the footnote confused me and cause those questions to chatgpt which I am not sure are even right.
The standard calls them table references, but that term is hardly very apt. In most languages, a variable reference is a special
case of an expression; syntactically, it’s just a variable name, used to denote the value of the variable in question. But an SQL
“table reference” isn’t a special case of a table expression—not in the sense in which the latter term is used in this book, and
(perhaps more to the point) not in the sense in which it’s used in SQL, either.
explain
ChatGPT said:
ChatGPT
In the passage, the author is critiquing the terminology used in SQL and contrasting it with concepts from other languages and the book's own framework. Here’s a breakdown of the explanation:
Standard Terminology:
SQL uses the term “table reference” to describe how tables or subqueries are named and used in queries. For example, in SQL, you might see something like FROM table_name or FROM subquery AS alias.
Variable References in Other Languages:
In many programming languages, a variable reference is a simple case of an expression. A variable reference is essentially just the variable name used to retrieve its value. For instance, if x is a variable, referencing x in an expression like x + 1 is straightforward.
Table Reference vs. Table Expression:
The author argues that calling something a “table reference” in SQL is not entirely fitting because it does not align well with the concept of a “table expression”. In the context of the book and SQL, a table expression refers to more complex constructs that represent tables or sets of data in a broader sense. This can include:
Subqueries: Which can generate a result set to be used in a query.
Derived Tables: Temporary tables created within a query.
Joins and Unions: Which combine or manipulate sets of data.
The term “table expression” implies a more abstract and complex structure than a simple reference. It suggests a way of representing and working with tables that goes beyond just naming them.
Inappropriateness of the Term:
The author suggests that “table reference” is not a good fit for describing these more complex constructs because it is too narrow and doesn’t capture the broader, more flexible nature of “table expressions.” In SQL, and in the book’s framework, “table expressions” are more comprehensive and include operations and constructs that involve complex manipulations of data.
In summary, the author feels that the term “table reference” in SQL does not adequately describe the more complex and abstract nature of table expressions as understood in the book and in SQL. The term “table expression” would be a better fit for representing these more intricate constructs.
You said:
But an SQL
“table reference” isn’t a special case of a table expression—not in the sense in which the latter term is used in this book, and
(perhaps more to the point) not in the sense in which it’s used in SQL, either.
why in sql a table reference is not a special case of a table expression?
ChatGPT said:
ChatGPT
In SQL, the term "table reference" is used to denote the straightforward identification and usage of tables or subqueries within a query. On the other hand, "table expression" refers to a broader concept that encompasses a variety of ways to represent and manipulate sets of data in queries.
Here's why a "table reference" is not considered a special case of a "table expression" in SQL:
Definition and Scope:
Table Reference: In SQL, this term usually refers to a simple mention of a table or a subquery. For example, FROM table_name or FROM (SELECT * FROM table) AS alias are straightforward references. These references do not inherently involve complex operations or transformations.
Table Expression: This term, especially as used in the book and in a broader context, includes a wider range of constructs that involve more complex operations on data. This includes derived tables, subqueries, common table expressions (CTEs), and more. Essentially, it represents a richer set of operations and manipulations.
Complexity:
Table Reference: It is a direct mention of a table or subquery without any inherent complexity beyond identifying the source of data.
Table Expression: It involves a more abstract and flexible way of representing and working with data. It can include:
Subqueries: Which may act as temporary tables.
Derived Tables: Tables created on-the-fly within a query.
Common Table Expressions (CTEs): Named temporary result sets.
Use in SQL:
Table Reference: It’s a specific, often simplistic way to indicate a data source.
Table Expression: It captures a broader, more dynamic concept in SQL. For example, a CTE or a complex subquery might represent a table expression, but a simple table reference does not have the same level of abstraction or flexibility.
Semantic Difference:
Table Reference does not involve the additional semantics or transformations that are characteristic of table expressions. Table expressions can include operations like joins, filters, and aggregations that are beyond mere referencing.
In summary, "table reference" in SQL is not a special case of "table expression" because it represents a simpler and more direct concept, whereas "table expression" encompasses a broader and more complex range of data representations and manipulations.
You said:
so is the author trying to say that it is not a reference since an expression cannot be used in its place?
