Adding or Modifying Columns

Let us understand details about adding or modifying columns using ALTER TABLE command.

  • New columns can be added to the existing table. However, if you want to add a column which cannot have null value then you need to follow these steps.

    • Add column to the table.

    • Update data in the column with some value.

    • Alter table to enforce not null constraint for the newly added column.

  • Existing columns can be dropped from the table, but it is not advisable to do so. If at all we have to drop the column, then there should be extra caution as some or the other application functionality can be broken.

  • We can modify the existing columns for defining it as not null or to change the data type.

  • Once the application is in production, all the operations related to modifying or dropping columns should be avoided. We can consider adding columns.

  • Let us perform these tasks to understand more about adding or modifying or dropping table columns.

    • Change the data type of user_id as SERIAL (we have to first create the sequence and then set the sequence generated value as default).

    • Define default value for user_email_validated and is_active to FALSE.

    • Change the data type of user_role to CHAR(1), set default value to ‘U’.

    • Add new column last_updated_ts with data type timestamp and also set default value to current timestamp.

%load_ext sql

%env DATABASE_URL=postgresql://itversity_retail_user:retail_password@localhost:5432/itversity_retail_db

env: DATABASE_URL=postgresql://itversity_retail_user:retail_password@localhost:5432/itversity_retail_db

%sql DROP SEQUENCE IF EXISTS users_user_id_seq

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%sql CREATE SEQUENCE users_user_id_seq

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%sql ALTER TABLE users ALTER COLUMN user_id SET DEFAULT nextval('users_user_id_seq')

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%%sql 

SELECT table_catalog, 
    table_name,
    column_name,
    data_type,
    character_maximum_length,
    column_default,
    is_nullable,
    ordinal_position
FROM information_schema.columns 
WHERE table_name = 'users'
ORDER BY ordinal_position

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
9 rows affected.
table_catalog table_name column_name data_type character_maximum_length column_default is_nullable ordinal_position
itversity_retail_db users user_id integer None nextval('users_user_id_seq'::regclass) YES 1
itversity_retail_db users user_first_name character varying 30 None NO 2
itversity_retail_db users user_last_name character varying 30 None NO 3
itversity_retail_db users user_email_id character varying 50 None NO 4
itversity_retail_db users user_email_validated boolean None None YES 5
itversity_retail_db users user_password character varying 200 None YES 6
itversity_retail_db users user_role character varying 1 None YES 7
itversity_retail_db users is_active boolean None None YES 8
itversity_retail_db users created_dt date None CURRENT_DATE YES 9 
%%sql

ALTER TABLE users
    ALTER COLUMN user_email_validated SET DEFAULT FALSE,
    ALTER COLUMN is_active SET DEFAULT FALSE

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%%sql

ALTER TABLE users
    ALTER COLUMN user_role SET DATA TYPE CHAR(1),
    ALTER COLUMN user_role SET DEFAULT 'U'
    

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%%sql

ALTER TABLE users
    ADD COLUMN last_updated_ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP    

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

Note

We can perform multiple column level operations using one ALTER TABLE command. Let us see an example here.

%%sql

DROP TABLE IF EXISTS users

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%%sql

CREATE TABLE users (
  user_id INT,
  user_first_name VARCHAR(30) NOT NULL,
  user_last_name VARCHAR(30) NOT NULL,
  user_email_id VARCHAR(50) NOT NULL,
  user_email_validated BOOLEAN,
  user_password VARCHAR(200),
  user_role VARCHAR(1),
  is_active BOOLEAN,
  created_dt DATE DEFAULT CURRENT_DATE
)

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%sql DROP SEQUENCE IF EXISTS users_user_id_seq

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%sql CREATE SEQUENCE users_user_id_seq

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%%sql

ALTER TABLE users
    ALTER COLUMN user_id SET DEFAULT nextval('users_user_id_seq'),
    ALTER COLUMN user_email_validated SET DEFAULT FALSE,
    ALTER COLUMN is_active SET DEFAULT FALSE,
    ALTER COLUMN user_role SET DATA TYPE CHAR(1),
    ALTER COLUMN user_role SET DEFAULT 'U',
    ADD COLUMN last_updated_ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
Done.

[]

%%sql 

SELECT table_catalog, 
    table_name,
    column_name,
    data_type,
    character_maximum_length,
    column_default,
    is_nullable,
    ordinal_position
FROM information_schema.columns 
WHERE table_name = 'users'
ORDER BY ordinal_position

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
10 rows affected.
table_catalog table_name column_name data_type character_maximum_length column_default is_nullable ordinal_position
itversity_retail_db users user_id integer None nextval('users_user_id_seq'::regclass) YES 1
itversity_retail_db users user_first_name character varying 30 None NO 2
itversity_retail_db users user_last_name character varying 30 None NO 3
itversity_retail_db users user_email_id character varying 50 None NO 4
itversity_retail_db users user_email_validated boolean None false YES 5
itversity_retail_db users user_password character varying 200 None YES 6
itversity_retail_db users user_role character 1 'U'::bpchar YES 7
itversity_retail_db users is_active boolean None false YES 8
itversity_retail_db users created_dt date None CURRENT_DATE YES 9
itversity_retail_db users last_updated_ts timestamp without time zone None CURRENT_TIMESTAMP YES 10 
%%sql 

SELECT * FROM information_schema.sequences 
WHERE sequence_name ~ 'users'

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
1 rows affected.
sequence_catalog sequence_schema sequence_name data_type numeric_precision numeric_precision_radix numeric_scale start_value minimum_value maximum_value increment cycle_option
itversity_retail_db public users_user_id_seq bigint 64 2 0 1 1 9223372036854775807 1 NO
Overview of Data Types Different Types of Constraints