Join Queries CS 146

Introduction: Join Queries  So far, our SELECT queries have retrieved data from a single table  Usually queries combine data from multiple tables:  List how much (pounds) of each product that was purchased today  List the customer name and product name for a specific purchase  Queries that retrieve data from multiple tables require joining the tables through primary key/foreign key relationships

Main Types of Join Queries  Inner Join  Retrieves all matching fields in joined tables  Also called equijoin or natural join  Outer Join  Retrieves all fields in one table, and matching fields in second table if they exist

Example Inner Join CANDY_CUSTOMER CANDY_PURCHASE CANDY_PRODUCT

Join Query Syntax (ANSI 1992)  The word "INNER" is optional SELECT Column1, Column2, … FROM Table1 INNER JOIN Table2 ON Table1.JoinColumn = Table2.JoinColumn WHERE SearchCondition(s) Join condition

Join Query Example (ANSI 1992)  Note:  Order of tables in FROM clause doesn’t matter  Order of tables in ON condition doesn’t matter

Join Query Syntax (ANSI 1986) SELECT Column1, Column2, … FROM Table1, Table2 WHERE Table1.JoinColumn = Table2.JoinColumn AND SearchCondition(s) Join condition

Join Query Example (ANSI 1986) Notice the join is specified in the WHERE clause, not in the FROM clause

Advantages of 1992 Syntax  Separates the join conditions and search conditions  Makes it impossible to omit a join condition  Required for this class

Qualifying Field Names  What if a join query retrieves a field that exists in both tables?

Qualifying Field Names  You qualify the field name in the SELECT clause  Preface the field name with the name of either table

 Shorthand way to write queries by abbreviating table names  Pros & cons? Table Aliases NOTE: Once you create a table alias, you have to use it everywhere…

Inner Join of 3 Tables  General syntax:  Note:  Placing each INNER JOIN and ON clause on a separate line makes the query easier to read and understand SELECT Column1, Column2, … FROM Table1 INNER JOIN Table2 ON Table1.JoinColumn = Table2.JoinColumn INNER JOIN Table3 ON Table2.JoinColumn = Table3.JoinColumn WHERE SearchCondition(s)

3 Table Inner Join Example

Joining N Tables  You can join any number of tables, provided primary key/foreign key relationships exist  Challenge:  Including all necessary tables in the query

 You can join any number of tables, provided primary key/foreign key relationships exist  Challenge: you need to include table in join queries to provide needed links even if you don't include fields in the SELECT clause… Joining N Tables

Example CANDY_PRODUCT prod_desc (D) prod_id (J) CANDY_CUSTOMER cust_name (S) cust_id (J) CANDY_PURCHASE prod_id (J) cust_id (J) SELECT prod_desc FROM candy_product INNER JOIN candy_purchase ON candy_product.prod_id = candy_purchase.prod_id INNER JOIN candy_customer ON candy_purchase.cust_id = candy_customer.cust_id WHERE cust_name = 'Bobby Bon Bons'

Designing Complex Join Queries  Terminology:  Display field:  Retrieved data field  Appears in the SELECT clause  Join field  Primary or foreign key used to join tables  Appears in a join condition  Search field  Used in a search condition  Appears in the WHERE clause  Join queries must include all tables that contain display, join, or search fields

Query Design Diagrams  Visual way to identify display, join, and search fields  Process: 1. Identify every table in the query 2. Identify every involved field in each table  Label whether it is a display, search, or join field 3. Create join condition links