It is possible to embed a SQL statement within another. When this is done on the WHERE or the HAVING statements, we have a subquery construct. What is subquery useful for? First, it can also be used to join tables. Also, there are cases where the only way to correlate two tables is through a subquery.
The syntax is as follows:
[Comparison Operator] could be equality operators such as =, >, <, >=, <=. It can also be a text operator such as "LIKE."
Let's use the same example as we did to illustrate SQL joins:
and we want to use a subquery to find the sales of all stores in the West region. To do so, we use the following SQL statement:
SELECT SUM(Sales) FROM Store_Information
In this example, instead of joining the two tables directly and then
adding up only the sales amount for stores in the West region, we first
use the subquery to find out which stores are in the West region, and
then we sum up the sales amount for these stores.
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