ChatGPT said:
ChatGPT
Yes, that's essentially what the author is getting at. The author is highlighting that in SQL, a "table reference" is not a special case of a "table expression" because the two concepts are fundamentally different in their roles and functionalities.
Here’s a more detailed explanation:
Table Reference:
Definition: A table reference in SQL is a direct mention of a table or a subquery within a query. It is used to specify the source of data but does not inherently involve complex operations or manipulations.
Usage: Examples include FROM table_name or FROM (SELECT * FROM table) AS alias. It serves as a straightforward reference to the data source.
Table Expression:
Definition: A table expression is a more abstract concept that includes various ways of representing and manipulating sets of data. It encompasses constructs like subqueries, derived tables, common table expressions (CTEs), and more.
Usage: These constructs can perform complex operations and transformations on data. For example, a CTE or a derived table can include complex joins, filters, and aggregations, representing a richer set of data manipulations.
Key Point:
Non-Special Case: The term "table reference" is not considered a special case of "table expression" because a simple table reference (like FROM table_name) does not involve the same level of complexity or abstraction as a table expression. Table expressions can represent more complex data manipulations that a simple table reference cannot encapsulate.
Implication:
Inappropriate Terminology: Since a table reference is a basic, direct reference to a table or subquery and does not have the complex manipulative capabilities associated with table expressions, the author argues that using "table reference" to describe a concept that is part of a broader, more abstract "table expression" framework is misleading. The term "table reference" does not adequately capture the richness and flexibility of what is meant by "table expression" in the broader sense.
You said:
Now, the BNF grammar defined later in this chapter refers to the items in the commalist in
a FROM clause—i.e., the items following the keyword FROM itself—as table specifications.
The standard calls them table references, but that term is hardly very apt. In most languages, a variable reference is a special
case of an expression; syntactically, it’s just a variable name, used to denote the value of the variable in question. But an SQL
“table reference” isn’t a special case of a table expression—not in the sense in which the latter term is used in this book, and
(perhaps more to the point) not in the sense in which it’s used in SQL, either.
explain
ChatGPT said:
ChatGPT
In this passage, the author is discussing the terminology used to describe different components of SQL queries and is critiquing how some terms are used.
Here’s a detailed breakdown:
Table Specifications:
In the BNF (Backus-Naur Form) grammar mentioned, the term “table specifications” refers to the items listed in the FROM clause of an SQL query. For example, in a query like SELECT * FROM table1, table2, table1 and table2 are table specifications.
Standard Terminology - Table References:
SQL standard terminology uses the term “table references” to describe these items in the FROM clause. In SQL, this is how tables or subqueries are referred to in the context of a query.
Comparison with Variable References:
In many programming languages, a variable reference is a straightforward way of referring to a variable. It is essentially a simple expression that represents the value of the variable. For instance, if you have a variable x, referencing x in an expression like x + 1 is direct and simple.
Critique of the Term "Table Reference":
The author argues that “table reference” is not an apt term for what it describes in SQL because it doesn’t fit well with the concept of “table expression” as understood in the book and SQL itself. Here’s why:
Not a Special Case: A “table reference” in SQL does not represent a special case of a “table expression”. In the book’s context, a table expression refers to a more complex and abstract concept that includes various ways of representing and manipulating data, such as:
Derived Tables: Temporary tables created within a query.
Subqueries: Used to generate result sets.
Common Table Expressions (CTEs): Named temporary result sets.
Complexity and Abstraction: While a table reference is a simple mention of a table or subquery, a table expression includes more sophisticated operations and manipulations. Therefore, calling a simple table reference a special case of a table expression does not capture the full scope of what a table expression represents.
Summary: The author is suggesting that the term “table reference” does not adequately capture the broader, more complex concept of “table expression.” In SQL, a table reference is a basic way of identifying a table or subquery, while a table expression involves more complex and abstract manipulations of data. Thus, “table reference” is considered an inadequate term for representing these more intricate constructs.
You said:
what is the point it shouldn't require more than 3 lines to explain
ChatGPT said:
ChatGPT
The author is arguing that “table reference” in SQL is an inadequate term because it describes a simple mention of tables or subqueries in a query, whereas “table expression” (as used in the book) refers to more complex constructs involving data manipulations. Therefore, a table reference is not a special case of a table expression, which encompasses a broader range of data representations.
