completed subqueries

Author: ptyadana

Complete SQL & Databases - ZTM/06_Advanced_SQL.sql

@@ -698,15 +698,176 @@ FROM city
 WHERE countrycode='BEL' OR countrycode='TUN' OR countrycode='NL';
 
 
+/* ----------------------------------------------------------------------------------------------------------- */
 
+/********************** 27) Subqueries ************************/
+/*
+	Subqueries can be used in SELECT, FROM, HAVING, WHERE.
+	
+	For HAVING and WHERE clause, subquery must return SINGLE value record.
+*/
 
+SELECT 
+	title, price, 
+	(SELECT AVG(price) FROM products) AS "global average price"
+FROM products;
 
 
+-- Subquery can returns A Single Result or Row Sets
+SELECT 
+	title, price, 
+	(SELECT AVG(price) FROM products) AS "global average price" -- return single result
+FROM (
+	SELECT * FROM products -- return row sets
+) AS "products_sub";
+
+/************ 29) Types of Subqueries *************/
+/*
+	Single Row
+	Multiple Row
+	Multiple Column
+	Correlated
+	Nested
+*/
+
+-- Single Row: returns Zero or One Row
+SELECT emp_no, salary
+FROM salaries
+WHERE salary > (
+	SELECT AVG(salary) FROM salaries
+);
 
+-- Multiple Row: returns One or More Rows
+SELECT title, price, category
+FROM products
+WHERE category IN (
+	SELECT category FROM categories
+	WHERE categoryname IN ('Comedy', 'Family', 'Classics')
+);
 
+-- Multiple Columns: returns ONE or More columns
+SELECT emp_no, salary, dea.avg AS "Department average salary"
+FROM salaries s
+JOIN dept_emp as de USING(emp_no)
+JOIN(
+	SELECT dept_no, AVG(salary) FROM salaries AS s2
+	JOIN dept_emp AS de2 USING(emp_no)
+	GROUP BY dept_no
+) AS dea USING (dept_no)
+WHERE salary > dea.avg;
+
+
+-- Correlated: Reference ONE or More columns in the OUTER statement - Runs against Each Row
+/* Get the most recent salary of employee */
+SELECT emp_no, salary AS "most recent salary", from_date
+FROM salaries AS s
+WHERE from_date = (
+	SELECT MAX(s2.from_date) AS max
+	FROM salaries AS s2
+	WHERE s2.emp_no = s.emp_no
+)
+ORDER BY emp_no;
 
 
+-- Nested : Subquery in Subquery
+SELECT orderlineid, prod_id, quantity
+FROM orderlines
+JOIN(
+	SELECT prod_id
+	FROM products
+	WHERE category IN(
+		SELECT category FROM categories
+		WHERE categoryname IN('Comedy', 'Family', 'Classics')
+	)
+) AS limited USING(prod_id);
 
+/*************** 30) Using Subqueries ************/
+SELECT 
+	first_name,
+	last_name,
+	birth_date,
+	AGE(birth_date)
+FROM employees
+WHERE AGE(birth_date) > (SELECT AVG(AGE(birth_date)) FROM employees);
+
+
+/* Show the salary with title of the employee using Subquery, instead of JOIN */
+SELECT emp_no, salary, from_date,
+	(SELECT title FROM titles AS t 
+	 WHERE t.emp_no=s.emp_no AND t.from_date=s.from_date)	
+FROM salaries s
+ORDER BY emp_no;
 
+EXPLAIN ANALYZE
+SELECT emp_no, salary AS "most recent salary", from_date
+FROM salaries AS s
+WHERE from_date = (
+	SELECT MAX(s2.from_date) AS max
+	FROM salaries AS s2
+	WHERE s2.emp_no = s.emp_no
+)
+ORDER BY emp_no;
 
 
+/********************** 32) Subqueries Operators *******************/
+/*
+		EXISTS : Check if the subquery returns any rows
+*/
+SELECT firstname, lastname, income
+FROM customers AS c
+WHERE EXISTS(
+	SELECT * FROM orders as o
+	WHERE c.customerid = o.customerid AND totalamount > 400
+) AND income > 90000
+
+/* 
+	IN : Check if the value is equal to any of the rows in the return (NULL yields NULL)
+	NOT IN : Check if the value is NOT equal to any of the rows in the return (NULL yields NULL)
+*/
+SELECT prod_id
+FROM products
+WHERE category IN(
+	SELECT category FROM categories
+	WHERE categoryname IN ('Comedy', 'Family', 'Classics')
+);
+
+
+SELECT prod_id
+FROM products
+WHERE category IN(
+	SELECT category FROM categories
+	WHERE categoryname NOT IN ('Comedy', 'Family', 'Classics')
+);
+
+/*
+		ANY / SOME : check each row against the operator and if any comparison matches, return TRUE.
+*/
+
+SELECT prod_id
+FROM products
+WHERE category = ANY(
+	SELECT category FROM categories
+	WHERE categoryname IN ('Comedy', 'Family', 'Classics')
+);
+
+/*
+			ALL : check each row against the operator and if all comparisions match, return true.
+*/
+SELECT prod_id, title, sales
+FROM products
+JOIN inventory as i USING(prod_id)
+WHERE i.sales > ALL(
+	SELECT AVG(sales) FROM inventory
+	JOIN products as p1 USING(prod_id)
+	GROUP BY p1.category
+);
+
+/*
+	Single Value Comparison
+*/
+SELECT prod_id
+FROM products
+WHERE category = (
+	SELECT category FROM categories
+	WHERE categoryname IN ('Comedy')
+);