Advanced SQL Advanced Database Dr. AlaaEddin Almabhouh

Slide 2 (of 30) Topic & Structure of Lesson  Using Aliases for Table Names  Using Joins to Combining Data from Multiple Tables  Combining Multiple Result Sets

SQL Aliases  SQL aliases are used to temporarily rename a table or a column heading.  Basically aliases are created to make column names more readable.  SQL Alias Syntax for Columns SELECT column_name AS alias_name FROM table_name  SQL Alias Syntax for Tables SELECT column_name(s) FROM table_name AS alias_name

Using Aliases for Table Names  Example 1 (without an alias name)  Example 2 (with an alias name) USE joindb SELECT buyer_name, s.buyer_id, qty FROM buyers AS b INNER JOIN sales AS s ON b.buyer_id = s.buyer_id GO USE joindb SELECT buyer_name, s.buyer_id, qty FROM buyers AS b INNER JOIN sales AS s ON b.buyer_id = s.buyer_id GO USE joindb SELECT buyer_name, sales.buyer_id, qty FROM buyers INNER JOIN sales ON buyers.buyer_id = sales.buyer_id GO USE joindb SELECT buyer_name, sales.buyer_id, qty FROM buyers INNER JOIN sales ON buyers.buyer_id = sales.buyer_id GO

SQL JOINs  An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.  Types of SQL JOINs:  INNER JOIN  LEFT JOIN  RIGHT JOIN  FULL JOIN  CROSS JOIN

Introduction to Joins  Selects Specific Columns from Multiple Tables  JOIN keyword specifies that tables are joined and how to join them  ON keyword specifies join condition  Queries Two or More Tables to Produce a Result Set  Use primary and foreign keys as join conditions  Use columns common to specified tables to join tables

SQL INNER JOIN Keyword  The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables.  SQL INNER JOIN Syntax SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name=table2.column_name

USE joindb SELECT buyer_name, sales.buyer_id, qty FROM buyers INNER JOIN sales ON buyers.buyer_id = sales.buyer_id GO USE joindb SELECT buyer_name, sales.buyer_id, qty FROM buyers INNER JOIN sales ON buyers.buyer_id = sales.buyer_id GO salesbuyer_idbuyer_idprod_idprod_idqtyqty buyersbuyer_namebuyer_name Adam Barr Sean Chai Eva Corets Erin O’Melia buyer_idbuyer_id Resultbuyer_namebuyer_name Adam Barr Erin O’Melia Eva Corets buyer_idbuyer_idqtyqty Erin O’Melia Example 1 Using Inner Joins

SQL LEFT JOIN Keyword  The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match.  SQL LEFT JOIN Syntax SELECT column_name(s) FROM table1 LEFT OUTER JOIN table2 ON table1.column_name=table2.column_name

USE joindb SELECT buyer_name, sales.buyer_id, qty FROM buyers LEFT OUTER JOIN sales ON buyers.buyer_id = sales.buyer_id GO USE joindb SELECT buyer_name, sales.buyer_id, qty FROM buyers LEFT OUTER JOIN sales ON buyers.buyer_id = sales.buyer_id GO salesbuyer_idbuyer_idprod_idprod_idqtyqty buyersbuyer_namebuyer_name Adam Barr Sean Chai Eva Corets Erin O’Melia buyer_idbuyer_id Result buyer_namebuyer_name Adam Barr Erin O’Melia Eva Corets buyer_idbuyer_idqtyqty Erin O’Melia Sean Chai NULL Example 1 Using Left Outer Joins

SQL RIGHT JOIN Keyword  The RIGHT JOIN keyword returns all rows from the right table (table2), with the matching rows in the left table (table1). The result is NULL in the left side when there is no match.  SQL RIGHT JOIN Syntax SELECT column_name(s) FROM table1 RIGHT OUTER JOIN table2 ON table1.column_name=table2.column_name

SQL FULL OUTER JOIN Keyword  The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).  The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins.  SQL FULL OUTER JOIN Syntax SELECT column_name(s) FROM table1 FULL OUTER JOIN table2 ON table1.column_name=table2.column_name

SQL CROSS JOIN Keyword  The SQL CROSS JOIN produces a result set which is the number of rows in the first table multiplied by the number of rows in the second table,  If no WHERE clause is used along with CROSS JOIN. This kind of result is called as Cartesian Product.  SQL FULL OUTER JOIN Syntax SELECT column_name(s) FROM table1 CROSS JOIN table2

USE joindb SELECT buyer_name, qty FROM buyers CROSS JOIN sales GO USE joindb SELECT buyer_name, qty FROM buyers CROSS JOIN sales GO Result buyer_namebuyer_name Adam Barr qtyqty Adam Barr 1003 Sean Chai 15 Sean Chai Sean Chai 11 Sean Chai 1003 Eva Corets sales buyer_idbuyer_idprod_idprod_idqtyqty buyers buyer_idbuyer_id buyer_namebuyer_name Adam Barr Sean Chai Eva Corets Erin O’Melia Example 1 Using Cross Joins

Joining More Than Two Tables SELECT buyer_name, prod_name, qty FROM buyers INNER JOIN sales ON buyers.buyer_id = sales.buyer_id INNER JOIN produce ON sales.prod_id = produce.prod_id GO SELECT buyer_name, prod_name, qty FROM buyers INNER JOIN sales ON buyers.buyer_id = sales.buyer_id INNER JOIN produce ON sales.prod_id = produce.prod_id GO produceprod_idprod_idprod_nameprod_name Apples Pears Oranges Bananas 5 5 Peaches buyersbuyer_idbuyer_id buyer_namebuyer_name Adam Barr Sean Chai Eva Corets Erin O’Melia salesbuyer_idbuyer_id prod_idprod_id qtyqty Result buyer_namebuyer_name Erin O’Melia Adam Barr Erin O’Melia Adam Barr Eva Corets prod_nameprod_name Apples Pears Oranges Peaches qtyqty Example 1

Joining a Table to Itself SELECT a.buyer_id AS buyer1, a.prod_id,b.buyer_id AS buyer2 FROM sales AS a JOIN sales AS b ON a.prod_id = b.prod_id WHERE a.buyer_id > b.buyer_id GO SELECT a.buyer_id AS buyer1, a.prod_id,b.buyer_id AS buyer2 FROM sales AS a JOIN sales AS b ON a.prod_id = b.prod_id WHERE a.buyer_id > b.buyer_id GO sales b buyer_idbuyer_idprod_idprod_idqtyqty sales a buyer_idbuyer_idprod_idprod_idqtyqty Result buyer1buyer1 4 4 prod_idprod_idbuyer2buyer Example 3

Combining Multiple Result Sets  The SQL UNION operator combines the result of two or more SELECT statements.  SQL UNION Syntax SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2

Combining Multiple Result Sets  Each Query Must Have:  Similar data types  Same number of columns  Same column order in select list Example: SELECT (firstname + ' ' + lastname) AS name, city, postalcode FROM employees UNION SELECT companyname, city, postalcode FROM customers SELECT (firstname + ' ' + lastname) AS name, city, postalcode FROM employees UNION SELECT companyname, city, postalcode FROM customers

Slide 13 (of 30) JOINS: Some notes  Inner is default and is usually omitted  Outer is also often omitted but should be included as good practice

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