What is the difference between DISTINCT and GROUP BY in SQL?

 DISTINCT and GROUP BY are both SQL clauses used for data retrieval and are used to reduce redundancy in query results, but they serve different purposes and have distinct use cases:

DISTINCT:

1. Purpose:

   • DISTINCT is used to eliminate duplicate rows from the result set based on the selected columns. It ensures that only unique rows are returned.

2. Usage:

   • You typically use DISTINCT in the SELECT statement to specify the columns for which you want to find unique values. It applies to all selected columns, and it does not change the structure of the result set.

3. Example:

   SELECT DISTINCT column1, column2 FROM table_name;
   

4. Result:

   • The result will include only distinct combinations of values from column1 and column2, but all rows are still shown.

GROUP BY:

1. Purpose:

   • GROUP BY is used for grouping rows with similar values in one or more columns and performing aggregate functions (e.g., SUM, AVG, COUNT) on each group. It allows you to perform calculations and summary operations on data subsets.

2. Usage:

   • GROUP BY is used in conjunction with aggregate functions. You specify one or more columns by which you want to group the data, and then you apply aggregate functions to other columns.

3. Example

   SELECT column1, COUNT(*) FROM table_name GROUP BY column1;
   

4. Result:

   • The result will group rows by the values in column1 and show the count of rows in each group. It doesn't show individual rows but instead shows aggregated data.

Key Differences:

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• Result Set Structure:

  • DISTINCT does not change the structure of the result set. It filters rows to ensure uniqueness but still shows all columns and rows.
  • GROUP BY alters the structure of the result set by creating groups based on specified columns and providing aggregated results for each group.

• Aggregation:

  • DISTINCT is not used for aggregation. It only removes duplicate rows.
  • GROUP BY is used for aggregation, allowing you to perform calculations on grouped data.

• Multiple Columns:

  • DISTINCT applies to all selected columns in the SELECT statement.
  • GROUP BY allows you to specify one or more columns for grouping while applying aggregate functions to other columns.

In summary, DISTINCT is used to filter out duplicate rows based on selected columns, while GROUP BY is used to group rows based on specified columns and perform aggregate calculations. The choice between them depends on your specific data analysis needs.