Here is a complex sub-query on how to write it using two tables explained. One is the customer table, and the other one is the order table.

Sub-queries are two types. Single row return and Multiple-row return. Below is the multiple-row return example that explains in detail.

Sub-query Question

A subquery should get everything from the Customer_Table if the customer has an order in the Order_Table of greater than $400.

Customer_Table

CUstomer_number	Customer_name
123	Rao
567	Kyte
897	Robo
901	Tup
991	Vek

Order_table

Order_number	Customer_number	Order_total
1001	123	100.76
1002	567	200
1003	897	310
1004	901	450
1005	991	566

Here is Sub-query

select *
from customer_table
where customer_number in
(select customer_number
from order_table
where order_total > 400);

How it works

The sub-query is inside the parenthesis. First, it gets the customer_number from the order_table (the order_total greater than 400).

The outer query gets all the details of customer_table for all the matching customer numbers.

Summary

I chose to use sub-query, which ultimately reduced complexity, and we got the desired result.

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