What are Correlated Subqueries?

A correlated subquery is a nested inner query whose results depend on the outer query for its values. The inner query gets executed once for each row evaluated by the outer query. In simple words, the result of the subquery will be dependent on the outer query. Don’t confuse correlated subqueries with the nested queries. In nested queries, the inner query is only executed one time whereas correlated subqueries get executed for every row returned by the outer query. The outer query can consist of UPDATE, DELETE, WHERE, and SELECT clauses in case of correlated subqueries.

Key points about correlated subqueries:

• The inner query executes once for every row in the outer query.

• The inner query has a reference to a column in the outer query in its WHERE clause.

• The results of the inner query are used by the outer query.

Syntax:

SELECT columns
FROM outer_table

WHERE condition_operator

      (SELECT columns
       FROM inner_table
       WHERE outer_table.column = inner_table.column);

The syntax of the Correlated subqueries mainly consist of 3 parts:

• In the first part we will have to specify the columns we want to retirive in the final result set from the outer query. Use the SELECT clause to select the required columns and FROM clause to select the outer table from which the main query retrieves rows.

• In the second part we will have to define the conditions for selecting rows from the outer table using the Where clause. The conditions can consist of comparisons logical operators or any valid conditions specified by the user.

• In the third part we will have to define our correlated subquery. This inner query will retrieve the columns from the inner table. The correlation will be established by specifying a condition that links a column from the outer query’s table to a column in the inner query’s table.

Correlate subquery is a great tool in SQL servers that allows users to fetch the required data from the tables without performing complex join operations. In SQL, a subquery is a query nested inside another query. A correlated subquery is a specific type of subquery that references columns from the outer query, creating a relationship between the two.

In this article, we will learn about the correlated subqueries in SQL Server step by step with various examples. We will learn how correlated subquery can be used as a substitute for complex join operations. So deep dive into this article and master the correlate subqueries in SQL server.