2/2
You said:
why a so called sql table reference is not a special case of a table expression?
ChatGPT said:
ChatGPT
An SQL "table reference" is not a special case of a "table expression" because it represents a straightforward mention of a table or subquery in a query without inherent complexity. In contrast, a "table expression" encompasses more complex constructs, such as derived tables, subqueries, and common table expressions (CTEs), which involve richer operations and manipulations of data. Thus, "table reference" is too simple to be considered a subset of the more abstract and flexible "table expression."
You said:
but the subquery may be complex??
ChatGPT said:
ChatGPT
Yes, a subquery can indeed be complex, but the distinction is about the nature of the terms:
Table Reference: This term refers to a simple mention of a table or subquery in the FROM clause. Even though a subquery can be complex, a table reference itself is just a way to identify and use a data source in a straightforward manner.
Table Expression: This term refers to a broader concept that includes complex constructs and operations on data, such as derived tables, common table expressions (CTEs), and more. It represents a richer and more abstract way of working with data beyond just naming tables or subqueries.
The key point is that while a subquery (used as a table reference) might be complex, the term "table reference" in SQL is used to describe its role as a data source in the query. It doesn't fully capture the abstract and multifaceted nature of a "table expression," which includes more sophisticated data manipulations and constructs.
You said:
so the difference is in that it can't use features that sql table expression can use?
ChatGPT said:
ChatGPT
Yes, the difference lies in the scope and capabilities:
Table Reference: In SQL, this term refers to a basic mention of a table or a subquery within the FROM clause. While a subquery used as a table reference can be complex, the table reference itself is simply a way to identify a data source without implying any specific data manipulation or transformation beyond the direct usage of the table or subquery.
Table Expression: This term encompasses a broader range of constructs and features. It includes not only simple table references but also more complex features such as:
Derived Tables: Temporary tables created on-the-fly within a query.
Common Table Expressions (CTEs): Named temporary result sets that can be referenced multiple times within a query.
Subqueries in SELECT, WHERE, or JOIN clauses: These can be part of more complex data manipulations and operations.
In essence, a table reference is a specific, often straightforward component used to identify tables or subqueries, whereas a table expression represents a more flexible and comprehensive concept involving various ways to construct and manipulate data within SQL queries.
You said:
is the author saying that calling them table references is wrong since, they aren't a simple reference, and may be subqueries?
ChatGPT said:
ChatGPT
Yes, the author is suggesting that calling them "table references" is inadequate because it oversimplifies the concept. Here’s why:
Not Just Simple References: The term "table reference" implies a straightforward, basic mention of a table or data source. However, in SQL, these references can include complex subqueries, which can have intricate structures and operations.
Complexity of Subqueries: Subqueries used as table references can be complex and involve significant data manipulation. The term "table reference" does not capture the full range of complexity and functionality that these subqueries might embody.
Broader Concept: The author argues that "table expressions" is a more appropriate term because it encompasses not just simple table references but also more complex constructs like derived tables and common table expressions (CTEs). These constructs allow for a richer set of data operations and manipulations.
In summary, the author believes that "table references" is not an ideal term because it doesn’t reflect the complexity and functionality of the constructs it describes, particularly when they involve complex subqueries. The term "table expressions" better captures the range of constructs and manipulations involved.
You said:
the author is actually suggesting table specification no?
ChatGPT said:
ChatGPT
Yes, the author is indeed suggesting that “table specification” is a more appropriate term than “table reference”.
Here’s why:
Complexity and Scope: The term “table specification” can encompass a wider range of constructs, including both simple table names and more complex subqueries. This term is more flexible and can account for the variety of ways tables or subqueries can be represented in SQL.
Inadequacy of "Table Reference": The term “table reference” is seen as inadequate because it implies a simpler notion of merely referencing a table. However, in practice, table references can involve complex subqueries and other constructs, which “table specification” can better describe.
Alignment with Grammar: The BNF grammar referred to in the book uses “table specification” to describe items in the FROM clause, which aligns better with the variety of data sources that can be included, including simple tables and complex subqueries.
In summary, the author suggests “table specification” as a more accurate term to reflect the full range of possible constructs and data sources in SQL, rather than just the simpler notion implied by “table reference.”