Different Methods to Find Unique Values in SQL

Hello friends! Today we’ll be learning different methods to find unique values in SQL. When working with large datasets in SQL, it is often essential to find unique values for analysis or reporting purposes. Thankfully, SQL provides several approaches to identify distinct or unique values within a column or a dataset.

1. DISTINCT keyword

One of the simplest and most commonly used methods is to use the DISTINCT keyword. This approach allows us to retrieve only unique values from a specified column or columns.

For example, to find unique values in the “name” column of a table called “employees,” you can write the following query:

SELECT DISTINCT name 
FROM employees;

2. GROUP BY clause

Another effective method is to use the GROUP BY clause in combination with aggregate functions. This approach groups the data by one or more columns and then applies aggregate functions like COUNT, SUM, or AVG to the grouped data.

To find unique values in a column using the GROUP BY clause, you can write a query like this:

SELECT name 
FROM employees 
GROUP BY name;

3. COUNT() function

The COUNT() function can be used to count the occurrences of each value in a column. By comparing the count to one, we can determine if a value is unique or not.

To retrieve unique values using the COUNT() function, you can use a subquery or a common table expression (CTE) as follows:

SELECT name 
FROM employees 
GROUP BY name 
HAVING COUNT(*) > 0 ;

4. EXISTS or NOT EXISTS clause

The EXISTS and NOT EXISTS clauses are useful when you want to find unique values that exist or do not exist in another table.

For example, to find unique values in the “name” column of the “employees” table that do not exist in the “managers” table, you can write the following query:

SELECT name FROM 
employees 
WHERE NOT EXISTS (    
SELECT 1    FROM managers    
WHERE employees.name = managers.name );

5. ROW_NUMBER() function

The ROW_NUMBER() function assigns a unique sequential number to each row within a specific partition. By filtering for rows where the row number is equal to 1, we can retrieve unique values.

To find unique values using the ROW_NUMBER() function, you can use a PARTITION BY clause as shown below:

WITH numbered_rows AS (
SELECT name, ROW_NUMBER() OVER (PARTITION BY name ORDER BY name) AS row_num    
FROM employees )
SELECT name 
FROM numbered_rows
WHERE row_num = 1;

6. UNION operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

To find unique values using the UNION operator, you can use a UNION as shown below:

SELECT COUNTRY FROM Customers 
UNION 
SELECT COUNTRY FROM Customers;

These are just a few of the many approaches to finding unique values in SQL. Choose the method that suits your specific requirements and the structure of your database.

By utilizing these methods effectively, you can easily handle and analyze large datasets while extracting valuable insights from your SQL databases.

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