SUB QUERIES

A SUB QUERY is a query within a query which can return one or more rows

SELECT * FROM SALES
WHERE CUSTOMER_ID =
(SELECT CUSTOMER_ID FROM CUSTOMER WHERE LAST_NAME = 'JOSEPH'

A SUBQUERY executes first before the main query, SO the customer id was returned from the the subquery then passed to the original query

When the subquery returns more than one value use the IN instead of the =

SELECT * FROM SALES
WHERE CUSTOMER_ID IN  
(SELECT CUSTOMER_ID FROM CUSTOMER WHERE REGION ='SOUTH')

SELECT * FROM SALES
WHERE CUSTOMER_ID IN
(SELECT CUSTOMER_ID FROM CUSTOMER WHERE LAST_NAME = 'JOSEPH' OR last_name = 'mann'

SUBQUERY ON MULTIPLE COLUMNS

SELECT sales_date, order_id, customer_id, product_id, unit_price
FROM sales
WHERE (product_id, unit_price) IN
(
SELECT product_id, unit_price
FROM sales
WHERE sales_date = '01-jan-2015'
)

USING subqueries in the FROM clause

Used mostly when we join detailed data with aggregated data

SELECT 
s.sales_date,
s.order_id, 
s.customer_id, 
s.salesperson_id, 
s.total_amount, 
st.sales_total
FROM sales s, 
(SELECT sales_date, sum(total_amount) AS sales_total 
FROM sales
GROUP BY sales_date) st
WHERE s.sales_date = st.sales_date;e

Problem: Select data from sales table where the selected TOTAL_AMOUNT are greater than the average TOTAL_AMOUNT of their respective customer

SELECT *
FROM SALES  a 
WHERE TOTAL_AMOUNT >(SELECT AVG(TOTAL_AMOUNT)
 FROM SALES b
 WHERE b.customer_id = a.customer_id)
ORDER BY CUSTOMER_ID;

SUBQUERY is executed for each row on the sales table

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