SQL Fundamentals

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Questions and Answers

What is a primary key defined through?

Relationship

Constraint (correct)

Attribute

Entity

What are foreign keys also defined as?

Constraint (correct)

Attribute

Entity

Relationship

What happens to entities in a database?

They become tables (correct)

They become attributes

They become constraints

They become relationships

What is required of primary key attributes?

They must be not null and unique Signup and view all the answers

How are many-to-many relationships represented in a database?

As two many-to-one relationships Signup and view all the answers

What SQL command is used to delete a table?

DROP TABLE Signup and view all the answers

What is used to define a foreign key?

The columns which make up the FK, the referenced table, and the columns which are referenced by the FK Signup and view all the answers

What happens to attributes in a database?

They become columns with the same name Signup and view all the answers

What is the purpose of the DROP TABLE command?

To remove a table from the database Signup and view all the answers

Which command is used to add a new column to a table?

ALTER TABLE r ADD A D Signup and view all the answers

What is the purpose of the UPDATE command?

To change row(s) in a table Signup and view all the answers

What is the use of the WHERE clause in UPDATE and DELETE commands?

To specify which rows to change or remove Signup and view all the answers

What is the command to add a constraint to a table?

ALTER TABLE r ADD CONSTRAINT Signup and view all the answers

What is the command to delete all tuples from a table?

DELETE FROM r Signup and view all the answers

What was SQL originally called?

Sequel Signup and view all the answers

Which of the following SQL data types is a fixed-length character string?

char(n) Signup and view all the answers

What is the purpose of the create table command in SQL?

To define an SQL relation Signup and view all the answers

What is the primary purpose of integrity constraints in SQL?

To restrict the set of allowed values for an attribute Signup and view all the answers

What is the name of the standard that most modern DBMS use in SQL?

SQL-92 Signup and view all the answers

What is the data type of the attribute 'budget' in the 'department' table?

numeric(12,2) Signup and view all the answers

What is the purpose of the 'primary key' constraint in SQL?

To specify a unique identifier for a table Signup and view all the answers

In which year was SQL-86 standard introduced?

1986 Signup and view all the answers

Study Notes

Overview to SQL

Originally called 'Sequel', Structured English Query Language
Designed and implemented at IBM, with ANSI Standards: SQL-86 or SQL1, SQL-92 or SQL2, SQL-99 or SQL3
Most modern DBMS use a variety of SQL, mostly based on SQL2, increasingly SQL3
Can be used from other languages and often extended to provide common programming constructs (if-then tests, loops, variables, etc.)

SQL Data Types

char(n): fixed-length character string
varchar(n): variable-length character string with specific maximum length n
int: integer
smallint: small integer
numeric(p, d): fixed-point number with user-specified precision
float(n): floating-point number, with precision of at least n digits
real, double precision

SQL Basic Schema Definition

Defines an SQL relation using the create table command
General form: create table r (A1 D1, A2 D2, ..., An Dn, ...);
r is the name of the relation, Ai is the name of an attribute, Di specifies the type of attribute Ai

Integrity Constraints

SQL prevents updates that violate an integrity constraint
Supports various integrity constraints, including:
- Primary key: primary key attributes are required to be not null and unique
- Foreign key: defined through constraints, specifying columns, referenced table, and referenced columns

Implementing E/R Designs

Entities and attributes: each entity becomes a table, with attributes as columns
Primary keys are defined through constraints
Relationships:
- 1:1: usually not used or treated as a special case of M:1
- M:1: represented as a foreign key from the M-side to the 1
- M:M: split into two M:1 relationships

Example

Creating tables for department, course, and instructor entities
Defining primary keys and foreign keys

More SQL

DROP TABLE: deletes a table
ALTER TABLE: changes a table
- Adds or removes a column
- Adds or removes a constraint
INSERT, UPDATE, and DELETE: add, change, or remove rows from a table
- Use WHERE clauses to specify which rows to change or remove
- Be careful with these, as incorrect WHERE clauses can destroy data

Deleting Tables

DROP TABLE removes a table
DELETE FROM removes all tuples from a table

Changing Tables

ALTER TABLE:
- Adds a new column: alter table r add A D;
- Removes an existing column: alter table r drop A;
- Adds a new constraint: alter table r add constraint;
- Removes an existing constraint: alter table r drop constraint;

INSERT and UPDATE

INSERT INTO: adds a row to a table
UPDATE: changes row(s) in a table
Both use WHERE clauses to specify which rows to change or add

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Description

This quiz covers the basics of SQL, including data types, schema definition, integrity constraints, and E-R designs. It also explores the history of SQL and its development.