Group By Vs Distinct in PostgreSQL

When working with PostgreSQL, efficiently organizing and retrieving data is critical for database performance and decision-making. Two commonly used clauses are DISTINCT and GROUP BY that serve distinct purposes in data retrieval. DISTINCT is used to filter out duplicate values, while GROUP BY is employed to aggregate data based on specific columns. Understanding these clauses' differences and use cases is vital for writing optimized queries and enhancing database performance.

In this article, we will explain DISTINCT and GROUP BY clauses in PostgreSQL, explore their syntax, provide examples with outputs, and highlight the best use cases for each. The article is designed to be SEO-friendly, with practical examples that will help us understand and apply these concepts effectively.

Understanding GROUP BY Clause

The GROUP BY clause is used with aggregate functions such as SUM, AVG, COUNT, MIN, and MAX to organize the result set based on one or more columns. By defining conditions, it arranges data into groups.

Syntax

SELECT column1, aggregate_function(column2)
FROM table_name
GROUP BY column1;

Key Terms

• column1: Specifies the column by which the result set will be grouped.
  
• aggregate_function(column2): Applies an aggregate function (e.g., SUM, AVG, COUNT, MIN, MAX) to column2 within each group.
  
• table_name: Refers to the name of the table from which data is queried.
  
• GROUP BY column1: Organizes the result set into groups based on the values in column1.

Understanding DISTINCT Clause

The DISTINCT clause is a useful tool in PostgreSQL that helps retrieve unique values from a particular column or combination of columns within a result set. It filters out duplicate values and improves the quality of data. The DISTINCT clause plays an important role in efficient data processing and retrieval, which leads to faster query execution times, especially when working with large datasets.

Syntax

SELECT DISTINCT column1, column2
FROM table_name;

Key Terms

• column1: Specifies the first column from which unique values will be retrieved.
  
• column2: Specifies the second column from which unique values will be retrieved, if applicable.
  
• table_name: Refers to the name of the table from which data is queried.
  
• DISTINCT: Filters out duplicate combinations of values from column1 and column2.

Difference between GROUP BY and DISTINCT clause in PostgreSQL

The Difference between "DISTINCT" and "GROUP BY" in PostgreSQL:

Setting Up Environment

To understand DISTINCT and GROUP BY Clause in PostgreSQL, we will first create a table name "student" which contains StudentID, Name, Age, the various and Grade as columns, then insert some values. We will perform various operations to understand it.

Query:

CREATE TABLE student (
    StudentID SERIAL PRIMARY KEY,
    Name VARCHAR(100),
    Age INT,
    Grade VARCHAR(10)
);
INSERT INTO student (Name, Age, Grade) VALUES ('Rohan', 18, 'A');
INSERT INTO student (Name, Age, Grade) VALUES ('Priya', 17, 'B');
INSERT INTO student (Name, Age, Grade) VALUES ('Aarav', 18, 'A');
INSERT INTO student (Name, Age, Grade) VALUES ('Isha', 17, 'B');
INSERT INTO student (Name, Age, Grade) VALUES ('Ananya', 18, 'A');

SELECT * FROM STUDENT;

Output

Examples of DISTINCT Clause

Let us look at some of the examples of the DISTINCT Clause in PostgreSQL to better understand the concept and its practical applications. The DISTINCT keyword is particularly useful when working with large datasets where duplicate entries need to be eliminated. It helps in generating concise and unique outputs, simplifying data analysis and reporting.

Example 1: Obtaining Unique Grades

Suppose we want to see the unique grades in the table as there are duplicate grades also. Using the DISTINCT clause eliminates all duplicate entries and displays only the unique values. We will run the below query.

Query:

SELECT DISTINCT Grade
FROM student;

Output

Explanation:

This query return the unique grades obtained by students. Each grade appears only once in the output, making it easier to identify all distinct grades present in the dataset.

Example 2: Obtaining Unique Ages

we can use the DISTINCT clause to filter out duplicates and get unique values. We can use the below query.

Query:

SELECT DISTINCT Age
FROM student;

Output

Explanation:

This query return the unique ages of the students. All the ages are listed only once which occurs once or more than once in the student table.

Examples of GROUP BY Clause

Let us look at some of the examples of GROUP BY Clause in PostgreSQL to better understand the concept. This clause is especially useful for performing aggregate operations on grouped data to extract meaningful insights.

Example 1: Total number of students in each Grade

Suppose we want to see the total number of students having the same grade for different grades, we can run the below query.

Query:

SELECT Grade, COUNT(*) AS total_students
FROM student
GROUP BY Grade;

Output

Explanation:

This query calculates the total number of students in each grade by adding the students having the same grade.

Example 2: Average age of students in each Grade

Suppose we want to know the average age of students who obtained the same grade. We can use the below query,

Query:

SELECT Grade, AVG(Age) AS avg_age
FROM student
GROUP BY Grade;

Output

Explanation:

This query calculates the average age of students grouped by their grades using the AVG() aggregate function. The GROUP BY clause ensures that the average is computed separately for each unique grade, providing summarized insights for each group.

Conclusion

Understanding the differences between DISTINCT and GROUP BY clauses in PostgreSQL is very important for efficient data operations in PostgreSQL. DISTINCT is used to get the Unique value from a column while GROUP BY is used to group the rows according to columns according to a condition by using aggregate functions such as sum, avg. DISTINCT is good for just retrieving the unique values from the column while GROUP BY is used to summarize the data.

FAQs

Is DISTINCT faster than GROUP BY?

The performance of DISTINCT vs. GROUP BY depends on the query and data distribution. Generally, DISTINCT can be slightly faster for simple deduplication, while GROUP BY is better suited for grouping data with aggregate functions.

What is the difference between DISTINCT and GROUP BY clause?

DISTINCT removes duplicate rows from the result set, focusing on unique combinations of selected columns. GROUP BY groups rows with the same values and is typically used with aggregate functions like SUM or COUNT.

What is the difference between SELECT COUNT(DISTINCT) and GROUP BY?

SELECT COUNT(DISTINCT column) counts the number of unique values in a column. GROUP BY groups rows based on column values and allows aggregations for each group, providing more detailed results.

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Article Tags :

• PostgreSQL
• Databases