SQL UPDATE statement allows you to update an existing record in the database.The UPDATE command uses a WHERE clause. If you don't use a WHERE clause, all rows will be updated.

In fact, the syntax for a basic UPDATE statement is very similar to a SELECT statement.

SQL statement

UPDATE Individual

SET UserName = 'funnyman'WHERE IndividualId = '6'

Source Table








Result

Now if we select this record, we can see the updated value.

IndividualI FirstNameLastNameUserName

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d

6 Benny Hill funnymanUpdating Multiple Fields

To update multiple fields, separate each field assignment with a comma.SQL statementUPDATE IndividualSET UserName = 'getserious', FirstName = 'Onetree'

WHERE IndividualId = '6'

Result

IndividualI


6 Onetree Hill getserious

Next lesson covers the DELETE statement.

SQL Select

The SELECT statement is probably the most commonly used in SQL. It simply retrieves data from thedatabase.

Lets have a look at a simple SELECT statement:


This SQL SELECT statement is attempting to retrieve all columns from a table called individuals.

How do we know it is trying to select all columns? Because it is using an asterisk (*). This is a quick way ofselecting all columns - it's much easier than writing out the names of all columns (especially if there are a lot

of columns).Of course, this SQL SELECT statement assumes that there is a table called individuals. If there wasn't, an

error would be generated.Lets have a look at the table the statement is trying to select data from:






5 Homer Gain noplacelikeBecause our select statement asks to display all columns and all records, we would see the following:







Select from Multiple Tables

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You can select from more than one table at a time. To do this, simply separate each table with a comma.

You should also qualify any references to columns by placing the table name in front, separated by a dot.We have another table called "Occupation", which contains the individual's occupation.

OccupationId IndividualId JobTitle

1 1 Engineer

2 2 Accountant

3 3 Cleaner

4 4 Attorney

5 5 Sales Executive

SQL statementWe will select from both the "Individual" table and the "Occupation" table. We will qualify any column

names by prefixing them with its table's name and a dot.SELECT * FROM Individual, OccupationWHERE Individual.FirstName = 'Homer'

Result

Individual

Id

FirstNa

me

LastNa

me

UserNam

e

Occupation

Id

Individual

IdJobTitle

1 Fred Flinstone freddo 1 1 Engineer

2 Homer Simpson homey 2 2 Accounta

nt

3 Homer Brown notsofamo

us

3 3 Cleaner

4 Ozzy Ozzbourn

e

sabbath 4 4 Attorney

5 Homer Gain noplacelike 5 5 Sales

ExecutiveDisplaying Less Columns

If you don't need every column to be displayed you can single out just the columns you're interested in. It's

good programming practice to do this - the more columns your program has to return, the more it will

impact its performance.To only display those columns you're interested in, simply replace the asterisk (*) with a comma separated

list of the column names.SQL statementSELECT IndividualId, LastName, UserName FROM Individual

WHERE FirstName = 'Homer'

Result

IndividualId LastName UserName

2 Simpson homey

3 Brown notsofamous

5 Gain noplacelike

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SQL Where

In the previous lesson, we used a SQL SELECT statement to retrieve all records from a database table. This

is fine if we want to see every record, but what if we were only interested in some records? For example,

what if we were only interested in individuals whose first name is "Homer"?We could use the WHERE clause.

Using the WHERE clause, you can filter out only those records that satisfy a given condition.

SQL WHERE SyntaxSELECT * FROM table_nameWHERE column_name = 'criteria'Example

SQL WHERE StatementSELECT * FROM Individual


Source Table







Result

Given there are 3 people with the first name of "Homer", the results will look

like this:

IndividualI




5 Homer Gain noplacelikeMultiple ConditionsYou can filter records based on more than one condition using operators. Two common operators are the

AND and OR operators.AND Operator

The AND operator filters the query to only those records that satisfy both the first condition and the second

condition.SELECT * FROM Individual


AND LastName = 'Brown'

Result

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OR Operator

The OR operator filters the query to only those records that satisfy either one or the other condition.


WHERE FirstName = 'Homer'OR LastName = 'Ozzbourne'

Result






SQL Order By

Using a SQL SELECT statement can retreive many hundreds or even thousands of records. In some cases

you might find it useful to sort the records by a given column. For example, when selecting records from theIndividual table, you might like to sort them by the LastName column.


