Dive into the world of SQL and explore the power of the SQL UNIQUE keyword. In this article, you will gain a deeper understanding of the purpose and usage of SQL UNIQUE, learning the essential role it plays in preventing duplicate entries within your database. With practical examples, you will discover how to create unique constraints while using ALTER TABLE to enforce the SQL UNIQUE rules effectively. Furthermore, this article will showcase the differences between SELECT UNIQUE and SELECT DISTINCT SQL queries, as well as demonstrating their real-life applications. Lastly, uncover the significance of counting unique values through SQL COUNT DISTINCT, while becoming familiar with common scenarios for dealing with SQL unique values in a database. Get ready to enhance your skills and understanding of the SQL UNIQUE keyword.
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Sign up for freeWhat is the difference between an SQL UNIQUE constraint and a SELECT DISTINCT query?
What does the SQL COUNT DISTINCT function do?
How does the SQL UNIQUE constraint prevent duplicate entries?
What is the purpose of SELECT DISTINCT and SELECT UNIQUE queries in SQL?
What is the purpose of the SQL UNIQUE keyword?
When creating a table with the SQL UNIQUE constraint, which statement should you use?
How can you apply the SQL UNIQUE constraint to an existing table?
A common use case for the SQL UNIQUE constraint is to ensure that each user's ____________ is unique in a database.
Which SQL keyword is considered obsolete and less commonly used: SELECT DISTINCT or SELECT UNIQUE?
In which situations is the SELECT DISTINCT query useful?
How can you retrieve unique product categories from a "products" table using SELECT DISTINCT?
What is the difference between an SQL UNIQUE constraint and a SELECT DISTINCT query?
What does the SQL COUNT DISTINCT function do?
How does the SQL UNIQUE constraint prevent duplicate entries?
What is the purpose of SELECT DISTINCT and SELECT UNIQUE queries in SQL?
What is the purpose of the SQL UNIQUE keyword?
When creating a table with the SQL UNIQUE constraint, which statement should you use?
How can you apply the SQL UNIQUE constraint to an existing table?
A common use case for the SQL UNIQUE constraint is to ensure that each user's ____________ is unique in a database.
Which SQL keyword is considered obsolete and less commonly used: SELECT DISTINCT or SELECT UNIQUE?
In which situations is the SELECT DISTINCT query useful?
How can you retrieve unique product categories from a "products" table using SELECT DISTINCT?
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Send FeedbackSQL UNIQUE is a constraint or rule applied to a column or a set of columns in an SQL database table. It ensures that each value within the specified column(s) remains unique, preventing any duplicate entries and maintaining data integrity.
For example, if you have a table named "users" with a UNIQUE constraint on the "email" column, the DBMS will not allow two rows with identical email addresses. When you attempt to insert a new row with a duplicate email address or update an existing row with an email address that already exists in the table, the DBMS will return an error.
For example, we have a table named "users" with the following columns: id, first_name, last_name, and email. We want to enforce the uniqueness of each user's email address. To do this, we can create the table using the following SQL CREATE TABLE statement:
CREATE TABLE users (
id INT PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50),
email VARCHAR(320) UNIQUE
);
For instance, let's say we have an existing table called "users" without a UNIQUE constraint on the "email" column. To add a UNIQUE constraint to the "email" column, you can use the following SQL ALTER TABLE statement:
ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email);
While the SQL UNIQUE constraint enforces uniqueness on columns, there might be situations where you need to retrieve unique values from a table column without altering the table structure. For this purpose, you can implement SELECT UNIQUE SQL queries to extract distinct data without modifying the original table.
For example, if you have a "products" table as shown below:
| Product ID | Category |
| 1 | Electronics |
| 2 | Electronics |
| 3 | Furniture |
| 4 | Books |
To retrieve unique product categories, you can use the SELECT DISTINCT query:
SELECT DISTINCT Category FROM products;
The output will be:
| Category |
| Electronics |
| Furniture |
| Books |
SELECT COUNT(DISTINCT column_name) FROM table_name WHERE optional_conditions;When you execute this query, the database management system (DBMS) will return the total count of unique values in the specified column, meeting the optional_conditions if any were provided.
For example, let's say you have a table called "orders" with the following columns: order_id, customer_id, and product_id. If you want to count the number of unique customers who have placed orders, you can write the following query:
SELECT COUNT(DISTINCT customer_id) FROM orders;
This query will return the total count of unique customer_id values found in the "orders" table.
SQL UNIQUE: A constraint applied to a column or a set of columns in an SQL database table ensuring uniqueness and preventing duplicate entries.
Select Unique SQL vs. Select Distinct: Both serve the same purpose when querying data, retrieving unique values without duplicates, but Select Distinct is more commonly used.
SQL COUNT DISTINCT: A powerful query that combines the COUNT function and the DISTINCT keyword to count unique occurrences of a specific value or group within one or multiple columns.
ALTER TABLE: Used to apply SQL UNIQUE constraint to an existing column or set of columns without altering the table structure.
Common Use Cases: Data analysis, measuring user engagement, calculating event attendance, detecting data anomalies, and evaluating marketing efforts.
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