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www.crummy.com /software /BeautifulSoup / Beautiful Soup is a Python package for parsing HTML and XML documents, including those with malformed markup. It creates a parse tree for documents that can be used to extract data from HTML, [ 3 ] which is useful for web scraping .
The Suppliers and Parts database is an example relational database that is referred to extensively in the literature [citation needed] and described in detail in C. J. Date's An Introduction to Database Systems, 8th ed. [1] It is a simple database comprising three tables: Supplier, Part and Shipment, and is often used as a minimal exemplar of the interrelationships found in a database.
For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either null values or values from a parent table's primary key or a candidate key. [2] In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.
Title Authors ----- ----- SQL Examples and Guide 4 The Joy of SQL 1 An Introduction to SQL 2 Pitfalls of SQL 1 Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Book table, one could re-write the query above in the following form:
For example, a "table" element could be defined in a new namespace with new semantics different from the HTML "table" element and the browser will be able to differentiate between the two. In providing namespaces, XHTML combined with CSS allow authoring communities to easily extend the semantic vocabulary of documents.
Since surrogate keys replace a table's identifying attributes—the natural key—and since the identifying attributes are likely to be those queried, then the query optimizer is forced to perform a full table scan when fulfilling likely queries. The remedy to the full table scan is to apply indexes on the identifying attributes, or sets of them.
The query evaluation, and thus query containment, is LOGCFL-complete and thus in polynomial time. [9] Acyclicity of conjunctive queries is a structural property of queries that is defined with respect to the query's hypergraph : [ 6 ] a conjunctive query is acyclic if and only if it has hypertree-width 1.
Query by Example (QBE) is a database query language for relational databases. It was devised by Moshé M. Zloof at IBM Research during the mid-1970s, in parallel to the development of SQL. [1] It is the first graphical query language, using visual tables where the user would enter commands, example elements and conditions.