Creating Table Without Constraints

Query:

CREATE TABLE students
(
id INT,
name VARCHAR(17),
course VARCHAR(15),
PRIMARY KEY (id)
); 

Explanation:

• In the above image we have created a table with the name students.
• We can see that the a table with the name students is created at the left side.
• As mentioned we have not added any constraints in the given table while creating the column.
• In the above table we have id , name and course as column names.
• The id is the primary key in the above table . Similiar to the sql the primary key is unique identification for any records in the table.
• The primary key should be unique and it should not have any null values while inserting the records in the table.

Creating a Table Using Check Constraint

Query:

CREATE TABLE employee
(
id INT,
name VARCHAR(255),
age INT CHECK( age > 18 ), 
salary INT,
PRIMARY KEY (id)
);

Explanation:

• In the above image we have created a table with the name employee.
• The employee table has the the column names id ,name , age and salary.
• The id is the primary key of the table employee and every record in the table is uniquely identified with the primary key.
• The age is created with the check constraint and the value in the column age should be greater than 18 .
• Now we will insert the records in the table.

Condition 1: Let’s Insert some records for checking Check Constraints

INSERT INTO employee (id, name, age, salary) 2 VALUES
(1,"Krishna", 25, 200000),
(2, "Rama",26,100000),
(3, "Hanuman", 20, 300000),
(4, "Shiva",19,400000);

Explanation:

• In the above employee table we have successfully inserted the record values.
• We have created unique key for every record in the insert operation.
• After that we have inserted every column value satisfying the specific datatype in the table.
• We have added the age value greater than 18 in every value.

Output:

Condition 2: Check values during Insertion by applying the CHECK Constraints.

Explanation:

• In the above image we have inserted records in the table employee.
• The insertion was not successfull because the records doesn’t satisfy the check constraint in the table.
• The age value is less than 18. So the record is not inserted successfully in the table.

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In this article, We will understand the Unique Constraint with examples and so on.