Joins CIS-182

Multi-Table Queries Multi-table queries allow data from different tables to be combined and compared Typically done by comparing a common value Comparison done between like data types May be primary key/foreign key fields but doesn’t have to be

Multi-table queries allow data from different tables to be combined and compared

Typically done by comparing a common value

Comparison done between like data types

May be primary key/foreign key fields but doesn’t have to be

Types of Multi-table Queries Joins : combine columns from multiple tables Combine publishers and titles to show title and publisher name Unions : combine rows from different lists Create a list of editors and authors who live in California (two different tables) Subqueries : use the results of one query to determine result of a second query Find all books that have a higher than average price

Joins : combine columns from multiple tables

Combine publishers and titles to show title and publisher name

Unions : combine rows from different lists

Create a list of editors and authors who live in California (two different tables)

Subqueries : use the results of one query to determine result of a second query

Find all books that have a higher than average price

Joins Inner Join: create a row for each match of values from two tables Outer Join: include all rows from one table and any related data found in a second table Cartesian Join: match each row in a table with every row in a second table Also referred to as a cross join

Inner Join: create a row for each match of values from two tables

Outer Join: include all rows from one table and any related data found in a second table

Cartesian Join: match each row in a table with every row in a second table

Also referred to as a cross join

Inner Joins Most common kind of join Get related data from two tables where values match May be done using FROM clause with JOIN or in WHERE clause Microsoft does not fully support WHERE clause in SQL Server 2008 Older database management systems relied on WHERE and don’t support JOIN

Most common kind of join

Get related data from two tables where values match

May be done using FROM clause with JOIN or in WHERE clause

Microsoft does not fully support WHERE clause in SQL Server 2008

Older database management systems relied on WHERE and don’t support JOIN

Inner Join With FROM Clause Use the keyword JOIN with or without INNER preceeding it Specify the fields to compare using ON Field names that are not unique must be qualified by the table they’re in

Use the keyword JOIN with or without INNER preceeding it

Specify the fields to compare using ON

Field names that are not unique must be qualified by the table they’re in

FROM Clause Examples SELECT * FROM Publishers JOIN Titles ON Publishers.pub_id = Titles.pub_id SELECT * FROM Publishers INNER JOIN Titles ON Publishers.pub_id = Titles.pub_id Optional Inner Join Keyword On Keyword

SELECT * FROM Publishers JOIN Titles ON Publishers.pub_id = Titles.pub_id

SELECT * FROM Publishers INNER JOIN Titles ON Publishers.pub_id = Titles.pub_id

Join With WHERE List tables in FROM clause, separated by comma WHERE clause defines the fields to compare

List tables in FROM clause, separated by comma

WHERE clause defines the fields to compare

WHERE Clause Example SELECT * FROM Publishers, Titles WHERE Publishers.pub_id = Titles.pub_id

SELECT *

FROM Publishers, Titles

WHERE Publishers.pub_id = Titles.pub_id

Using An Alias To shorten references to tables, can use an Alias Keyword AS is optional SELECT * FROM Publishers AS P JOIN Titles AS T ON P.pub_id = T.pub_id

To shorten references to tables, can use an Alias

Keyword AS is optional

SELECT *

FROM Publishers AS P JOIN Titles AS T ON P.pub_id = T.pub_id

Predicting Results With no criteria specified, the table on the many side will determine the number of rows returned An entry on the one-side can show up over and over – once for each related entry in the many table If return publisher name and title, the publisher name will repeat once for each title associated with that publisher

With no criteria specified, the table on the many side will determine the number of rows returned

An entry on the one-side can show up over and over – once for each related entry in the many table

If return publisher name and title, the publisher name will repeat once for each title associated with that publisher

More than Two Tables When have more than two tables in query, tables combined in order found unless use parentheses Parentheses control order of execution if found SELECT * FROM publishers p JOIN titles t ON p.pub_id = t.pub_id JOIN titleauthors ta ON t.title_id = ta.title_id 1 st Combine Publishers and Titles 2 nd Combine Publishers/Titles With TitleAuthors

When have more than two tables in query, tables combined in order found unless use parentheses

Parentheses control order of execution if found

SELECT *

FROM publishers p JOIN titles t ON p.pub_id = t.pub_id JOIN titleauthors ta ON t.title_id = ta.title_id

Outer Joins Used when rows from one table should be part of the result even if there are no related rows in a second table Create a list of all publishers, including any titles the publisher has published (include a publisher where there ISN’T an entry in titles) Direction must be specified Left/Right specify which table has the rows which should always be included Full specifies that rows from both tables should be included even if no match between rows

Used when rows from one table should be part of the result even if there are no related rows in a second table

Create a list of all publishers, including any titles the publisher has published (include a publisher where there ISN’T an entry in titles)

Direction must be specified

Left/Right specify which table has the rows which should always be included

Full specifies that rows from both tables should be included even if no match between rows

Outer Join Example Include all publishers, even if they haven’t published a book LEFT refers to the table on the left side of the JOIN keyword and equal sign SELECT * FROM Publishers LEFT OUTER JOIN Titles ON Publishers.pub_id = Titles.pub_id

Include all publishers, even if they haven’t published a book

LEFT refers to the table on the left side of the JOIN keyword and equal sign

SELECT *

FROM Publishers LEFT OUTER JOIN

Titles ON Publishers.pub_id =

Titles.pub_id

Cartesian Join Typically used to generate lots of data quickly Match each row from table 1 with every row from table 2 Result is (table 1 row count)*(table 2 row count)

Typically used to generate lots of data quickly

Match each row from table 1 with every row from table 2

Result is (table 1 row count)*(table 2 row count)

Cartesian Join Examples Using table list: SELECT * FROM Publishers, Titles Using CROSS JOIN keywords: SELECT * FROM Publishers CROSS JOIN Titles

Using table list:

SELECT *

FROM Publishers, Titles

Using CROSS JOIN keywords:

SELECT *

FROM Publishers CROSS JOIN Titles