SQL Inserting, Updating, and Selecting
Overview
In this lesson, we'll cover different ways to manipulate and select data from SQL database tables.
Objectives
Use the
INSERT INTO
command to insert data (i.e. rows) into a database tableUse
SELECT
statements to select data from a database tableUse the
WHERE
Clause to select data from specific table rowsUse comparison operators, like
<
or>
, to select specific dataUse
UPDATE
statements to update data within a database tableUse
DELETE
statements to delete data from a database table
Setting Up Our Database
In this code along, we'll be creating a cats
table in a pets_database.db
. So, let's navigate to our terminal and get started.
First let's create our pets_database
by running the following the command in the terminal:
Now that we have a database, let's create our cats
table along with id
, name
, age
and breed
columns.
Okay, let's start storing some cats.
Code Along I: INSERT INTO
In your terminal, in the sqlite prompt, type the following:
We use the INSERT INTO
command, followed by the name of the table to which we want to add data. Then, in parentheses, we put the column names that we will be filling with data. This is followed by the VALUES
keyword, which is accompanied by a parentheses enclosed list of the values that correspond to each column name.
Important: Note that we didn't specify the "id" column name or value. Since we created the cats
table with an "id" column whose type is INTEGER PRIMARY KEY
, we don't have to specify the id column values when we insert data. Primary Key columns are auto-incrementing. As long as you have defined an id column with a data type of INTEGER PRIMARY KEY
, a newly inserted row's id column will be automatically given the correct value.
Let's add a few more cats to our table. This time we'll do this via our text editor. Create a file, 01_insert_cats_into_cats_table.sql
. Use two INSERT INTO
statements to insert the following cats into the table:
name
age
breed
"Lil' Bub"
5
"American Shorthair"
"Hannah"
1
"Tabby"
Hint: You can use double single quotes ('
) to escape the apostrophe in 'Lil'' Bub'
.
Each INSERT INTO
statement gets its own line in the .sql
file in your text editor. Each line needs to end with a ;
. Run the file with the following code in your terminal:
NOTE: This is a bash command, run this from your bash not sqlite console.
Now, we'll learn how to SELECT
data from a table, which will help us to confirm that we inserted the above data correctly.
Selecting Data
Now that we've inserted some data into our cats
table, we likely want to read that data. This is where the SELECT
statement comes in. We use it to retrieve database data, or rows.
Code Along II: SELECT FROM
A basic SELECT
statement works like this:
We specify the names of the columns we want to SELECT and then tell SQL the table we want to select them FROM.
We want to select all the rows in our table, and we want to return the data stored in any and all columns in those rows. To do this, we could pass the name of each column explicitly:
Which should give us back:
A faster way to get data from every column in our table is to use a special selector, known commonly as the 'wildcard', *
selector. The *
selector means: "Give me all the data from all the columns for all of the cats" Using the wildcard, we can SELECT
all the data from all of the columns in the cats table like this:
Now let's try out some more specific SELECT
statements:
Selecting by Column Names
To select just certain columns from a table, use the following:
That should return the following:
You can even select more than one column name at a time. For example, try out:
Top-Tip: If you have duplicate data (for example, two cats with the same name) and you only want to select unique values, you can use the DISTINCT
keyword. For example:
Selecting Based on Conditions: The WHERE
Clause
WHERE
ClauseWhat happens when we want to retrieve a specific table row? For example the row that belongs to Maru? Or to retrieve all the baby cats who are younger than two years old? We can use the WHERE
keyword to select data based on specific conditions. Here's an example of a boilerplate SELECT
statement using a WHERE
clause.
Let's retrieve just Maru from our cats
table:
That statement should return the following:
We can also use comparison operators, like <
or >
to select specific data. Let's give it a shot. Use the following statement to select the young cats:
Advanced: The SQL statements we're learning here will eventually be used to integrate the applications you'll build with a database. For example, it's easy to imagine a web application that has many users. When a user signs into your app, you'll need to access your database and select the user that matches the credentials an individual is using to log in.
Updating Data
Let's talk about updating, or changing, data in our table rows. We do this with the UPDATE
keyword.
Code Along III: UPDATE
A boilerplate UPDATE
statement looks like this:
The UPDATE
statement uses a WHERE
clause to grab the row you want to update. It identifies the table name you are looking in and resets the data in a particular column to a new value.
Let's update one of our cats. Turns out Maru's friend Hannah is actually Maru's friend Hana. Let's update that row to change the name to the correct spelling:
One last thing before we move on: deleting table rows.
Deleting Data
To delete table rows, we use the DELETE
keyword.
Code Along IV: DELETE
A boilerplate DELETE
statement looks like this:
Let's go ahead and delete Lil' Bub from our cats
table (sorry Lil' Bub):
Notice that this time we selected the row to delete using the Primary Key column. Remember that every table row has a Primary Key column that is unique. Lil' Bub was the second row in the database and thus had an id of 2
.
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