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Chapter 1. Data and Tables: A place for everything

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Don’t you just hate losing things? Whether it’s your car keys, that 25% off coupon for Urban Outfitters, or your application’s data, there’s nothing worse than not being able to keep up with what you need... when you need it. And when it comes to your applications, there’s no better place to store your important information than in a table. So turn the page, come on in, and take a walk through the world of relational databases.

Defining your data

Greg knows many lonely single people. He likes keeping track of what his friends are up to, and enjoys introducing them to each other. He has lots of information about them scrawled on sticky notes like this:

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Greg’s been using his system for a very long time. Last week he expanded his connections to include people who are seeking new jobs, so his listings are growing quickly. Very quickly...

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Brain Power

Is there a better way to organize this information? What would you do?

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Exactly right. A database is just what we need.

But before you can get into creating databases, you’re going to need to have a better idea of what kinds of data you’re going to want to store and some ways of categorizing it.

Look at your data in categories

Let’s look at your data in a different way. If you cut each note into pieces, then spread the pieces out horizontally you’d get something that looked like this:

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Then if you cut up another sticky note with the categories you just noticed, and put the pieces above their corresponding information, you’d have something that looks a lot like this:

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Here’s that same information nicely displayed in a TABLE in columns and rows.

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last_name

first_name

email

birthday

profession

location

status

interests

seeking

Branson

Ann

7-1-1962

Aeronautical Engineer

San Antonio, TX

Single, but involved

RPG, Programming

New Job

Hamilton

Jamie

9-10-1966

System Administrator

Sunnyvale, CA

Single

Hiking, Writing

Friends, Women to date

Soukup

Alan

12-2-1975

Aeronautical Engineer

San Antonio, TX

Married

RPG, Programming

Nothing

Mendoza

Angelina

8-19-1979

Unix System Administrator

San Francisco, CA

Married

Acting, Dancing

New Job

What’s in a database?

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Database Detour

Before we get into the details of what tables, rows, and columns are, let’s step back and look at the bigger picture. The first SQL structure you need to know about is the container that holds all your tables known as a database.

Note

A database is a container that holds tables and other SQL structures related to those tables.

Every time you search online, go shopping, call information, use your TiVo, make a reservation, get a speeding ticket, or buy groceries, a database is being asked for information, otherwise known as being queried.

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Database Detour

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Database Detour

Your database viewed through x-ray specs...

Note

The information inside the database is organized into tables.

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Database Detour

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Database Detour

Databases contain connected data

All of the tables in a database should be connected in some way. For example, here are the tables that might be in a database holding information about doughnuts:

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Database Detour

Here’s an example of what an address book table containing your personal information might look like. You’ll often see the word field used instead of column. They mean the same thing. Also, row and record are often used interchangeably.

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Exactly. You can identify categories for the type of data you’re collecting for each person. Your categories then become your columns. Each sticky note becomes a row. You can take all that information from your stickies and turn it into a table.

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Take command!

Start up your SQL relational database management system (RDBMS) and open a command-line window or graphical environment that allows you to communicate with your RDBMS. Here’s our terminal window after we start MySQL.

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First you’re going to need to create a database to hold all your tables.

  1. Type in the line of code below to create your database called gregs_list.

    Note

    Spaces aren’t allowed in the names of databases and tables in SQL, so an underscore can be used instead.

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    Watch it!

    Did you read the intro?

    We’re using MySQL to command our databases, so commands in your Database Management System (DBMS) might look a little different. See Appendix B for instructions on installing MySQL on your server.

  2. Now you need to tell your RDBMS to actually use the database you just created:

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Setting the table: the CREATE TABLE statement

Let’s see all this in action with the doughnut data. Say you were having trouble remembering what type of doughnuts a snack in your list was just from its name, you might create a table to save having to remember them instead. Below is a single command to type into your console window. When you’ve typed it, you can press RETURN to tell your SQL RDBMS to carry out the command.

doughnut_list

doughnut_name

doughnut_type

Blooberry

filled

Cinnamondo

ring

Rockstar

cruller

Carameller

cruller

Appleblush

filled

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Creating a more complicated table

Remember the columns for Greg’s table? We’ve jotted them down on a sticky note. You’ll need those to write your CREATE TABLE command.

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Brain Power

In which two ways do the column names on the sticky note differ from those in the table above? Why are they significant?

Look how easy it is to write SQL

You’ve seen that to create a table you categorize your data into columns. Then you come up with the right data type and length for each column. After you estimate how long each column needs to be, writing the code is straightforward.

Create the my_contacts table, finally

Now you know exactly what each line is doing, you can type in the CREATE TABLE command. You can enter it one line at a time, copying the code at the top of this page.

Or you can enter it all as one really long single line:

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Whichever way you choose to enter it, before you hit return after the semicolon, make sure you haven’t missed any characters:

last_name VARCHAR(3) is a very different column than lastname VARCHAR(30)!

Your table is ready

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Actually, you’ll need a few more data types for other kinds of data, like numbers.