ORDER BY LastName

Source Table


1 Fred Flinstone freddo2 Homer Simpson homey




Result

IndividualI






2 Homer Simpson homeyDescending Order

By default, ORDER BY sorts the column in ascending order - that is, from lowest values to highest values.

You could also explicitly state this using the ASC keyword, but it's not necessary.If you want highest values to appear first, you can use the DESC keyword.

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SQL statement

SELECT * FROM IndividualORDER BY LastName DESC

Result

IndividualI





3 Homer Brown notsofamousSorting By Multiple Columns

You can sort by multiple columns by stating each column in the ORDER BY clause, separating eachcolumn name with a comma. SQL will first order the results by the first column, then the second, and so on

for as many columns that are included in the ORDER BY clause.

SQL statement

SELECT * FROM IndividualORDER BY FirstName, LastName

Result

IndividualI







SQL Top

In the preceeding lessons on the SELECT statement, the examples have returned all records that have

matched our SELECT criteria. This is great if you want to look at every record, but, what if you only want

to look at the first few records?

Sounds like you need the SQL TOP clause.

The TOP clause allows us to specify how many rows to return. This can be useful on very large tables whenthere are thousands of records. Returning thousands of records can impact on performance, and if you are

working with a production database, this could have an adverse impact on the users.Note: The SQL TOP clause is Transact-SQL, and not part of ANSI SQL. Therefore, depending on your

database system, you may not be able to use this clause.

SQL statement

SELECT TOP 3 * FROM Individual

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Source Table






Result

IndividualI





Specifying a Percentage

You have the option of specifying a percentage of the result set instead of an absolute value. You do this

with the PERCENT keyword.

SQL statementSELECT TOP 40 PERCENT * FROM Individual

Result

IndividualI




SQL TOP and the ORDER BY clause

If you are using the TOP clause along with the ORDER BY clause, the TOP clause is applied to the ordered

result set.

Therefore, if we add an ORDER BY to the above query, we end up with something like this:SQL statement

SELECT TOP 40 PERCENT * FROM IndividualORDER BY LastName DESC

Result

IndividualI




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SQL Distinct

Once a table starts getting a lot of data in it, some columns will contain duplicate values. For example, many

Individuals share first names and surnames. Most of the time this isn't a problem. But sometimes you will

want to find out how many unique values there are in a table. To do this you can use the DISTINCTkeyword.

SQL statement

SELECT DISTINCT(FirstName) FROM Individual

Source Table







Result

Using the DISTINCT keyword, all customers with a name of "Homer" are

counted as one.

FirstName

Fred

Homer

Ozzy

SQL In

The SQL IN operator assists you in providing multiple values in your WHERE clause. This can provide

very useful when you need to compare your value to a list of values. Often this list could be the result of aquery from another table.

SQL SyntaxSELECT * FROM table_name

WHERE column_name IN (value1,value2,...)

Example


WHERE LastName IN ('Simpson','Ozzbourne','Flinstone')

Source Table

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Result

IndividualI




4 Ozzy Ozzbourne sabbathYou might have noticed that this returns the same result as the following SQL statement:

SELECT * FROM IndividualWHERE LastName = 'Simpson'OR LastName = 'Ozzbourne'

OR LastName = 'Flinstone'

Yes, we could just have easily used that statement but the SQL IN statement is more concise.SQL IN and Subqueries

Now, where the SQL IN operator becomes really useful is when you need to compare a value against the

result of another query.For example, lets say we have another table called "Publisher". This table contains users who are allowed to

contribute to the website via an administration console. All users in the Publisher table are also in the

Individual table, but not all users in the Individual table are in the Publisher table.Source Tables

Individual Table




3 Homer Brown notsofamous4 Ozzy Ozzbourne sabbath


Publisher Table


1 Administrator

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2 Contributor

3 Contributor

4 Contributor

Our task is to return a list of usernames from all publishers who have an access level of "Contributor".

You may notice that the usernames are in the Individual table but the access level is stored in the Publishertable. Also, there could potentially be many contributors. This is a good candidate for the SQL IN operator!

SQL statementSELECT UserName FROM Individual

WHERE IndividualId IN(SELECT IndividualId

FROM Publisher

WHERE AccessLevel = 'Contributor')

Result

UserName

homey

notsofamous

sabbathIn this example there aren't many records in the Publisher table, but imagine if there were thousands - the INstatement is great for this sort of thing.

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