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Answer: The FOREIGN KEY Constraint

Explore how to apply the FOREIGN KEY constraint in SQL to ensure data consistency between related tables. Learn to modify table structures, add or drop foreign keys, and understand their role in preventing invalid data. This lesson equips you with the skills needed to handle foreign key constraints confidently.

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Solution

The solution is given below:

MySQL
/* Modifying the EmpID in the Skills table to be NOT NULL */
ALTER TABLE   Skills
MODIFY COLUMN EmpID INT NOT NULL;
/* Adding the foreign key constraint on EmpID in the Skills table */
ALTER TABLE     Skills
ADD FOREIGN KEY (EmpID) REFERENCES Employees (EmpID);
/* Describe the structure of the table */
DESC Skills;

Explanation

The explanation of the solution code is given below:

  • Lines 2–3: The ALTER TABLE statement makes changes in the structure of the Skills table. It modifies the EmpID column to be NOT NULL. A NULL value in this column can lead to orphaned rows, which can ...