Sql basics joi ns and common commands (1)

SQL BasicsCommon Commands and JOINs

Common Commands

SharkTbl

WhaleTblfe

Common Command Example Tables

ID Name Street

1 Alf 111 Street

2 Barry 222 Street

3 Charlie 333 Street

4 Dirk 444 Street

ID Name Street Greeting

1 Antenna Assessor 5 Ave howdy

2 Bear Barber 6 Ave hello

3 Chorizo Chef 7 Ave hi

4 Database Diver 8 Ave salve citizen

Multi Table SELECTs

They look like this:

SELECT * FROM SharkTbl, WhaleTbl;

But you can run into trouble if they share common column names.

Table Defined Columns

This is going to break...

SELECT Name, Street FROM SharkTbl, WhaleTbl;

...because both SharkTbl and WhaleTbl have columns named Name and Street.

Result:


For more than one table, use this format:

SELECT <tbl 1>.<col 1>, <tbl 2>.<col 2>FROM <tbl 1>, <tbl 2>

Here's how it looks in action:

SELECT SharkTbl.Name, WhaleTbl.Street FROM SharkTbl, WhaleTbl;


If some columns are unique and some are ambiguous, you can only include table definitions where you must, like so:

SELECT Greeting, SharkTbl.NameFROM SharkTbl, WhaleTbl;

The "AS" Column Alias Command

You use this to clean up table names.

Structure:

SELECT <tbl 1>.<col 1> AS <alias 1>FROM <tbl 1>, <tbl 2>

The "AS" Column Alias Command

Example:

SELECT SharkTbl.Name AS Pudding, WhaleTbl.Street AS BrowniesFROM SharkTbl, WhaleTbl;

Changes The Column Names:

"AND" vs "OR"

"AND" vs "OR"

AND Concept:

If A and B, then true

You've got to have both A and B before true

"AND" vs "OR"

OR Concept:

If A or B, then true

Having either A or B will make it true

You don't need both, just one or the other.

"AND" vs "OR"

This makes OR a more wide search

This makes AND a narrow and exact search

"AND" vs "OR"

Lets run some examples in phpMyAdmin starting with this query:

SELECT Name, Street FROM SharkTblWHERE Street = '222 Street'OR Name = 'Barry';

The "AND" Command

What It Looks Like:

SELECT Name, Street FROM SharkTblWHERE Street = '222 Street'AND Name = 'Barry';

The "AND" Command

Rules:

ANDs are an extension of the WHERE command.

You can use as many as you please, one per condition.

The "OR" Command

What It Looks Like:

SELECT Name, Street FROM SharkTblWHERE Street = '222 Street'OR Name = 'Barry';

ORs are also an extension of the WHERE command.

COUNT Command

Say you want a count of the number of rows returned instead of the raw data.

Structure:

SELECT COUNT(<col 1>) FROM <tbl 1>;

COUNT Command

Example:

SELECT COUNT(Name) AS NameCountFROM SharkTbl;

Returns:

COUNT Command

Lets try it without the count to double check:

SELECT NameFROM SharkTbl;

Yup, it works as 4 rows are returned:

JOINsThe Greatest SQL Command

JOIN Example Tables

PersonTbl

JobTbl

ID Name JobID

1 Alf 3

2 Barry 3

3 Charlie 1

4 Dirk 2

ID Name

1 Antenna Assessor

2 Bear Barber

3 Chorizo Chef

4 Database Diver

The Shape Of Things In Our World

● Oceans○ Swimming Creatures

■ Orcas● Transient● Resident● Offshore

■ Sharks● Hammer Heads● Great White Sharks

■ Other Less Interesting Fish○ Reefs

■ Coral ■ Cement


● Orders○ Products

■ Product Attributes■ Manufacturers■ Billing Codes■ Part Numbers

○ Addresses○ Tracking Numbers○ Faxes○ Notes


So, just about anything you look at can be broken down into "A has Bs which have Cs":

● As○ have Bs

■ which in turn have Cs○ have Fs○ have Gs○ have Hs


To turn this pattern into databases:

1. Make A, B and C into tables

2. Relate the tables to one another

3. Describe any object or relation of objects


These three things allow table relations:

1. Primary Keys

2. Foreign Keys

3. JOINs in your SQL statements

Primary Keys

Remember that tables have IDs?

Like so:

All the IDs we have seen so far are formally known as Primary Keys.

ID Name

1 Alf

2 Barry

3 Charlie

4 Dirk

Primary Keys

Primary keys are the IDs used to identify a row.

