learn-co-curriculum / ds-skills2-filtering-and-ordering

Filtering and Ordering

Introduction

In this lesson, we'll cover how to write SQL queries to retrieve and add specific data to SQL database tables.

Objectives

You will be able to:

• Limit the number of records returned by a query using LIMIT
• Filter results using BETWEEN and IS NULL
• Order the results of your queries by using ORDER BY (ASC & DESC)

What is a SQL Query?

The term "query" refers to any SQL statement that retrieves data from your database. In fact, we've already written a number of SQL queries using basic SELECT statements. We've already seen how to retrieve single units of data, or rows, with queries like these:

To select all of the rows from a cats table:

SELECT * FROM cats;

To select only rows representing data meeting certain conditions:

SELECT * FROM cats WHERE name = "Maru";

What if, however, we wanted to select the oldest cat? Or all of the cats that don't currently belong to an owner? Or all of the cats with short names?

Data storage isn't very useful if we can't manipulate, view, and analyze that data. Luckily for us, SQL is actually a powerful tool for doing just that.

In this exercise, we'll walk through executing a handful of common and handy SQL queries.

Code Along: SQL Queries

Creating our Database

In this code along, we'll be creating a cats table in a pets_database.db.

First let's create our pets_database by running the following commands.

import sqlite3 
connection = sqlite3.connect('pets_database.db')
cursor = connection.cursor()

# create database connection here

Good work. Now if we look at our file directory, a new file should have appeared called pets_database.db! This is the binary representation of the database. You can think of this like a .jpg file. It won't open up in a text editor, but it does open up in the image viewer app. It is the same way for .db files. They won't open in your editor, but they can be read by the appropriate database engine.

Now that we have a database and a cursor object that is connected to our database, let's create our cats table along with id, name, age, breed, and owner_id columns.

cursor.execute('''CREATE TABLE cats ( id INTEGER PRIMARY KEY, name TEXT, age INTEGER, breed TEXT, owner_id INTEGER );''')

# create table here

Let's add some cats to our cats table to make this more interesting:

cursor.execute('''INSERT INTO cats (name, age, breed, owner_id) VALUES ("Maru", 3 , "Scottish Fold", 1);''')
cursor.execute('''INSERT INTO cats (name, age, breed, owner_id) VALUES ("Hana", 1 , "Tabby", 1);''')
cursor.execute('''INSERT INTO cats (name, age, breed) VALUES ("Lil\' Bub", 5, "American Shorthair");''')
cursor.execute('''INSERT INTO cats (name, age, breed) VALUES ("Moe", 10, "Tabby");''')
cursor.execute('''INSERT INTO cats (name, age, breed) VALUES ("Patches", 2, "Calico");''')

# insert data here

Let's check out our cats table now:

cursor.execute('''SELECT * FROM cats;''').fetchall()

Note: the method .fetchall() returns a list where each record is represented as a tuple, which you can think of as a list-like object. If we would like to retrieve an element from a tuple, we simply access it by-index -- similar to how we access the elements of a normal Python list. (i.e. example_tuple[0] - returns element at index 0)

# select all data from cats data here

This should return:

[(1, 'Maru', 3, 'Scottish Fold', 1),
 (2, 'Hana', 1, 'Tabby', 1),
 (3, "Lil' Bub", 5, 'American Shorthair', None),
 (4, 'Moe', 10, 'Tabby', None),
 (5, 'Patches', 2, 'Calico', None)]

ORDER BY

The first query modifier we'll explore is ORDER BY. This modifier allows us to order the table rows returned by a certain SELECT statement. Here's a boilerplate SELECT statement that uses ORDER BY:

cursor.execute('''SELECT column_name FROM table_name ORDER BY column_name ASC|DESC;''').fetchall()

Let's select our cats and order them by age:

cursor.execute('''SELECT * FROM cats ORDER BY age;''').fetchall()

# select all cats order by age here

This should return the following:

[(2, 'Hana', 1, 'Tabby', 1),
 (5, 'Patches', 2, 'Calico', None),
 (1, 'Maru', 3, 'Scottish Fold', 1),
 (3, "Lil' Bub", 5, 'American Shorthair', None),
 (4, 'Moe', 10, 'Tabby', None)]             

When using ORDER BY, the default is to order in ascending order. If we want to specify though, we can use ASC for "ascending" or DESC for "descending." Let's try to select all of our cats and sort them by age in descending order.

cursor.execute('''SELECT * FROM cats ORDER BY age DESC;''').fetchall()

# select cats order by age descending here

This should return

[(4, 'Moe', 10, 'Tabby', None),
 (3, "Lil' Bub", 5, 'American Shorthair', None),
 (1, 'Maru', 3, 'Scottish Fold', 1),
 (5, 'Patches', 2, 'Calico', None),
 (2, 'Hana', 1, 'Tabby', 1)]        

LIMIT

What if we want the oldest cat? If we want to select extremes from a database table––for example, the employee with the highest paycheck or the patient with the most recent appointment––we can use ORDER BY in conjunction with LIMIT.

LIMIT is used to determine the number of records you want to return from a dataset. For example:

SELECT * FROM cats ORDER BY age DESC LIMIT 1;

Note: When we would only like the first result (or one result as is the case in the example above) we can use the sqlite3 method .fetchone() which, instead of returning a list of results, returns the first result (or the record at index 0). We can use this in-place of or in conjunction with LIMIT 1 in order to get back a single element.

cursor.execute('''SELECT * FROM cats ORDER BY age DESC LIMIT 1;''').fetchone()
# OR
# cursor.execute('''SELECT * FROM cats ORDER BY age DESC;''').fetchone() # returns the same element as the above

This part of the statement: SELECT * FROM cats ORDER BY age DESC returns all of the cats in order from oldest to youngest. Setting a LIMIT of 1 returns just the first, i.e. oldest, cat on the list.

