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Questions and Answers
What is the primary purpose of domains in SQL?
What is the primary purpose of domains in SQL?
- To define table relationships
- To specify the datatype of attributes (correct)
- To manage query performance
- To outline user permissions
Which of the following SQL domain types is classified as elementary?
Which of the following SQL domain types is classified as elementary?
- User-defined domains
- Composite types
- Exact numeric domains (correct)
- Temporal intervals
Which SQL domain would be appropriate for storing a boolean value?
Which SQL domain would be appropriate for storing a boolean value?
- bit (correct)
- integer
- character varying
- decimal
What does the 'precision' in approximate numeric domains specify?
What does the 'precision' in approximate numeric domains specify?
Which of the following is NOT an elementary domain defined in SQL?
Which of the following is NOT an elementary domain defined in SQL?
Which SQL standard was the first one established in 1986?
Which SQL standard was the first one established in 1986?
Which of these options represents a temporal instant in SQL?
Which of these options represents a temporal instant in SQL?
What is the historical significance of 'SEQUEL' in the context of SQL?
What is the historical significance of 'SEQUEL' in the context of SQL?
What is a schema in SQL?
What is a schema in SQL?
What SQL statement is used to create a new table?
What SQL statement is used to create a new table?
What does the 'not null' constraint specify in a table definition?
What does the 'not null' constraint specify in a table definition?
When defining a domain in SQL, what is meant by 'elementary domain'?
When defining a domain in SQL, what is meant by 'elementary domain'?
What does the 'on delete set null' clause achieve in SQL table relationships?
What does the 'on delete set null' clause achieve in SQL table relationships?
Which statement correctly defines a default value in a table?
Which statement correctly defines a default value in a table?
In the given example of creating a table, what is the primary key?
In the given example of creating a table, what is the primary key?
What is the correct syntax to define a domain that defaults to null?
What is the correct syntax to define a domain that defaults to null?
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Study Notes
SQL
- SQL stands for Structured Query Language.
- It is a language that functions as both a data manipulation language (DML) and a data definition language (DDL).
- SQL was initially proposed as SEQUEL by IBM Research in 1974.
- The first implementation of SQL was SQL/DS by IBM in 1981.
- SQL has been standardized since 1983.
- The first standard was published in 1986, and then revised in 1989 (SQL-89).
- The second standard was published in 1992 (SQL-2 or SQL-92).
- The third standard was published in 1999 (SQL-3 or SQL-99).
- Most database systems support the base functionality of the SQL standard and offer their own proprietary extensions.
Domains
- Domains define the datatype of attributes.
- There are two categories of domains: elementary and user-defined.
Elementary Domains
- Elementary domains are predefined by the SQL standard.
- Character: This domain represents single characters or strings, and allows for variable-length strings.
- Bit: This domain represents single boolean values or strings of boolean values, allowing for variable-length strings.
- Exact Numeric Domains: Represents exact values, including integers or values with a fractional part.
- Approximate Numeric Domains: Represents approximate real values based on a floating point representation.
- Temporal Instants: These domains represent specific points in time.
- Temporal Intervals: These domains represent durations of time between two instants.
Schema Definition
- A schema is a collection of database objects, which can include domains, tables, indexes, assertions, views, and privileges.
- A schema has a unique name and is owned by a specific user (the authorization).
Table Definition
- An SQL table is composed of:
- An ordered set of attributes
- A set of constraints (which may be empty)
- The
create tablestatement is used to define a relation schema and create an empty table instance.
User Defined Domains
- User-defined domains are similar to the variable types defined in programming languages.
- A user-defined domain is defined by its name, elementary domain, default value, and a set of constraints.
- The
create domainstatement is used to define a user-defined domain.
Default Domain Values
- Default domain values define the value that an attribute should assume if a value is not explicitly provided during row insertion.
- The
defaultkeyword is used to set a default value, which can be a constant, an expression, ornull. - The
userkeyword represents the login of the user executing the command.
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