constraints

Q1. What does a primary key do?

• A primary key is used to store large amounts of data.
• A primary key uniquely identifies each record in a database table.
• A primary key can be duplicated across records.
• A primary key is a type of foreign key.

Q2. Which of the following constraints prevents NULL values in a column?

• DEFAULT
• NOT NULL
• PRIMARY KEY
• UNIQUE

Q3. Which of the following constraints can be used to provide a default value for a column?

• DEFAULT constraint
• PRIMARY KEY constraint
• CHECK constraint
• UNIQUE constraint

Q4. Which of the following is TRUE about UNIQUE constraint?

• The UNIQUE constraint can be applied to multiple columns but allows one duplicate value.
• The UNIQUE constraint allows duplicate values in a column.
• The UNIQUE constraint prevents duplicate values in a column.
• The UNIQUE constraint is only applicable to primary keys.

Q5. Which constraint ensures referential integrity between two tables?

• Primary key constraint
• Unique constraint
• Check constraint
• Foreign key constraint

Q6. What happens if we try to insert a NULL into a NOT NULL column?

• The operation will succeed without any issues.
• A default value will be inserted instead.
• The operation will fail with an error.
• The NULL value will be converted to an empty string.

Q7. Which statement correctly creates a table with a default value?

• CREATE TABLE example (id INT, name VARCHAR(100) DEFAULT 'unknown');
• CREATE TABLE example (id INT DEFAULT NULL, name VARCHAR(100));
• CREATE TABLE example (id INT DEFAULT 1, name TEXT);
• CREATE TABLE example (id INT DEFAULT 100, name VARCHAR(100));

Q8. Which SQL command is used to add a foreign key to an existing table?

• ALTER TABLE table_name ADD CONSTRAINT fk_name FOREIGN KEY (column_name) REFERENCES other_table (other_column);
• CREATE TABLE table_name ADD FOREIGN KEY (column_name);
• ALTER TABLE table_name ADD FOREIGN KEY (column_name) TO other_table;
• UPDATE table_name SET column_name REFERENCES other_table (other_column);

Q9. Which of the following is NOT allowed in a primary key?

• NULL values
• Empty strings
• Non-unique values
• Duplicate values

What is the result of this query?

CREATE TABLE emp (

id INT PRIMARY KEY,

name VARCHAR(20),

email VARCHAR(50) UNIQUE

);

• A. Only email must be unique

• B. Only id must be unique
  

• C. Both id and email must be unique
  

• D. All columns must be unique