Create a MySQL table 'employees' ensuring a unique 'employee_id' and foreign keys referencing another table with ON UPDATE CASCADE and ON DELETE RESTRICT actions

MySQL Create Tables: Exercise-17 with Solution

17. 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 UPDATE CASCADE action allows you to perform cross-table update and ON DELETE RESTRICT action reject the deletion. The default action is ON DELETE RESTRICT.

Assume that the structure of the 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 email addresses of employees with a maximum length of 25 characters, marked as NOT NULL
    EMAIL varchar(25) NOT NULL,

    -- Column to store phone numbers of employees with a maximum length of 20 characters, marked as DEFAULT NULL
    PHONE_NUMBER varchar(20) DEFAULT NULL,

    -- Column to store the hire date of employees, marked as NOT NULL
    HIRE_DATE date NOT NULL,

    -- Column to store job IDs of employees with a maximum length of 10 characters, marked as NOT NULL
    JOB_ID varchar(10) NOT NULL,

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

    -- Column to store commission percentages of employees with decimal precision of 2, 2, marked as DEFAULT NULL
    COMMISSION_PCT decimal(2,2) DEFAULT NULL,

    -- Column to store manager IDs of employees with decimal precision of 6, 0, marked as DEFAULT NULL
    MANAGER_ID decimal(6,0) DEFAULT NULL,

    -- Column to store department IDs of employees with decimal precision of 4, 0, marked as DEFAULT NULL
    DEPARTMENT_ID decimal(4,0) DEFAULT NULL,

    -- Creating a FOREIGN KEY constraint on the DEPARTMENT_ID column, referencing the departments table
    FOREIGN KEY(DEPARTMENT_ID) REFERENCES departments(DEPARTMENT_ID),

    -- Creating a FOREIGN KEY constraint on the JOB_ID column, referencing the jobs table
    FOREIGN KEY(JOB_ID) REFERENCES jobs(JOB_ID)
)
-- 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    |       |
| EMAIL          | varchar(25)  | NO   |     | NULL    |       |
| PHONE_NUMBER   | varchar(20)  | YES  |     | NULL    |       |
| HIRE_DATE      | date         | NO   |     | NULL    |       |
| JOB_ID         | varchar(10)  | NO   |     | NULL    |       |
| SALARY         | decimal(8,2) | YES  |     | NULL    |       |
| COMMISSION_PCT | decimal(2,2) | YES  |     | NULL    |       |
| MANAGER_ID     | decimal(6,0) | YES  |     | NULL    |       |
| DEPARTMENT_ID  | decimal(4,0) | YES  | MUL | NULL    |       |
+----------------+--------------+------+-----+---------+-------+
11 rows in set (0.01 sec)

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, name, email, phone number, hire date, job, salary, commission percentage, manager ID, and department ID.
• Sets constraints on certain columns like NOT NULL, DEFAULT NULL, and defines a primary key on the 'EMPLOYEE_ID'.
• Establish foreign key constraints on the 'DEPARTMENT_ID' and 'JOB_ID' columns, referencing the corresponding columns in the "departments" and "jobs" tables.
• Set the storage engine to InnoDB for transactional support.

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Previous: Write a SQL statement to create Table Exercises: Create a table employees including columns, and make sure that, the employee_id column contain only unique value, and set some foreign keys referencing to another table.
Next: Write a SQL statement 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.