Podcast
Questions and Answers
What is a database schema?
What is a database schema?
The logical structure of the database.
What is meant by the term 'domain' in the context of attributes?
What is meant by the term 'domain' in the context of attributes?
The set of allowed values for each attribute.
The order of tuples in a relation is relevant.
The order of tuples in a relation is relevant.
False
What is a superkey?
What is a superkey?
Signup and view all the answers
Give an example of a candidate key for the instructor relation.
Give an example of a candidate key for the instructor relation.
Signup and view all the answers
Which of these are pure relational query languages? (Select all that apply)
Which of these are pure relational query languages? (Select all that apply)
Signup and view all the answers
Match the following operations with their symbols:
Match the following operations with their symbols:
Signup and view all the answers
What language was renamed to Structured Query Language (SQL)?
What language was renamed to Structured Query Language (SQL)?
Signup and view all the answers
Which of the following is NOT a version of SQL according to the ANSI and ISO standards?
Which of the following is NOT a version of SQL according to the ANSI and ISO standards?
Signup and view all the answers
What does DML stand for in SQL?
What does DML stand for in SQL?
Signup and view all the answers
What SQL command is used to define an SQL relation?
What SQL command is used to define an SQL relation?
Signup and view all the answers
The SQL data-definition language (DDL) allows the specification of information about ______.
The SQL data-definition language (DDL) allows the specification of information about ______.
Signup and view all the answers
Study Notes
Database Structure
- Relational Databases: Store data in tables with rows & columns
-
Attributes (Columns): Represent individual properties of data, like
ID
,name
,salary
- Tuples (Rows): Represent individual entities or records, like a specific instructor's details
- Domains: Define possible values for each attribute.
Database Schema and Instance
-
Database Schema: Defines the logical structure of a database, including its tables and attributes. Example:
instructor(ID, name, dept_name, salary)
- Database Instance: Represents a snapshot of actual data at a specific time, filling in the tables defined by the schema.
Keys
-
Superkey: A set of attributes whose values uniquely identify a tuple in any possible relation. Example:
{ID}
or{ID, name}
are superkeys for theinstructor
relation. -
Candidate Key: A minimal superkey, meaning no subset is also a superkey. Example:
{ID}
is a candidate key forinstructor
. - Primary Key: One of the candidate keys chosen to uniquely identify tuples in a table.
-
Foreign Key Constraints: Enforce relationships between tables. A value in one table (referencing relation) must correspond to a value in another table (referenced relation). Example:
dept_name
ininstructor
references thedepartment
table.
Schema Diagrams
- Diagrammatically represent the structure of a database, showing tables and their relationships. Often use symbols to depict relationships like foreign key constraints.
Relational Query Languages
-
Procedural vs Non-Procedural:
- Procedural languages specify step-by-step instructions.
- Non-procedural (declarative) languages describe what to retrieve, not how.
-
"Pure" Query Languages:
- Relational Algebra: A procedural query language with set operations and relational operators.
- Tuple Relational Calculus: A non-procedural language that uses logical formulas to define queries.
- Domain Relational Calculus: Similar to tuple calculus, but uses variables representing domain values.
- Equivalence: These pure languages are equivalent in expressive power, meaning they can express the same queries.
Relational Algebra
- Purpose: A procedural query language using operations to manipulate relations.
-
Basic Operations:
- Select (σ): Retrieves tuples meeting a specified condition.
- Project (Π): Extracts specific columns from a relation.
- Union (∪): Combines two relations, removing duplicates.
- Set Difference (-): Returns tuples in one relation, but not in another.
- Cartesian Product (x): Creates all possible combinations of tuples from two relations.
- Rename (ρ): Assigns a new name to a relation or attribute.
SQL Overview
- SQL (Structured Query Language) is a language for interacting with relational databases.
- It's used for querying data, inserting new data, deleting data, and modifying existing data.
- SQL has been standardized by ANSI and ISO.
- Commercial database systems support most SQL-92 features and offer proprietary extensions beyond standard SQL.
History of SQL
- SQL originated as "Sequel" at IBM's San Jose Research Laboratory.
- It was developed as part of the System R project.
- SQL has undergone multiple revisions as standards have evolved: SQL-86, SQL-89, SQL-92, SQL:1999, SQL:2003.
SQL Parts
- SQL has several components, including:
- Data Manipulation Language (DML): for querying, inserting, deleting, and updating data.
- Data Definition Language (DDL): for defining database schemas, constraints, and other database objects.
- Integrity: for specifying constraints to maintain database integrity.
- View Definition: for defining views (virtual tables).
- Transaction Control: for managing transactions.
- Embedded SQL and Dynamic SQL: for embedding SQL statements within other programming languages.
- Authorization: for managing access rights to database objects.
Data Definition Language (DDL)
- SQL DDL allows the specification of information about relations, including:
- Schema: Defines the structure of relations, including attribute names and data types.
- Data Types: Specifies the type of values each attribute can hold.
- Integrity Constraints: Rules that ensure data validity.
- Indexes: Structures used to optimize data retrieval.
- Security and Authorization: Controls access to database objects.
- Physical Storage Structure: Defines how data is physically stored on disk.
Domain Types in SQL
- char(n): Fixed-length character strings, with user-specified maximum length n.
- varchar(n): Variable-length character strings, with user-specified maximum length n.
- int: Integers (dependent on machine precision).
- smallint: A subset of integers, dependent on machine precision.
- numeric(p,d): Fixed-point numbers, with user-specified precision of p digits, with d digits to the right of the decimal point.
- real, double precision: Floating-point and double-precision floating-point numbers.
- float(n): Floating-point numbers, with user-specified precision of at least n digits.
Create Table Construct
- SQL relations are defined using the
create table
command:-
create table r (A1 D1, A2 D2, ..., An Dn, (integrity-constraint1), ..., (integrity-constraintk))
-
r
is the name of the relation. - Each
Ai
is an attribute name in the relation's schema. -
Di
is the data type of values in theAi
domain. - The command also allows specifying integrity constraints.
-
-
- Example:
-
create table instructor (...
-
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Description
Test your knowledge on relational databases, their structure, and key concepts such as attributes, tuples, and keys. This quiz will help you understand the differences between database schema and instance, as well as the types of keys like superkeys and candidate keys.