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What is Correlated Subquery in SQL ?

A correlated subquery is a Subquery that uses the values of the outer query and it is evaluated for each row processed by the outer query .

A correlated subquery always depends on outer query for its value. Because of this dependency, a correlated subquery cannot be executed independently like as simple subquery .

Lets look at an example of correalted subquery.

Here, we have two tables – Orders and orders_details as given below.

Select OrderId, OrderDate, customerName, Orderqty from orders

Select OrderId, UnitPrice from Orders_details 


Following statement uses correlated subquery to returns the details of the orders table-  orders_details whose OrderQty price is equal to max unitprice in a table – orders 

select OrderId ,OrderDate ,CustomerName ,Orderqty from orders odr
where Orderqty in (select max(UnitPrice) as qty from orders_details od
where Odr.OrderId =od.OrderId
Group by orderId



Subquery in SQL

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