Want to get familiar with PostgreSQL? This one will suffice

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This article was published by Angel Yu in Cloud + Community

What is PostgreSQL?

PostgreSQL is a free object-relational database server distributed under a flexible BSD-style license. It provides users with an alternative to other open source database systems and proprietary systems. We suggest you use the cloud to build the database, save trouble operation such as data migration, database see: cloud.tencent.com/product/cdb…

In this article, we’ll discuss how to create and manage tables in the postgreSQL interface. You’ll learn how to properly configure tables and use them to store your information.

How do I install and log in to PostgreSQL on Ubuntu

We will install PostgreSQL on Ubuntu, but it should work in the default repository for most other distributions.

Enter the following commands to install:

sudo apt-get update
sudo apt-get install postgresql postgresql-contrib
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After installation, create a new user to manage the database we will create:

sudo adduser postgres_user
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Log in to the default PostgreSQL user (called “Postgres”) to create a database and assign it to a new user:

sudo su  -  postgres
PSQL
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You will be put into the PostgreSQL command prompt.

Create a new user that matches the system user you created. Then create the database managed by the user:

CREATE USER postgres_user The password is'password';
CREATE DATABASE my_postgres_db OWNER postgres_user ;
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Run the following command to exit the screen:

\q
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Exit the default “Postgres” user account and log in to the user you created using the following command:

exit
sudo su - postgres_user
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Use the following command to log in to the database you created:

psql my_postgres_db
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We are now ready to look at table management.

Table creation syntax in PostgreSQL

We don’t have any tables in our database yet. We can verify this with this command:

\d
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No relations found.
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We can create a new table by using the following syntax:

CREATE TABLE new_table_name (
    table_column_title TYPE_OF_DATA column_constraints,
    next_column_title TYPE_OF_DATA column_constraints,
    table_constraint
    table_constraint
) INHERITS existing_table_to_inherit_from;
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All columns in an existing table are inherited in addition to those listed in the previous definition. The sections in parentheses are divided into two parts: column definitions and table constraints.

PostgreSQL column and table definitions

Column definitions follow the following syntax pattern:

column_name data_type (optional_data_length_restriction) column_constraints
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Column names should be self-explanatory.

PostgreSQL Indicates the data type

The data type can be any of the following:

• Boolean: Declare true or false values using “Boolean” or “bool”.
• Character values
  • Char: holds one character
  • Char (#) : Saves the number of characters. Space will be inserted to fill any extra space.
  • Varchar (#) : Contains a maximum of # characters.
• An integer value
  • Smallint: An integer between -32768 and 32767.
  • Int: An integer between -214783648 and 214783647.
  • Serial: an integer that is automatically filled.
• Floating point value
  • Float (#) : a floating point number with at least # precision points.
  • Real: 8-byte floating point number
  • Numeric (#, after_dec) : A real number with # digits, followed by after_dec digits
• Date and time values
  • Date: stores the date value
  • Time: indicates the storage time value
  • Timestamp: Stores date and time values
  • Timestamptz: Stores a timestamp that contains time zone data
  • Interval: Stores the difference between two timestamp values
• The geometric data
  • Point: Stores the coordinates of a pair of defined points
  • Line: Stores a set of points that map to a line
  • Lseg: Stores data that defines a line segment
  • Box: Stores the data that defines the rectangle
  • Polygon: Stores data that defines any enclosed space
• Equipment specification
  • Inet: storage IP address
  • Macaddr: indicates the MAC address of the storage device

PostreSQL column and table constraints

Column definitions can also have constraints that provide rules for the data types found in the column. The following can be used as space-delimited values following data types:

• NOT NULL: Columns cannot have NULL values
• UNIQUE: The column values of any record cannot be the same. Null is always treated as a unique value
• PRIMARY KEY: a combination of the above two constraints. Each table can only be used once
• CHECK: Ensures that the condition of the value in the column is true
• REFERENCES: Values must exist in a column of another table

After the columns are defined, table scope constraints can be declared. Table scope constraints can be UNIQUE, PRIMARY KEY, CHECK, or REFERENCES.

How to create a table in PostgreSQL

We will create a table called “PG_equipment” that defines various playground equipment. Enter the following table definitions:

CREATE TABLE pg_equipment (
    equip_id serial PRIMARY KEY,
    type varchar (50) NOT NULL,
    color varchar (25) NOT NULL,
    location varchar(25) check (location in ('north'.'south'.'west'.'east'.'northeast'.'southeast'.'southwest'.'northwest')),
    install_date date
    );
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NOTICE:  CREATE TABLE will create implicit sequence "pg_equipment_equip_id_seq" for serial column "pg_equipment.equip_id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "pg_equipment_pkey" for table "pg_equipment"
CREATE TABLE
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We can view our new table by typing “\ d” at the prompt:

\d
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                       List of relations
 Schema |           Name            |   Type   |     Owner     
--------+---------------------------+----------+---------------
 public | pg_equipment              | table    | postgres_user
 public | pg_equipment_equip_id_seq | sequence | postgres_user
(2 rows)
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List the table, as well as the sequence created by the Equip_ID serial Data type statement.

How do I change table data in PostgreSQL

We can change the definition of a table using the following general syntax:

ALTER TABLE table_name Action_TO_Take;
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For example, we can add a column to our “PG_equipment” table by typing:

ALTER TABLE pg_equipment ADD COLUMN functioning bool;
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ALTER TABLE
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We can view additional columns by typing:

\d pg_equipment
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    Column    |         Type          |                            Modifiers                            
--------------+-----------------------+-----------------------------------------------------------------
 equip_id     | integer               | not null default nextval('pg_equipment_equip_id_seq'::regclass)
 type         | character varying(50) | not null
 color        | character varying(25) | not null
 location     | character varying(25) | 
 install_date | date                  | 
 functioning  | boolean               |
 . . .
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To add a default value, provide the following command:

ALTER TABLE pg_equipment ALTER COLUMN functioning SET DEFAULT 'true';
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If we want to ensure that this value is not null, we can do this:

ALTER TABLE pg_equipment ALTER COLUMN functioning SET NOT NULL;
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To rename the column, use the following syntax:

ALTER TABLE pg_equipment RENAME COLUMN functioning TO working_order;
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To delete the column we just created, type the following command:

ALTER TABLE pg_equipment DROP COLUMN working_order;
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We can rename the entire table with the following command:

ALTER TABLE pg_equipment RENAME TO playground_equip;
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Delete a PostgreSQL table

We can delete the table we created by typing the following command:

DROP TABLE playground_equip;
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DROP TABLE
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If we supply this command to a table that does not exist, we see the following error:

ERROR: table "playground_equip" does not exist
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To avoid this error, we can tell postgreSQL to delete the table and return it successfully in any way. We do this by issuing the following command:

DROP TABLE IF EXISTS playground_equip;
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NOTICE:  table "playground_equip" does not exist, skipping
DROP TABLE
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This time, it tells us we can’t find the table, but continues instead of throwing an error.

conclusion

You should now know how to create and manage simple tables in PostgreSQL. These skills are useful if you are learning how to control PostgreSQL from the command line. For more Linux tutorials, please keep an eye on the Tencent Cloud + community.

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Reference: How To Create, Remove, & Manage Tables in PostgreSQL on a Cloud Server

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