Create a MySQL table 'employees' with a unique 'employee_id' and foreign keys referencing another table using ON DELETE NO ACTION and ON UPDATE NO ACTION actions, rejecting deletion and updates

MySQL Create Tables: Exercise-20 with Solution

20. Write a MySQL query to create a table employees including columns employee_id, first_name, last_name, job_id, salary and make sure that, the employee_id column does not contain any duplicate value at the time of insertion, and the foreign key column job_id, referenced by the column job_id of jobs table, can contain only those values which are exists in the jobs table. The InnoDB Engine have been used to create the tables. The specialty of the statement is that, The ON DELETE NO ACTION and the ON UPDATE NO ACTION actions will reject the deletion and any updates.

Assume that the structure of two table jobs and InnoDB Engine have been used to create the table jobs.

CREATE TABLE IF NOT EXISTS jobs ( 
JOB_ID integer NOT NULL UNIQUE PRIMARY KEY, 
JOB_TITLE varchar(35) NOT NULL DEFAULT ' ', 
MIN_SALARY decimal(6,0) DEFAULT 8000, 
MAX_SALARY decimal(6,0) DEFAULT NULL
)ENGINE=InnoDB;


+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| JOB_ID     | int(11)      | NO   | PRI | NULL    |       |
| JOB_TITLE  | varchar(35)  | NO   |     |         |       |
| MIN_SALARY | decimal(6,0) | YES  |     | 8000    |       |
| MAX_SALARY | decimal(6,0) | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+

Sample Solution:

-- Creating a table named 'employees' if it doesn't already exist to store employee information

CREATE TABLE IF NOT EXISTS employees(
    -- Column to store employee IDs with decimal precision of 6, 0, marked as NOT NULL and serving as the PRIMARY KEY
    EMPLOYEE_ID decimal(6,0) NOT NULL PRIMARY KEY,

    -- Column to store first names of employees with a maximum length of 20 characters, marked as DEFAULT NULL
    FIRST_NAME varchar(20) DEFAULT NULL,

    -- Column to store last names of employees with a maximum length of 25 characters, marked as NOT NULL
    LAST_NAME varchar(25) NOT NULL,

    -- Column to store job IDs of employees as INTEGER, marked as NOT NULL
    JOB_ID INTEGER NOT NULL,

    -- Column to store salaries of employees with decimal precision of 8, 2, marked as DEFAULT NULL
    SALARY decimal(8,2) DEFAULT NULL,

    -- Creating a FOREIGN KEY constraint on the JOB_ID column, referencing the jobs table
    FOREIGN KEY(JOB_ID) REFERENCES jobs(JOB_ID)
    -- Setting actions for ON DELETE (NO ACTION) and ON UPDATE (NO ACTION)
    ON DELETE NO ACTION 
    ON UPDATE NO ACTION
)
-- Setting the storage engine to InnoDB for transactional support
ENGINE=InnoDB;

Let execute the above code in MySQL command prompt

Here is the structure of the table:

mysql> DESC employees;
+----------------+--------------+------+-----+---------+-------+
| Field          | Type         | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+-------+
| EMPLOYEE_ID    | decimal(6,0) | NO   | PRI | NULL    |       |
| FIRST_NAME     | varchar(20)  | YES  |     | NULL    |       |
| LAST_NAME      | varchar(25)  | NO   |     | NULL    |       |
| JOB_ID         | varchar(10)  | NO   |     | NULL    |       |
| SALARY         | decimal(8,2) | YES  |     | NULL    |       |
+----------------+--------------+------+-----+---------+-------+

Explanation:

According to the MySQL code above:

• Create a table named "employees" if it doesn't already exist to store employee information.
• Define columns for employee details such as ID, first name, last name, job ID, and salary.
• Sets constraints on certain columns like NOT NULL, DEFAULT NULL, and defines a primary key on the 'EMPLOYEE_ID'.
• Establish a foreign key constraint on the 'JOB_ID' column, referencing the "jobs" table.
• Specifies that no action should be taken (ON DELETE NO ACTION and ON UPDATE NO ACTION) if a referenced record in the "jobs" table is deleted or updated.
• Set the storage engine to InnoDB for transactional support.

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