MySQL 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. The InnoDB Engine have been used to create the tables

MySQL Create Tables: Exercise-16 with Solution

16. Write a SQL statement to create a table employees including columns employee_id, first_name, last_name, email, phone_number hire_date, job_id, salary, commission, manager_id and department_id and make sure that, the employee_id column does not contain any duplicate value at the time of insertion, and the foreign key column department_id, reference by the column department_id of departments table, can contain only those values which are exists in the departments table and another 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.

"A foreign key constraint is not required merely to join two tables. For storage engines other than InnoDB, it is possible when defining a column to use a REFERENCES tbl_name(col_name) clause, which has no actual effect, and serves only as a memo or comment to you that the column which you are currently defining is intended to refer to a column in another table." - Reference dev.mysql.com

Assume that the structure of two tables departments and jobs.

+-----------------+--------------+------+-----+---------+-------+
| Field           | Type         | Null | Key | Default | Extra |
+-----------------+--------------+------+-----+---------+-------+
| DEPARTMENT_ID   | decimal(4,0) | NO   | PRI | 0       |       |
| DEPARTMENT_NAME | varchar(30)  | NO   |     | NULL    |       |
| MANAGER_ID      | decimal(6,0) | YES  |     | NULL    |       |
| LOCATION_ID     | decimal(4,0) | YES  |     | NULL    |       |
+-----------------+--------------+------+-----+---------+-------+


+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| JOB_ID     | varchar(10)  | NO   | PRI |         |       |
| JOB_TITLE  | varchar(35)  | NO   |     | NULL    |       |
| MIN_SALARY | decimal(6,0) | YES  |     | NULL    |       |
| MAX_SALARY | decimal(6,0) | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+

Sample Solution:

CREATE TABLE IF NOT EXISTS employees ( 
EMPLOYEE_ID decimal(6,0) NOT NULL PRIMARY KEY, 
FIRST_NAME varchar(20) DEFAULT NULL, 
LAST_NAME varchar(25) NOT NULL, 
EMAIL varchar(25) NOT NULL, 
PHONE_NUMBER varchar(20) DEFAULT NULL, 
HIRE_DATE date NOT NULL, 
JOB_ID varchar(10) NOT NULL, 
SALARY decimal(8,2) DEFAULT NULL, 
COMMISSION_PCT decimal(2,2) DEFAULT NULL, 
MANAGER_ID decimal(6,0) DEFAULT NULL, 
DEPARTMENT_ID decimal(4,0) DEFAULT NULL, 
FOREIGN KEY(DEPARTMENT_ID) 
REFERENCES  departments(DEPARTMENT_ID),
FOREIGN KEY(JOB_ID) 
REFERENCES  jobs(JOB_ID)
)ENGINE=InnoDB;

Let execute the above code in MySQL 5.6 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)

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Previous: Write a SQL statement to create a table employees including columns employee_id, first_name, last_name, email, phone_number hire_date, job_id, salary, commission, manager_id and department_id and make sure that, the employee_id column does not contain any duplicate value at the time of insertion and the foreign key columns combined by department_id and manager_id columns contain only those unique combination values, which combinations are exists in the departments 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. 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.