Oracle SQL Tutorial 12 - Intro to Constraints

Summary

In this video, Caleb dives into the concept of Oracle constraints, explaining their importance and usage in database management. He covers several key constraints: Not Null, Primary Key, Foreign Key, Unique, Check, and Default. Caleb explains how each constraint works to maintain data integrity, such as ensuring uniqueness, defining relationships between tables, and setting limits on column values. Through practical examples, including a banking app scenario, he demonstrates how these constraints help manage data effectively. Caleb also teases upcoming videos where he will show how to implement these constraints in Oracle databases.

Takeaways

• 😀 Constraints define rules for the type of data allowed in a database column.
• 😀 The NOT NULL constraint ensures a column always has a value and no nulls are allowed.
• 😀 The PRIMARY KEY constraint combines NOT NULL and UNIQUE, ensuring each value is unique and non-null.
• 😀 The FOREIGN KEY constraint enforces relationships between tables, requiring values in one column to match those in another table's primary key.
• 😀 The CHECK constraint allows specific rules, such as limiting the range of values in a column, like setting a transaction amount between $1 and $1,000.
• 😀 The UNIQUE constraint ensures all values in a column are distinct and prevents duplicate values in that column.
• 😀 The DEFAULT keyword is used to assign a default value to a column, like setting an account balance to zero when creating a new account.
• 😀 Primary keys should remain immutable to maintain data integrity and uniquely identify rows.
• 😀 Foreign keys reference primary keys in other tables, ensuring that data entered corresponds to valid existing data in another table.
• 😀 The CHECK constraint can be used to enforce more specific rules, such as enforcing conditions on numeric values or text entries.
• 😀 Caleb will dive deeper into implementing these constraints in upcoming videos, focusing on practical application in Oracle databases.

Q & A

What is the purpose of a database constraint?

A database constraint is essentially a rule that determines what kind of data is allowed in a column. It enforces specific rules on the database to ensure data integrity and consistency.

What does the 'NOT NULL' constraint do?

The 'NOT NULL' constraint ensures that a column always requires a value. It prevents null values, meaning no empty or undefined data can be entered into that column.

How does a primary key work in a database?

A primary key uniquely identifies each row in a database table. It combines the characteristics of 'NOT NULL' and 'UNIQUE', meaning it must always have a value and it must be unique across all rows in the table.

What is the difference between a foreign key and a primary key?

A primary key uniquely identifies rows within its own table, while a foreign key in another table references the primary key of a different table. The foreign key constraint ensures that data in one table corresponds to data in another table.

What happens if a value that doesn't exist in the referenced table is inserted into a foreign key column?

If an invalid value is inserted into a foreign key column (one that doesn't match any existing primary key value in the referenced table), the database will reject the insertion and generate an error.

What does the 'CHECK' constraint do in a database?

The 'CHECK' constraint allows you to specify a condition or rule for a column's values. It can be used to restrict the allowed data, such as ensuring values are within a specific range (e.g., between 10 and 100).

Can you give an example of how the 'CHECK' constraint could be used in a banking application?

In a banking application, a 'CHECK' constraint could enforce a minimum and maximum transaction limit, such as ensuring a transfer amount is between $1 and $1,000.

What is the role of the 'DEFAULT' keyword in a database?

The 'DEFAULT' keyword is used to set a default value for a column. If no value is provided during an insert operation, the column will automatically be populated with the default value. For example, a new account could have a default balance of zero.

What is the significance of using uppercase for keywords like 'NOT NULL', 'PRIMARY KEY', and 'FOREIGN KEY'?

Keywords such as 'NOT NULL', 'PRIMARY KEY', and 'FOREIGN KEY' are conventionally written in uppercase to differentiate them from other elements in the query, making them stand out and easier to identify.

How does a 'UNIQUE' constraint work?

The 'UNIQUE' constraint ensures that all values in a specified column are distinct, meaning no two rows can have the same value in that column. This is useful for ensuring the uniqueness of certain types of data, like email addresses or user IDs.