Table of contents
Intro
Lets review all of the Table-related functions in one place.
Create tables in the database
Top create a table in your database use the CREATE TABLE command. Here, we define the schema of a table. We create users table which has two attributes/fields/columns, name and email. Each of these attributes are defined as character strings with max of 128 characters
people=> CREATE TABLE users(
people(> name VARCHAR(128),
people(> email VARCHAR(128)
people(> );
CREATE TABLE
people=>
List tables in the database
To list tables/relations in the database use to he following command:
people=> \dt
Did not find any relations.
if you haven't created any tables in your database yet, you will get "Did not find any relations." message. Otherwise, you will get list of existing tables like what follows:
people=> \dt
List of relations
Schema | Name | Type | Owner
--------+-------+-------+-------
public | users | table | pg4e
(1 row)
people=>
Get the schema of a table
Lets suppose we have forgotten what was the schema (the definition of the table and its attributes/columns), and we want to review the schema one more time. In this case we can use the following command:
people=> \d+ users
Table "public.users"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
--------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
name | character varying(128) | | | | extended | | |
email | character varying(128) | | | | extended | | |
Access method: heap
people=>
Basic CRUD commands
INSERT/create row(s) into the table
Suppose we wan to insert several rows/objects into the table. We have to use the following command:
people=> INSERT INTO users (name, email) VALUES ('Peter', 'peter@example.edu');
INSERT 0 1
people=> INSERT INTO users (name, email) VALUES ('Terry', 'terry@example.edu');
INSERT 0 1
people=> INSERT INTO users (name, email) VALUES ('Eliza', 'eliza@example.edu');
INSERT 0 1
people=> INSERT INTO users (name, email) VALUES ('John', 'john@example.edu');
INSERT 0 1
READ/SELECT row(s) from a table
In order to read the rows in a table we should use SELECT command:
people=> SELECT * FROM users;
name | email
-------+-------------------
Peter | peter@example.edu
Terry | terry@example.edu
Eliza | eliza@example.edu
John | john@example.edu
(4 rows)
We can include some criteria in the SELECT command as well:
people=> SELECT * FROM users WHERE email='terry@example.edu';
name | email
-------+-------------------
Terry | terry@example.edu
(1 row)
UPDATE row(s) in a table
In order to update one/more piece(s) of information in one/more row(s), we use UPDATE command like what follows:
people-> UPDATE users SET name='Johnny' WHERE email='john@example.edu';
UPDATE 1
Hint: this command will update any number of rows that meet its criteria.
Lets see how this command changed the last entry of users table:
people=> SELECT * FROM users;
name | email
--------+-------------------
Peter | peter@example.edu
Terry | terry@example.edu
Eliza | eliza@example.edu
Johnny | john@example.edu
(4 rows)
DELETE row(s) from a table
To remove one/more row(s) from a table, use the DELETE command like what follows:
people=> DELETE FROM users WHERE email='terry@example.edu';
DELETE 1
Hint: this command will delete any number of rows that meet its criteria.
Lets see how this command removed an entry of users table:
people=> SELECT * FROM users;
name | email
--------+-------------------
Peter | peter@example.edu
Eliza | eliza@example.edu
Johnny | john@example.edu
(3 rows)
ALTER TABLE
What happens if you missed something in the time of creation of a table in database? Is it over? ABSOLUTELY NOT! SQL lets us change the table schema in the times needed.
When you use ALTER TABLE and change the definition of one field, a SQL-based database applies that change to both the table's schema and to all of the existing data in that table, all while the system is running and functional. So there are a couple of actions like add, change, drop a column that you can do using ALTER TABLE key words. Lets suppose that we have created the following table and we want to make changes to this table.
CREATE TABLE users(
name VARCHAR(128),
email VARCHAR(128)
);
First, lets take a look at the table's schema:
people=# \d+ users
Table "public.users"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
--------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
name | character varying(128) | | | | extended | | |
email | character varying(128) | | | | extended | | |
Access method: heap
Add a column to a table
In order to add a column with " address VARCHAR(128) " definition to the users table, we have to use the following code. You can see the changes into the table's schema
people=# ALTER TABLE users ADD COLUMN address VARCHAR(128);
ALTER TABLE
people=# \d+ users
Table "public.users"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
---------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
name | character varying(128) | | | | extended | | |
email | character varying(128) | | | | extended | | |
address | character varying(128) | | | | extended | | |
Access method: heap
Change type of a column
To change type of address column from VARCHAR(128) to TEXT, we have to use to following code:
people=# ALTER TABLE users ALTER COLUMN address TYPE TEXT;
ALTER TABLE
people=# \d+ users
Table "public.users"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
---------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
name | character varying(128) | | | | extended | | |
email | character varying(128) | | | | extended | | |
address | text | | | | extended | | |
Access method: heap
Drop a column of a table
In order to drop the address column of users table, we have to use the following code:
people=# ALTER TABLE users DROP address;
ALTER TABLE
people=# \d+ users
Table "public.users"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
--------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
name | character varying(128) | | | | extended | | |
email | character varying(128) | | | | extended | | |
Access method: heap
UPDATE
What happens if we wanted to update content of one/several row(s)? In this case we can you UPDATE key word.
Update content of row(s) in a table
First Lets take a look at the current contents of the users table:
people=# SELECT * FROM users;
name | email
--------+-------------------
Peter | peter@example.edu
Eliza | eliza@example.edu
Johnny | john@example.edu
Terry | terry@example.edu
(4 rows)
Now, lets change the email address of the user named Johnny, and watch the results:
people=# UPDATE users SET email='johnny@example.edu' WHERE name='johnny';
UPDATE 1
people=# SELECT * FROM users;
name | email
--------+--------------------
Peter | peter@example.edu
Eliza | eliza@example.edu
Terry | terry@example.edu
Johnny | johnny@example.edu
(4 rows)