Execute the above statement and you should see:

(4, 'Moe', 10, 'Tabby', None)            

Let's get the two oldest cats:

SELECT * FROM cats ORDER BY age DESC LIMIT 2;

# select the two oldest cats here

Execute that statement and you should see:

[(4, 'Moe', 10, 'Tabby', None), (3, "Lil' Bub", 5, 'American Shorthair', None)]          

BETWEEN

As we've already established, being able to sort and select specific data sets is important. Continuing on with our example, let's say we urgently need to select all of the cats whose age is between 1 and 3. To create such a query, we can use BETWEEN. Here's an boilerplate SELECT statement using BETWEEN:

SELECT column_name(s) FROM table_name WHERE column_name BETWEEN value1 AND value2;

Let's try it out on our cats table:

SELECT name FROM cats WHERE age BETWEEN 1 AND 3;

# find all records between ages 1 and three here

This should return:

[('Maru',), ('Hana',), ('Patches',)]

NULL

Let's say the administrator of our Pets Database has found a new cat. This kitty doesn't have a name yet, but should be added to our database right away. We can add data with missing values using the NULL keyword.

Let's insert our new cat into the database. Our abandoned kitty has a breed, but no name or age as of yet:

INSERT INTO cats (name, age, breed) VALUES (NULL, NULL, "Tabby");

# insert new record here
# retrieve all records from the cat table here

Now, if we look at our cats data with SELECT * FROM cats;, we should see:

[(1, 'Maru', 3, 'Scottish Fold', 1),
 (2, 'Hana', 1, 'Tabby', 1),
 (3, "Lil' Bub", 5, 'American Shorthair', None),
 (4, 'Moe', 10, 'Tabby', None),
 (5, 'Patches', 2, 'Calico', None),
 (6, None, None, 'Tabby', None)]                  

We can even select the mysterious, nameless kitty with the following query:

SELECT * FROM cats WHERE name IS NULL;

# select cats where the name field is null here

This should return the following:

[(6, None, None, 'Tabby', None)]

COUNT

Now, we'll talk about a SQL aggregate function, COUNT.

SQL aggregate functions are SQL statements that retrieve minimum and maximum values from a column, sum values in a column, get the average of a column's values, or count a number of records that meet certain conditions. You can learn more about these SQL aggregators here and here.

For now, we'll just focus on COUNT. COUNT will count the number of records that meet certain condition. Here's a standard SQL query using COUNT:

 "SELECT COUNT([column name]) FROM [table name] WHERE [column name] = [value]"

Let's try it out and count the number of cats who have an owner_id of 1:

SELECT COUNT(owner_id) FROM cats WHERE owner_id = 1;

# retrieve the count of cats whose owner id = 1 here

This should return:

(2,)

GROUP BY

Lastly, we'll talk about the handy aggregate function GROUP BY. Like its name suggests, it groups your results by a given column.

Let's take our table of cats

id          name        age         breed          owner_id  
----------  ----------  ----------  -------------  ----------
1           Maru        3           Scottish Fold  1         
2           Hana        1           Tabby          1         
3           Lil\' Bub   5           American Shor            
4           Moe         10          Tabby                    
5           Patches     2           Calico                   
6                                   Tabby                    

Here, we can see at a glance that there are three tabby cats and one of every other breed — but what if we had a larger database where we couldn't just tally up the number of cats grouped by breed? That's where — you guessed it! — GROUP BY comes in handy.

SELECT breed, COUNT(breed) FROM cats GROUP BY breed;

# execute GROUP BY sql statement here

This should return

[('American Shorthair', 1), ('Calico', 1), ('Scottish Fold', 1), ('Tabby', 3)]

GROUP BY is a great function for aggregating results into different segments — you can even use it on multiple columns!

SELECT breed, owner_id, COUNT(breed) FROM cats GROUP BY breed, owner_id;

# execute multiple column group by statement here

This should return:

[('American Shorthair', None, 1),
 ('Calico', None, 1),
 ('Scottish Fold', 1, 1),
 ('Tabby', None, 2),
 ('Tabby', 1, 1)]

Note on SELECT

We are now familiar with this syntax:

SELECT name FROM cats;

However, you may not know that this can be written like this as well:

SELECT cats.name FROM cats;

Both return:

[('Maru',), ('Hana',), ("Lil' Bub",), ('Moe',), ('Patches',), (None,)] 

SQLite allows us to explicitly state the tableName.columnName we want to select. This is particularly useful when we want data from two different tables.

Imagine we have another table called dogs with a column for the dog names:

CREATE TABLE dogs (
	id INTEGER PRIMARY KEY,
	name TEXT
);

INSERT INTO dogs (name) VALUES ("Clifford");

If we want to get the names of all the dogs and cats, we can no longer run a query with just the column name. SELECT name FROM cats,dogs; will return Error: ambiguous column name: name.

Instead, we must explicitly follow the tableName.columnName syntax.

SELECT cats.name, dogs.name FROM cats, dogs;

You may see this in the future. Don't let it trip you up.

View Basic SQL Queries on Learn.co and start learning to code for free.

Summary

In this lesson, you expanded your SQL knowledge by learning how to modify your data using statements like ORDER BY. Additionally, you learned how to filter and limit your results.