In Oracle SQL, subqueries, also known as nested queries or subselects, are queries embedded within other SQL statements. Subqueries are used to retrieve data that will be used as a part of the main query’s condition or result set. Here’s a brief description of subqueries in Oracle:

1. Purpose:

• Subqueries allow you to break down complex queries into smaller, more manageable parts. They are used to retrieve data dynamically based on the results of another query.

2. Types of Subqueries:

• Single-Row Subqueries: These subqueries return a single value and are typically used in conditions where a single value is expected, such as comparisons using =, >, <, etc.
• Multiple-Row Subqueries: These subqueries return multiple rows and can be used with operators like IN, ANY, ALL, etc., to compare a set of values.
• Correlated Subqueries: These subqueries reference columns from the outer query, allowing you to create dependencies between the inner and outer queries.
• Scalar Subqueries: Subqueries that return a single value, often used in the SELECT clause to retrieve a calculated value.
• Inline Views (Derived Tables): Subqueries that are placed in the FROM clause, effectively creating a temporary table that can be used in the main query.

3. Syntax:

• The basic syntax of a subquery looks like this: SELECT column1 FROM table1 WHERE column2 operator (SELECT column3 FROM table2 WHERE condition);
• The subquery is enclosed in parentheses and is typically placed in the WHERE clause, although it can also be used in other parts of the SQL statement.

4. Use Cases:

• Subqueries are used for various purposes, including:
  • Filtering data based on a condition calculated from another table.
  • Retrieving data to be used in the SELECT clause to perform calculations.
  • Evaluating conditions for INSERT, UPDATE, or DELETE statements.
  • Creating dynamic and flexible queries that adapt to changing data.

5. Performance Considerations:

• While subqueries are powerful, they can impact query performance. Correlated subqueries, in particular, can be resource-intensive because they execute once for each row in the outer query.
• Proper indexing and query optimization techniques are important when working with subqueries to ensure efficient query execution.

Subqueries are a versatile tool in SQL and are used extensively for various tasks, such as filtering, aggregating, and deriving data from multiple tables or conditions. They enhance the flexibility and readability of SQL queries, making them a fundamental concept in database querying and reporting.