*Queries Involving More Than One Relation*

On July 15, 2014, In The Database Language SQL by Admin

Views (1491)

Much of the power of relational algebra comes from its ability to combine two or more relations through joins, products, unions, intersections, and differences. We get all of these operations in SQL. The set-theoretic operations - union, intersection, and difference - appear directly in SQL, as we shall learn in Union, Intersection, and Difference of Queries. First, we shall learn how the select-from-where statement of SQL allows us to perform products and joins.

### Products and Joins in SQL

SQL has a simple way to couple relations in one query: list each relation in the FROM clause. Then, the SELECT and WHERE clauses can refer to the attributes of any of the relations in the FROM clause.Example 1 : Assume we want to know the name of the producer of Star Wars. To answer this question we need the following two relations from our running example:

Movie(title, year, length, inColor, studioName, producerC#)

MovieExec(name, address, cert#, netWorth)

The producer certificate number is given in the Movie relation, so we can do a simple query on Movie to get this number. We could then do a second query on the relation MovieExec to find the name of the person with that certificate number.

However, we can phrase both these steps as one query about the pair of relations Movie and MovieExec as follows:

SELECT name

FROM Movie, MovieExec

WHERE title = Star Wars AND producerC# = cert#;

This query asks us to consider all pairs of tuples, one from Movie and the other from MovieExec. The conditions on this pair are stated in the WHERE clause:

1. The title component of the tuple from Movie must have value 'Star Wars'.

2. The producerC# attribute of the Movie tuple must be the same certificate number as the cert# attribute in the MovieExec tuple. That is, these two tuples must refer to the same producer.

Whenever we find a pair of tuples satisfying both conditions, we produce the name attribute of the tuple from MovieExec as part of the answer. If the data is what we expect, the only time both conditions will be met is when the tuple from Movie is for Star Wars, and the tuple from MovieExec is for George Lucas. Then and only then will the title be correct and the certificate numbers agree. Thus, George Lucas should be the only value produced. This process is suggested in Figure 1. We take up in more detail how to interpret multirelation queries in Interpreting Multirelation Queries.

### Tags

- relational algebra
- sql
- attributes
- tuples
- Queries in PSM
- Scrolling Cursors
- Protecting Against Concurrent Updates
- Modifications by Cursor
- Cursors
- Instead-Of Triggers
- Triggers in SQL
- Schema-Level Constraints and Triggers
- Tuple-Based CHECK Constraints
- Constraints on Attributes and Tuples
- Declaring Foreign-Key Constraints
- Keys Declared With UNIQUE
- Constraints and Triggers
- Interpreting Queries Involving Views
- Modifying Views
- View Definitions
- Introduction to Selection of Indexes
- Default Values / Indexes
- Simple Table Declarations
- Defining a Relation Schema in SQL
- Deletion / Updates
- Database Modifications
- Grouping / HAVING Clauses
- Full-Relation Operations
- Natural Joins / Outerjoins
- Subqueries in FROM Clauses
- Conditions Involving Tuples
- Subqueries
- Union, Intersection, and Difference of Queries
- Interpreting Multirelation Queries
- Tuple Variables
- Disambiguating Attributes
- The Truth-Value UNKNOWN
- Null Values and Comparisons Involving NULL
- Dates and Times
- Comparison of Strings
- Selection in SQL
- Projection in SQL
- The Database Language SQL
- Additional Constraint Examples
- Referential Integrity Constraints
- Constraints on Relations
- Extending the Projection Operator
- Grouping
- Extended Operators of Relational Algebra
- Selection on Bags / Product of Bags / Joins of Bags
- Union, Intersection, and Difference of Bags
- Relational Operations on Bags
- A Linear Notation for Algebraic Expressions
- Dependent and Independent Operations
- Renaming
- Combining Operations to Form Queries
- Selection / Cartesian Product
- Set Operations on Relations
- An Algebra of Relational Operations
- Relational Algebra
- Attribute Lists
- Information Integration Via Semistructured Data
- Semistructured Data Representation
- Object-Oriented Versus Object-Relational
- Nested Relations
- The Object-Relational Model
- What If There Is No Key
- Representing ODL Relationships
- Representing Other Type Constructors
- Representing Set-Valued Attributes
- Nonatomic Attributes in Classes
- Declaring Keys in ODL
- Subclasses in ODL / Multiple Inheritance in ODL
- Types in ODL
- Methods in ODL
- Multiplicity of Relationships
- Relationships in ODL / Inverse Relationships
- Attributes in ODL
- Introduction to ODL
- The Type System
- Other Data Models
- Decomposition into Fourth Normal Form
- Reasoning About Multivalued Dependencies
- Definition of Multivalued Dependencies
- Multivalued Dependencies
- Third Normal Form
- Boyce-Codd Normal Form
- Decomposing Relations
- Projecting Functional Dependencies
- Closing Sets of Functional Dependencies
- The Transitive Rule
- Why the Closure Algorithm Works
- Computing the Closure of Attributes
- Trivial Functional Dependencies
- The Splitting/Combining Rule
- Rules About Functional Dependencies
- Keys of Relations
- Using Null Values to Combine Relations - Comparison of Approaches
- An Object-Oriented Approach
- Converting Subclass Structures to Relations
- Handling Weak Entity Sets
- Combining Relations
- From E/R Relationships to Relations
- From Entity Sets to Relations
- From E/R Diagrams to Relational Designs
- Relation Instances
- Equivalent Representations of a Relation
- Tuples / Domains
- Attributes / Schemas
- The Relational Data Model
- Summary of The Entity-Relationship Data Model
- Weak Entity Set Notation
- Requirements for Weak Entity Sets
- Representing Keys in the E/R Model
- Keys in the E/R Model
- The Modeling of Constraints
- Picking the Right Kind of Element
- Design Principles
- Subclasses in the E/R Model
- Converting Multiway Relationships to Binary
- Attributes on Relationships
- Multiway Relationships
- Elements of the E/R Model
- Database System Implementation
- Database Programming
- Database Design
- The Query Processor
- Multimedia Data
- Relational Database Systems