Learn all SQL key constraints in 3 minutes
Keys in SQL
In SQL, keys are used to uniquely identify records in a table, establish relationships between tables, and enforce data integrity.
What are Keys?
Keys are columns or combinations of columns that help identify and link data across tables. There are several types of keys, each with a specific purpose.
Types of Keys
1. Primary Key: Uniquely identifies each record in a table, ensuring no duplicates.
2. Foreign Key: Links a table to another table's primary key, establishing relationships.
3. Unique Key: Ensures all values in a column are unique, but allows nulls.
4. Composite Key: A combination of columns that uniquely identify a record.
Why Keys Matter
1. Data Integrity: Keys prevent data duplication and inconsistencies.
2. Efficient Data Retrieval: Keys speed up query performance by indexing data.
3. Relationship Establishment: Keys link tables, enabling complex queries.
Example: Using Keys
Suppose you have a users table and an orders table:
users table: user_id (Primary Key), username
orders table: order_id (Primary Key), user_id (Foreign Key), order_date
Next, we'll explore how to create tables with keys and establish relationships in SQL.
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