Working with Tables and Data Types

How to create tables and define data types, with an introduction to primary keys and constraints

In this lesson, we’ll explore how to structure data in tables and choose appropriate data types for each column. You’ll also learn about primary keys and constraints, essential concepts for organizing and securing data in a relational database.

1. Understanding Tables in SQL

In a relational database, data is organized into tables. A table represents a specific entity or concept, like “Users” or “Products.” Each row in the table represents a unique instance of that entity (e.g., a specific user), while each column represents a characteristic or attribute of that entity (e.g., name, age, or email).

Example Table Structure

Here’s an example of a simple table called users:

| id | name       | age | email              |
|----|------------|-----|--------------------|
| 1  | Alice      | 30  | [email protected]  |
| 2  | Bob        | 25  | [email protected]    |

In this table:

• id uniquely identifies each user.
• name, age, and email store specific details about each user.

2. Creating a Table with CREATE TABLE

The CREATE TABLE statement defines the structure of a new table, specifying each column’s name and data type.

Basic Syntax

CREATE TABLE table_name (
    column1 data_type constraints,
    column2 data_type constraints,
    ...
);

Example: Creating a users Table

Here’s how to create the users table with columns for id, name, age, and email.

CREATE TABLE users (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,
    age INTEGER,
    email TEXT
);

• id: An integer that uniquely identifies each user. It’s designated as the PRIMARY KEY.
• name: A text field that cannot be null (NOT NULL), ensuring each user has a name.
• age: An integer storing the user’s age. No constraints are applied, so this column is optional.
• email: A text field storing the user’s email address.

Note: Constraints like PRIMARY KEY and NOT NULL help ensure data integrity. We’ll discuss them in more detail below.

3. Choosing Data Types

SQL provides various data types to represent different types of data. Choosing the correct data type for each column is essential for efficient storage and data integrity.

Common Data Types

Data Type Examples in SQL

• INTEGER for columns storing whole numbers, like age.
• TEXT for columns storing alphanumeric data, like name or email.
• REAL for decimal numbers, like a price or salary.
• DATE for date values, like birth_date or hire_date.

Tip: Choose the most specific data type to save space and improve performance.

4. Adding Constraints

Constraints are rules applied to columns to enforce data integrity. They help ensure that the data is accurate, consistent, and meaningful.

Common Constraints

Adding Constraints in SQL

• PRIMARY KEY: Ensures each row has a unique identifier.

    id INTEGER PRIMARY KEY
  

• NOT NULL: Requires the column to have a value.

    name TEXT NOT NULL
  

• UNIQUE: Ensures all entries in the column are distinct.

    email TEXT UNIQUE
  

Example Table with Constraints

CREATE TABLE users (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,
    age INTEGER DEFAULT 18,
    email TEXT UNIQUE
);

In this example:

• id is a primary key, ensuring each user has a unique identifier.
• name cannot be null, so every user must have a name.
• age defaults to 18 if no value is provided.
• email must be unique across all users.

5. Using AUTO_INCREMENT for Primary Keys (MySQL) or AUTOINCREMENT (SQLite)

In some databases, you can set a primary key column to automatically generate a unique number for each row. In SQLite, this is achieved with AUTOINCREMENT.

Example with AUTOINCREMENT (SQLite)

CREATE TABLE users (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT NOT NULL,
    email TEXT UNIQUE
);

Now, each new row added to the users table will automatically assign a unique id.

6. Modifying a Table with ALTER TABLE

Once a table is created, you can modify its structure using the ALTER TABLE command. Common modifications include adding, dropping, or renaming columns.

Adding a New Column

ALTER TABLE users
ADD COLUMN phone TEXT;

This adds a phone column to the users table.

Dropping a Column

If your database supports it (e.g., MySQL), you can remove columns.

ALTER TABLE users
DROP COLUMN phone;

Practice Exercise: Creating a New Table

1. Define a table structure for a table called products with the following columns:
   • product_id: integer, primary key with auto-increment.
   • product_name: text, not null.
   • price: real.
   • stock: integer with a default value of 0.
2. Write the SQL to create this table:

   CREATE TABLE products (
       product_id INTEGER PRIMARY KEY AUTOINCREMENT,
       product_name TEXT NOT NULL,
       price REAL,
       stock INTEGER DEFAULT 0
   );
   

3. Verify the table structure by running:

   .schema products
   

   This will display the products table’s structure if you’re using SQLite.

Summary of Key Commands

With this knowledge, you’re ready to create well-structured tables and define columns with data types and constraints. In the next lesson, we’ll cover how to add, modify, and delete data in your tables.

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