Difference between SELECT and DISTINCT SELECT commands in SQL.

The SELECT and SELECT DISTINCT commands are both used in SQL to retrieve data from a database table, but they serve slightly different purposes. Let's explore the differences between these two commands:

1. SELECT: The SELECT command is used to retrieve data from one or more columns of a table. It returns all rows that meet the specified conditions, including duplicate values.

Example:

SELECT column1, column2
FROM table_name
WHERE condition;

In this example, the query will return all rows that satisfy the given condition, including repeated values in the specified columns.

2. SELECT DISTINCT: The SELECT DISTINCT command is used to retrieve unique values from one or more columns of a table. It eliminates duplicate values and returns only distinct values.

Example:

SELECT DISTINCT column1
FROM table_name
WHERE condition;

In this example, the query will return only unique values from the specified column, removing any duplicates.

Key Differences:

Purpose:

• SELECT: Retrieves all values, including duplicates, that satisfy the specified conditions.
• SELECT DISTINCT: Retrieves only unique values, eliminating duplicates.

Result Set:

• SELECT: Returns all rows that meet the conditions, including all instances of duplicate values.
• SELECT DISTINCT: Returns a result set with only distinct values from the specified column(s).

Usage:

• SELECT: Use when you want to retrieve all values from one or more columns, regardless of duplication.
• SELECT DISTINCT: Use when you want to retrieve only unique values from a specific column.

Performance:

• SELECT: May retrieve a larger result set, potentially impacting query performance.
• SELECT DISTINCT: Generally retrieves a smaller result set, which can improve query performance in cases with many duplicates.

Column Limitation:

• SELECT: Can retrieve multiple columns and their values.
• SELECT DISTINCT: Typically used with a single column to retrieve unique values from that column.

Summary:

In summary, the main difference between SELECT and SELECT DISTINCT lies in their handling of duplicate values. Use SELECT when you want to retrieve all values, even if they are duplicated, and use SELECT DISTINCT when you want to retrieve only unique values from a specific column. The choice between them depends on the specific requirements of your SQL query and the data you're working with.