Chapter 2, Level 2: Constraints

Question 1

What are SQL constraints?

Answer 1

SQL constraints are used specify rules for data in a table. They are used to prevent bad or unwanted data from getting into the tables.
If there is any violation between the constraint (‘rule’) and the data action (e.g. inserting a null value when the constraint forbids a null value), then the action is aborted by the constraint.

Question 2

What are the six constraints?

Answer 2

• NOT NULL
• UNIQUE
• PRIMARY KEY
• FOREIGN KEY
• CHECK
• DEFAULT

Question 3

What statement would add a constraint preventing null values from being entered into a table column?

Answer 3

ALTER TABLE table_name
(
CONSTRAINT not_null_name NOT NULL (column_name)
);

Question 4

What is the Primary Key Constraint?

Answer 4

This constraint (rule) means that a column cannot be NULL but must be UNIQUE.

Question 5

What is the difference between a PRIMARY KEY vs. NOT NULL + UNIQUE constraints?

Answer 5

A PRIMARY KEY can only be defined once per table.

Question 6

When a column in one table references the primary key of another table, it’s called a ______?

Answer 6

Foreign Key

Question 7

A row in a table containing a FOREIGN KEY that references a missing row in another table is called a ______ record?

Answer 7

An orphan record

Question 8

The ______ constraint is used to validate the value that can be placed in a column.

Answer 8

CHECK