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Subquery - (PostgreSQL Table)

In PostgreSQL, a subquery is a query that is nested inside another query. It can be used to perform a variety of tasks, such as selecting data based on another query's result set, performing mathematical calculations, and more. In this tutorial, we'll discuss the syntax, example, output, explanation, use, important points, and summary of subqueries in PostgreSQL.

Syntax

SELECT column1, column2, ...
FROM table1
WHERE column1 
  (SELECT column_name(s) FROM table_name WHERE condition);

In this syntax, the SELECT statement is the outer query, and the subquery is enclosed inside parentheses. The subquery retrieves data from the table_name table based on the specified condition. The outer query then uses the result set from the subquery to filter the results.

Example

Let's take a look at an example of using a subquery in PostgreSQL.

SELECT *
FROM orders
WHERE customer_id IN (SELECT customer_id FROM customers WHERE country = 'USA');

In this example, we're selecting all orders from the orders table where the customer_id is found in the result set of a subquery. The subquery selects customer_id from the customers table where country is "USA". This means that only orders from US customers will be returned.

Explanation

A subquery is essentially a SELECT statement that is nested inside another SELECT statement. The inner SELECT statement is executed first, and its results are used by the outer SELECT statement to filter the results further.

In the above example, the inner SELECT statement is SELECT customer_id FROM customers WHERE country = 'USA'. This subquery returns all customer_id values for customers from the USA. The outer SELECT statement then filters the results of the orders table to include only those orders where the customer_id is found in the result set of the subquery.

Use

Subqueries are used in PostgreSQL to perform a variety of tasks, such as filtering data based on a result set, performing mathematical calculations, and more. They can be used in a variety of situations where a SELECT statement needs to be nested inside another SELECT statement.

Important Points

  • A subquery can return a single value or multiple values.
  • Subqueries can be used in the SELECT, FROM, and WHERE clauses of a SQL statement.
  • A subquery must be enclosed in parentheses.
  • Subqueries can also be nested inside other subqueries.

Summary

In this tutorial, we discussed subqueries in PostgreSQL. We explored the syntax, example, output, explanation, use, and important points of using subqueries. With this knowledge, you can now use subqueries in PostgreSQL to filter, aggregate, and manipulate data more effectively.

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