PostgreSQL Create Table: Create a table to set default value, a minimum value and a NULL as default value for specific columns

9. Write a SQL statement to create a table named jobs, including job_id, job_title, min_salary and max_salary, and make sure that, the default value for job_title is blank and min_salary is 8000 and max_salary is NULL will be entered automatically at the time of insertion if no value assigned for the specified columns.

Sample Solution:

Code:

-- This SQL statement creates a new table named 'jobs' if it does not already exist,
-- defining the structure and constraints for the table columns.

CREATE TABLE IF NOT EXISTS jobs ( 
    JOB_ID varchar(10) NOT NULL UNIQUE, -- Defines a column 'JOB_ID' to store job IDs as strings with a maximum length of 10 characters. The column cannot contain NULL values, and each value must be unique across all rows.
    JOB_TITLE varchar(35) NOT NULL DEFAULT ' ', -- Defines a column 'JOB_TITLE' to store job titles as strings with a maximum length of 35 characters. The column cannot contain NULL values, and if no value is provided during insertion, it defaults to an empty string.
    MIN_SALARY decimal(6,0) DEFAULT 8000, -- Defines a column 'MIN_SALARY' to store minimum salary amounts as decimal numbers with a precision of 6 digits and no decimal places. If no value is provided during insertion, it defaults to 8000.
    MAX_SALARY decimal(6,0) DEFAULT NULL -- Defines a column 'MAX_SALARY' to store maximum salary amounts as decimal numbers with a precision of 6 digits and no decimal places. If no value is provided during insertion, it defaults to NULL.
);

Explanation:

• The CREATE TABLE IF NOT EXISTS statement creates a new table only if it does not already exist in the database.
• The table is named 'jobs'.
• Each column definition specifies the column name, data type, and optional constraints.
• varchar(10) indicates a variable-length character string with a maximum length of 10 characters for the 'JOB_ID' column.
• varchar(35) indicates a variable-length character string with a maximum length of 35 characters for the 'JOB_TITLE' column.
• decimal(6,0) indicates a decimal number with a precision of 6 digits and no decimal places for both 'MIN_SALARY' and 'MAX_SALARY' columns.
• The NOT NULL constraint ensures that 'JOB_ID' and 'JOB_TITLE' columns cannot be left empty.
• The UNIQUE constraint ensures that each value in the 'JOB_ID' column must be unique across all rows in the table.
• The DEFAULT constraint specifies default values for columns when no value is provided during insertion. 'JOB_TITLE' defaults to an empty string, 'MIN_SALARY' defaults to 8000, and 'MAX_SALARY' defaults to NULL.

Output:

postgres=# CREATE TABLE IF NOT EXISTS jobs (
postgres(# JOB_ID varchar(10) NOT NULL UNIQUE,
postgres(# JOB_TITLE varchar(35) NOT NULL DEFAULT ' ',
postgres(# MIN_SALARY decimal(6,0) DEFAULT 8000,
postgres(# MAX_SALARY decimal(6,0) DEFAULT NULL
postgres(# );
CREATE TABLE

Here is the command to see the structure of the created table :

postgres=# \d jobs
                            Table "public.jobs"
   Column   |         Type          |                Modifiers
------------+-----------------------+-----------------------------------------
 job_id     | character varying(10) | not null
 job_title  | character varying(35) | not null default ' '::character varying
 min_salary | numeric(6,0)          | default 8000
 max_salary | numeric(6,0)          | default NULL::numeric
Indexes:
    "jobs_job_id_key" UNIQUE CONSTRAINT, btree (job_id)

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Next: Write a SQL statement to create a table named countries, including columns country_id, country_name and region_id and make sure that the country_id column will be a key field which will not contain any duplicate data at the time of insertion.