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A SQL correlated subquery is just a query that carries over values from the outer query. This means that there is a WHERE statement using data from the outer query, or the first query. These?subqueries have a tendency to be inefficient because they have to be carried out repeatedly.
For example, let’s say we need to generate a list of employees whose birthdays are in September. The outer query might be ask for their names and social security numbers, but the correlated subquery searches through all employees to find only those whose birthdays are in September.
How Can You Make a SQL Correlated?Subquery Efficient?
One way to solve the inefficiency problem is to use JOIN statements. JOIN statements combine rows from different tables that have a common element. It isn’t that a JOIN is faster than a where, but in some cases, we can eliminate repeated WHEREs by using a JOIN statement.
Here’s?our correlated subquery example that will hopefully clear up any confusion:
WHERE ID IN (SELECT ID
WHERE SALARY > 50000)
In this case, our subquery is trying to list only those employees with salaries in excess of 50,000. However, it may be more efficient to use JOIN statements so that we only need to execute the query once no matter how many employees there are. In the case of a large company, we could could use JOIN statements to find employees in each department who earn more than 50,000, so that we need only execute each statement once.
Replacing an Correlated Subquery in SQL with Join Statements
We can use JOIN statements to make?this kind of subquery more efficient. This works when we have several different tables we can draw data from rather than repeat the same correlated subquery.