Column types and constraints in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a column type in Flask-SQLAlchemy?

A column type defines the kind of data a column can hold, like Integer, String, or DateTime. It tells the database what type of information to expect.

beginner

What does the nullable=False constraint do in a column?

It means the column cannot have empty or NULL values. Every row must have a value for that column.

beginner

How do you set a column to be a primary key in Flask-SQLAlchemy?

You add primary_key=True to the column definition. This makes the column unique and identifies each row.

beginner

What is the purpose of the unique=True constraint?

It ensures that all values in the column are different. No two rows can have the same value in that column.

beginner

Name three common column types used in Flask-SQLAlchemy.

Common types include Integer for numbers, String for text, and DateTime for date and time values.

Which column type would you use to store a person's name in Flask-SQLAlchemy?

A String

B Integer

C Boolean

D DateTime

Names are text, so the String type is the right choice.

What does primary_key=True do in a column definition?

A Allows NULL values

B Sets a default value

C Makes the column optional

D Marks the column as unique and identifies each row

Primary key uniquely identifies each row and cannot be NULL.

If you want to make sure no two users have the same email, which constraint should you use?

A nullable=False

B unique=True

C primary_key=True

D default='[e-mail]'

unique=True ensures all values in the column are different.

What happens if you set nullable=False on a column?

A The column is ignored by the database

B The column can have empty values

C The column must always have a value

D The column automatically increments

nullable=False means the column cannot be empty or NULL.

Which of these is NOT a valid column type in Flask-SQLAlchemy?

A ArrayList

B String

C Integer

D Float

ArrayList is not a column type in Flask-SQLAlchemy.

Explain how to define a column with a type and constraints in Flask-SQLAlchemy.

Think about how you tell the database what kind of data to expect and rules for that data.

Why are constraints like unique and nullable important when designing database columns?

Consider how rules help keep your data clean and trustworthy.

Practice

1. Which SQLAlchemy column type should you use in Flask to store text data of variable length?

easy

A. db.String

B. db.Integer

C. db.Boolean

D. db.DateTime

Solution

1. Step 1: Understand column types for text data

   Text data of variable length is best stored using a string type that allows flexible length.

2. Step 2: Match the correct SQLAlchemy type

   db.String is designed for variable-length text, unlike db.Integer or db.Boolean.

3. Final Answer:

   db.String -> Option A

4. Quick Check:

   Variable-length text = db.String [OK]

Hint: Text data uses db.String type in Flask models [OK]

Common Mistakes:

• Choosing db.Integer for text data
• Using db.Boolean for strings
• Confusing db.DateTime with text types

2. Which of the following is the correct way to declare a non-nullable integer column named age in a Flask model?

easy

A. age = db.Column(db.Integer, nullable=True)

B. age = db.Column(db.Boolean, nullable=False)

C. age = db.Column(db.String, nullable=False)

D. age = db.Column(db.Integer, nullable=False)

Solution

1. Step 1: Identify the correct type for age

   Age should be stored as an integer, so db.Integer is correct.

2. Step 2: Set the nullable constraint

   To make the column non-nullable, use nullable=False.

3. Final Answer:

   age = db.Column(db.Integer, nullable=False) -> Option D

4. Quick Check:

   Integer + non-nullable = age = db.Column(db.Integer, nullable=False) [OK]

Hint: Use nullable=False to prevent empty values [OK]

Common Mistakes:

• Setting nullable=True when non-null is needed
• Using wrong data type like String for age
• Confusing Boolean with Integer type

3. Given this Flask model column declaration:

email = db.Column(db.String(120), unique=True, nullable=False)

What does the unique=True constraint do?

medium

A. Prevents duplicate email values in the table

B. Makes the email column optional

C. Allows multiple rows to have the same email

D. Automatically converts emails to lowercase

Solution

1. Step 1: Understand the unique constraint

   The unique=True constraint ensures no two rows have the same value in this column.

2. Step 2: Apply this to the email column

   This means duplicate emails are not allowed, keeping each email distinct.

3. Final Answer:

   Prevents duplicate email values in the table -> Option A

4. Quick Check:

   unique=True means no duplicates [OK]

Hint: unique=True means no two rows share the same value [OK]

Common Mistakes:

• Thinking unique=True allows duplicates
• Confusing unique with nullable
• Assuming unique changes data format

4. What is wrong with this Flask model column declaration?

price = db.Column(db.String, nullable=False, unique='yes')

medium

A. The nullable argument cannot be used with unique

B. The unique argument should be a boolean, not a string

C. The column type db.String cannot be used for price

D. The column name price is a reserved keyword

Solution

1. Step 1: Check the unique argument type

   The unique parameter expects a boolean value (True or False), not a string.

2. Step 2: Identify the error in the code

   Passing unique='yes' is invalid and will cause an error.

3. Final Answer:

   The unique argument should be a boolean, not a string -> Option B

4. Quick Check:

   unique must be True or False, not 'yes' [OK]

Hint: unique must be True/False, never a string [OK]

Common Mistakes:

• Using string values for boolean parameters
• Thinking nullable and unique conflict
• Assuming price is reserved keyword

5. You want to create a Flask model column for a user's phone number that must be exactly 10 digits, unique, and cannot be empty. Which declaration is best?

hard

A. phone = db.Column(db.String, unique=False, nullable=False)

B. phone = db.Column(db.Integer, unique=True, nullable=False)

C. phone = db.Column(db.String(10), unique=True, nullable=False)

D. phone = db.Column(db.String(15), nullable=True)

Solution

1. Step 1: Choose the correct type for phone numbers

   Phone numbers often contain leading zeros, so db.String(10) is better than db.Integer.

2. Step 2: Apply constraints for uniqueness and non-null

   Use unique=True to prevent duplicates and nullable=False to disallow empty values.

3. Final Answer:

   phone = db.Column(db.String(10), unique=True, nullable=False) -> Option C

4. Quick Check:

   String(10) + unique + non-null = phone = db.Column(db.String(10), unique=True, nullable=False) [OK]

Hint: Use String with length, unique=True, nullable=False for fixed phone numbers [OK]

Common Mistakes:

• Using Integer type losing leading zeros
• Allowing nullable=True when value is required
• Ignoring unique constraint for phone numbers