# Subquery

Subqueries

Sub-query is actually a one-time view is not stored in the hard disk. In the use of it as a general view can be operated.

Scalar query

Different from ordinary sub-queries return only a single data (line a), the only difference is, in fact, return scalar. That returns a scalar sub-query is called scalar query, can be used anywhere you can use a scalar.

For example, aggregate functions can not be used in the WHERE clause:

1. error

SELECT product_id, product_name, sale_price
FROM Product
WHERE sale_price > AVG(sale_price);

1. correct

SELECT product_id, product_name, sale_price
FROM Product
WHERE sale_price > (SELECT AVG(sale_price) FROM Product);

Correlated subquery

In short, there is no clear, so just to stick two pieces of code in order to understand its usage

1. Select higher than average in the same commodity in commodity

SELECT product_type, product_name, sale_price
FROM Product AS P1
WHERE sale_price > (
    SELECT AVG(sale_price) FROM Product AS P2
    WHERE P1.product_type = P2.product_type
    GROUP BY product_type
);

1. Select the category of goods and where the average sale price

SELECT product_id,product_name,product_type,sale_price,
(SELECT AVG(sale_price) FROM Product AS P2 WHERE P1.product_type = P2.product_type GROUP BY product_type)
FROM Product AS P1;

Also note that this alias is also a scope. Variable Scope can refer to general programming language.

Also to be noted that some special SQL, such as Oracle can not be used ASto define an alias.