*Conditions Involving Tuples*

On October 24, 2014, In The Database Language SQL by Admin

Views (944)

A tuple in SQL is represented by a parenthesized list of scalar values. Examples are (123, 'foo') and (name, address, networth). The first of these has constants as components; the second has attributes as components. Mixing of constants and attributes is permitted.

If a tuple t has the same number of components as a relation R, then it makes sense to compare t and R in expressions of the type listed in "Subqueries" under "Conditions Involving Relations". Examples are t IN R or t <> ANY R. The latter comparison means that there is some tuple in R other than t. Note that when comparing a tuple with members of a relation R, we must compare components using the assumed standard order for the attributes of R.

**Example 1 :**In Figure 1 is an SQL query on the three relations

We should analyze any query with subqueries from the inside out. Thus, let us start with the innermost nested subquery: lines (7) through (9). This query examines the tuples of the relation StarsIn and finds all those tuples whose starName component is 'Harrison Ford'. The titles and years of those movies are returned by this subquery. Recall that title and year, not title alone, is the key for movies, so we need to produce tuples with both attributes to identify a movie uniquely. Thus, we would expect the value produced by lines (7) through (9) to look something like Figure 2.

Now, look at the middle subquery, lines (4) through (6). It searches the Movie relation for tuples whose title and year are in the relation suggested by Figure 2. For each tuple found, the producer's certificate number is returned, so the result of the middle subquery is the set of certificates of the producers of Harrison Ford's movies.

Finally, look at the "main" query of lines (1) through (3). It examines the tuples of the MovieExec relation to find those whose cert# component is one of the certificates in the set returned by the middle subquery. For each of these tuples, the name of the producer is returned, giving us the set of producers of Harrison Ford's movies, as desired.

Incidentally, the nested query of Figure 1 can, like many nested queries, be written as a single select-from-where expression with relations in the FROM clause for each of the relations mentioned in the main query or a subquery. The IN relationships are replaced by equalities in the WHERE clause. For example, the query of Figure 3 is essentially that of Figure 1. There is a difference regarding the way duplicate occurrences of a producer - e.g., George Lucas - are handled.

### Tags

- tuple
- attributes
- nested query
- subquery
- sql
- scalar value
- Queries in PSM
- Using Shared Variables
- System Aspects of SQL
- Instead-Of Triggers
- Schema-Level Constraints and Triggers
- Tuple-Based CHECK Constraints
- Constraints on Attributes and Tuples
- Deferring the Checking of Constraints
- Maintaining Referential Integrity
- Declaring Foreign-Key Constraints
- Keys Declared With UNIQUE
- Constraints and Triggers
- 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
- Correlated Subqueries
- Subqueries
- Union, Intersection, and Difference of Queries
- Interpreting Multirelation Queries
- Tuple Variables
- Disambiguating Attributes
- Queries Involving More Than One Relation
- 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
- Extended Operators of Relational Algebra
- 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
- Natural Joins / Theta-Joins
- Selection / Cartesian Product
- Set Operations on Relations
- An Algebra of Relational Operations
- Relational Algebra
- Attribute Lists
- Semistructured Data Representation
- Object-Oriented Versus Object-Relational
- References
- Nested Relations
- What If There Is No Key
- Representing ODL Relationships
- Representing Other Type Constructors
- Representing Set-Valued Attributes
- Nonatomic Attributes in Classes
- From ODL Designs to Relational Designs
- 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
- 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
- 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
- Attributes on Relationships
- Instances of an E/R Diagram
- Elements of the E/R Model
- Database Programming
- Relational Database Systems