Suppose we added a price column to our doughnut table. We wouldn’t want to store that as a VARCHAR. Values stored as VARCHARs are interpreted as text, and you won’t be able to perform mathematical operations on them But there are more data types you haven’t met yet...

Brain Power

Before going further, come up with other types of data that need a data type other than VARCHAR or DATE.

Take a meeting with some data types

These are a few of the most useful data types. It’s their job to store your data for you without mucking it up. You’ve already met VARCHAR and DATE, but say hello to these.

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Watch it!

These data type names may not work with your SQL RDBMS!

Unfortunately, there are no universally accepted names for various data types. Your particular SQL RDBMS might use different names for one or more of these types. Check your documentation to find the correct names for your RDBMS.

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Good call. Checking your work is important.

To see how the my_contacts table you created looks, you can use the DESC command to view it:

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You try it.

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Your table, DESCribed

When you’ve entered the DESC command. You’ll see something that looks similar to this:

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Brain Power

What do you think? What sorts of problems could adding a new column create?

You can’t recreate an existing table or database!

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That’s a very good idea, and you’ll want to use a text editor throughout this book.

That way, you can copy and paste the statements into your SQL console whenever you need to. This will keep you from having to retype everything. Also, you can copy and edit old SQL statements to make new ones.

Out with the old table, in with the new

  1. Getting rid of a table is much easier than creating a table. Use this simple command:

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    DROP TABLE will work whether or not there is data in your table, so use the command with extreme caution. Once your table is dropped, it’s gone, along with any data that was in it.

    DROP TABLE deletes your table and any data in it!

  2. Now you can enter your new CREATE TABLE statement:

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A bunch of SQL keywords and data types, in full costume, are playing the party game “Who am I?” They give you a clue, and you try to guess who they are, based on what they say. Assume they always tell the truth about themselves. If they happen to say something that could be true for more than one guy, then write down all for whom that sentence applies. Fill in the blanks next to the sentence with the names of one or more attendees.

Tonight’s attendees:

CREATE DATABASE, USE DATABASE, CREATE TABLE, DESC, DROP TABLE, CHAR, VARCHAR, BLOB, DATE, DATETIME, DEC, INT

Answers in Who am I?.

To add data to your table, you’ll use the INSERT statement

This pretty much does what it says in the name. Take a look at the statement below to see how each part works. The values in the second set of parentheses have to be in the same order as the column names.

The command below isn’t a real command, it’s a template of a statement to show you the format of an INSERT statement.

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Create the INSERT statement

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Watch it!

Order matters!

The values should be listed in exactly the same order as the column names.

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Exactly right.

Here’s an INSERT statement you might use if you had a table of doughnut purchases. Notice how, in the values, the numbers that match the dozens of donuts purchased and price columns have no quotes.

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Variations on an INSERT statement

There are three variations of INSERT statements you should know about.

  1. Changing the order of columns

    You can change the order of your column names, as long as the matching values for each column come in that same order!

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  2. Omitting column names

    You can leave out the list of column names, but the values must be all there, and all in the same order that you added the columns in. (Double-check the order in Create the INSERT statement if you’re unsure.)

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  3. Leaving some columns out

    You can insert a few columns and leave some out.

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Brain Power

What do you think shows up in the table in columns that you don’t assign a value to?

Columns without values

Let’s insert a record into the my_contacts database from this incomplete sticky note:

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Because the sticky is missing some data, Greg will have to enter an incomplete record. But that’s okay, he’ll be able to add in the missing information later.

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Peek at your table with the SELECT statement

So you want to see what your table looks like? Well, DESC won’t cut it anymore, because it only shows the structure of the table and not the information inside of it. Instead, you should use a simple

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Relax

Don’t worry what the SELECT statement does for now.

We’ll be looking at it in a lot more detail in Chapter 2. For now, just sit back and marvel at the beauty of your table when you use the statement.

Now try it yourself. You’ll have to stretch out your window to see all the results nicely laid out.

Brain Power

Now you know that NULL appears in any columns with no assigned value. What do you think NULL actually means?

Controlling your inner NULL

There are certain columns in your table that should always have values. Remember the incomplete sticky note for Pat, with no last name? She (or he) isn’t going to be very easy to find when you have twenty more NULL last name entries in your table. You can easily set up your table to not accept NULL values for columns.

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NOT NULL appears in DESC

Here’s how the my_contacts table would look if you set all the columns to have NOT NULL values.

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Fill in the blanks with DEFAULT

If we have a column that we know is usually a specific value, we can assign it a DEFAULT value. The value that follows the DEFAULT keyword is automatically inserted into the table each time a row is added if no other value is specified. The default value has to be of the same type of value as the column.

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Using a DEFAULT value fills the empty columns with a specified value.

Your SQL Toolbox

You’ve got Chapter 1 under your belt, and you already know how to create databases and tables, as well as how to insert some of the most common data types into them while ensuring columns that need a value get a value.

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