PersonTbl

^ Primary key of the PersonTbl

ID Name

1 Alf

2 Barry

3 Charlie

4 Dirk

Primary Keys are going to reference rows in this table

Foreign Keys are going to reference rows in another table

Sometimes called Secondary Keys

Foreign Keys

Foreign Keys

They look like this:

JobID is a Foreign Key...

...and ID is the Primary Key

ID Name JobID

1 Alf 3

2 Barry 3

3 Charlie 1

4 Dirk 2

Foreign Keys

Why? Because:

JobID talks about another table

...but ID talks about this table

ID Name JobID

1 Alf 3

2 Barry 3

3 Charlie 1

4 Dirk 2

Formal:

Foreign Keys are keys that relate back to the Primary Keys of other tables.

Boiled Down:

You can use the JobID in PersonTbl to "look up" values in JobTbl

Foreign Keys

Use A Foreign Key, Get A Value


So, what just happened?

● PersonTbl's 4th row had a JobID of 2

● We looked the JobTbl row with ID of 2

● The name of JobTbl ID = 2 is Bear Barber

● So JobID 2 is the same as Bear Barber!


Big Concept:

The arrow is the JOIN action.


The arrow drawn between PersonTbl and JobTbl represents a JOIN.

A JOIN needs to know which Foreign Key in PersonTbl relates to which Primary Key in JobTbl.

Breaking Down A JOIN

Structure:

SELECT * FROM <table 1>JOIN <table 2>ON <table 1>.<col 1> = <table 2>.<col 2>


SELECT * FROM <table 1>JOIN <table 2>ON <table 1>.<col 1> = <table 2>.<col 2>

What each part does:

SELECT...FROM - Read all cols from table 1JOIN - Read all cols from table 2 as wellON - Use these keys to link the two tables


In English:

Select all columns from the one or more tables named where the primary key of one table is equal to the secondary key of the other.

Further boiled down:

Replace the JobID in PersonTbl with its name in JobTbl


Live SQL Example:

SELECT PersonTbl.ID, PersonTbl.Name AS PersonName, JobTbl.Name AS JobName FROM PersonTblJOIN JobTblON PersonTbl.JobID = JobTbl.ID;


Result:

Boiled Down Concept:

We replaced JobIDs with Job Names using JOIN

Implicit vs Explicit JOIN Statements

Implicit:

SELECT * FROM PersonTbl, JobTblWHERE PersonTbl.JobID = JobTbl.ID

Explicit:

SELECT * FROM PersonTblINNER JOIN JobTbl on JobTbl.ID = PersonTbl.JobID

Implicit vs Explicit JOIN Statements

My examples have all be explicit.

Explicit is better, because the reader does not have to guess if or how you are joining.

You write it out on this line:

INNER JOIN JobTbl on JobTbl.ID = PersonTbl.JobID

JOINs Put Another Way

When getting your head around JOINs, it is good to hear the same thing in many different ways.

Here's a lady on youtube explaining it:

http://www.youtube.com/watch?v=oWHO4lJlX54




Real World Example From MadCow

Goal:

Query all the available sizes of the ComfortGel Original

PartsTbl

PartsAttributeTbl


PNum PName

1802 ComfortGel Original Nasal CPAP Mask with Headgear

PAID PNum Description

44 4266 Petite

45 4266 Small

46 4266 Medium

47 4266 Large


The SQL:

SELECT PartsTbl.PNum, PartAttributeTbl.PAID, PartsTbl.PName, PartAttributeTbl.Description FROM PartsTblJOIN PartAttributeTbl ON PartsTbl.PNum = PartAttributeTbl.PNumWHERE PartsTbl.PNum = 1802


The Result:


Break Down This SQL:

SELECT PartsTbl.PNum, PartAttributeTbl.PAID, PartsTbl.PName, PartAttributeTbl.Description FROM PartsTbl

It Means:

Select columns from PartsTbl



JOIN PartAttributeTbl

It Means:

We are also wanting columns from the PartAttributeTbl



ON PartsTbl.PNum = PartAttributeTbl.PNum

It Means:

Match up the PNums in both tables



WHERE PartsTbl.PNum = 1802

It Means:

Look only for PNum 1802, which is the ComfortGel Original's PartTbl PNum ID.

For The Next SQL Class...

● Advanced JOINs○ INNER○ LEFT○ RIGHT

● UNIONS● More Helpful Commands● Advanced Real World Examples

Classes Next Thursday

Thurs Feb 21st @ Noon

John Nelson of IT presenting Virtualization 101.

Someone from Marketing presenting...something.

Date post:	25-Dec-2014
Category:	Documents
Upload:	johnnygoodman
View:	202 times
Download:	7 times

Sql basics joi ns and common commands (1)

Documents