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SQL Syntax
The syntax of the SQL programming language is defined and maintained by ISO/IEC SC 32 as part of ''ISO/IEC 9075''. This standard is not freely available. Despite the existence of the standard, SQL code is not completely portable among different database systems without adjustments. Language elements , width=500, caption=A chart showing several of the SQL language elements that compose a single statement. This adds one to the population of the USA in the country table. The SQL language is subdivided into several language elements, including: * ''Keywords'' are words that are defined in the SQL language. They are either reserved (e.g. , and ), or non-reserved (e.g. , and ). List of SQL reserved words. * ''Identifiers'' are names on database objects, like tables, columns and schemas. An identifier may not be equal to a reserved keyword, unless it is a delimited identifier. Delimited identifiers means identifiers enclosed in double quotation marks. They can contain characte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Syntax (programming Languages)
In computer science, the syntax of a computer language is the rules that define the combinations of symbols that are considered to be correctly structured Statement (computer science), statements or Expression (computer science), expressions in that language. This applies both to programming languages, where the document represents source code, and to markup languages, where the document represents data. The syntax of a language defines its surface form. Text-based user interface, Text-based computer languages are based on sequences of Character (computing), characters, while visual programming languages are based on the spatial layout and connections between symbols (which may be textual or graphical). Documents that are syntactically invalid are said to have a syntax error. When designing the syntax of a language, a designer might start by writing down examples of both legal and illegal String (computer science), strings, before trying to figure out the general rules from these ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Session (computer Science)
In computer science and networking in particular, a session is a time-delimited two-way link, a practical (relatively high) layer in the TCP/IP protocol enabling interactive expression and information exchange between two or more communication devices or ends – be they computers, automated systems, or live active users (see login session). A session is established at a certain point in time, and then ‘torn down’ - brought to an end - at some later point. An established communication session may involve more than one message in each direction. A session is typically stateful, meaning that at least one of the communicating parties needs to hold current state information and save information about the session history to be able to communicate, as opposed to stateless communication, where the communication consists of independent requests with responses. An established session is the basic requirement to perform a connection-oriented communication. A session also is the b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Query Plan
A query plan (or query execution plan) is a sequence of steps used to access data in a SQL relational database management system. This is a specific case of the relational model concept of access plans. Since SQL is declarative, there are typically many alternative ways to execute a given query, with widely varying performance. When a query is submitted to the database, the query optimizer evaluates some of the different, correct possible plans for executing the query and returns what it considers the best option. Because query optimizers are imperfect, database users and administrators sometimes need to manually examine and tune the plans produced by the optimizer to get better performance. Generating query plans A given database management system may offer one or more mechanisms for returning the plan for a given query. Some packages feature tools which will generate a graphical representation of a query plan. Other tools allow a special mode to be set on the connection to cau ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Database Management System
In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and analyze the data. The DBMS additionally encompasses the core facilities provided to administer the database. The sum total of the database, the DBMS and the associated applications can be referred to as a database system. Often the term "database" is also used loosely to refer to any of the DBMS, the database system or an application associated with the database. Before digital storage and retrieval of data have become widespread, index cards were used for data storage in a wide range of applications and environments: in the home to record and store recipes, shopping lists, contact information and other organizational data; in business to record presentation notes, project research and notes, and contact information; in schools as flash ca ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Select (SQL)
The SQL SELECT statement returns a result set of rows, from one or more tables. A SELECT statement retrieves zero or more rows from one or more database tables or database views. In most applications, SELECT is the most commonly used data manipulation language (DML) command. As SQL is a declarative programming language, SELECT queries specify a result set, but do not specify how to calculate it. The database translates the query into a " query plan" which may vary between executions, database versions and database software. This functionality is called the " query optimizer" as it is responsible for finding the best possible execution plan for the query, within applicable constraints. The SELECT statement has many optional clauses: * SELECT list is the list of columns or SQL expressions to be returned by the query. This is approximately the relational algebra projection operation. * AS optionally provides an alias for each column or expression in the SELECT list. Thi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Newline
A newline (frequently called line ending, end of line (EOL), next line (NEL) or line break) is a control character or sequence of control characters in character encoding specifications such as ASCII, EBCDIC, Unicode, etc. This character, or a sequence of characters, is used to signify the end of a line (text file), line of text and the start of a new one. History In the mid-1800s, long before the advent of teleprinters and teletype machines, Morse code operators or telegraphists invented and used Prosigns for Morse code, Morse code prosigns to encode white space text formatting in formal written text messages. In particular, the International Morse code, Morse prosign (mnemonic break text), represented by the concatenation of literal textual Morse codes "B" and "T" characters, sent without the normal inter-character spacing, is used in Morse code to encode and indicate a ''new line'' or ''new section'' in a formal text message. Later, in the age of modern teleprinters, st ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Comment (computer Programming)
In computer programming, a comment is text embedded in source code that a translator (compiler or interpreter (computing), interpreter) ignores. Generally, a comment is an annotation intended to make the code easier for a programmer to understand often explaining an aspect that is not readily apparent in the program (non-comment) code. For this article, ''comment'' refers to the same concept in a programming language, markup language, configuration file and any similar context. Some development tools, other than a source code translator, do parse comments to provide capabilities such as API documentation generator, document generation, static analysis, and version control integration. The comparison of programming languages (syntax)#Comments, syntax of comments varies by programming language yet there are repeating patterns in the syntax among languages as well as similar aspects related to comment content. The flexibility supported by comments allows for a wide degree of cont ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SQL CASE
In SQL, null or NULL is a special marker used to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL null serves to fulfill the requirement that all ''true relational database management systems (RDBMS)'' support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent null in database theory. In SQL, NULL is a reserved word used to identify this marker. A null should not be confused with a value of 0. A null indicates a lack of a value, which is not the same as a zero value. For example, consider the question "How many books does Adam own?" The answer may be "zero" (we ''know'' that he owns ''none'') or "null" (we ''do not know'' how many he owns). In a database table, the column reporting this answer would start with no value (marked by null), and it would not be updated with the value zero un ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SQL-92
SQL-92 (also called SQL 2) was the third revision of the SQL database query language. Unlike SQL-89, it was a major revision of the standard. Aside from a few minor incompatibilities, the SQL-89 standard is forward-compatible with SQL-92. The standard specification itself grew about five times compared to SQL-89. Much of it was due to more precise specifications of existing features; the increase due to new features was only by a factor of 1.5–2. Many of the new features had already been implemented by vendors before the new standard was adopted. However, most of the new features were added to the "intermediate" and "full" tiers of the specification, meaning that conformance with SQL-92 entry level was scarcely any more demanding than conformance with SQL-89. The next revision is SQL:1999 (SQL3). Related official standard * ANSI X3.135-1992 * ISO/IEC 9075:1992 * FIPS PUB 127-2 New features Significant new features include: C. J. Date with Hugh Darwen: ''A Guide to th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Skyline Operator
The skyline operator is the subject of an optimization problem and computes the Pareto optimum on tuples with multiple dimensions. This operator is an extension to SQL proposed by Börzsönyi et al. to filter results from a database to keep only those objects that are not dominated by any other point on all dimensions. The name ''skyline'' comes from the view on Manhattan from the Hudson River, where those buildings can be seen that are not hidden by any other. A building is visible if it is not dominated by a building that is taller or closer to the river (two dimensions, distance to the river minimized, height maximized). Another application of the skyline operator involves selecting a hotel for a holiday. The user wants the hotel to be both cheap and close to the beach. However, hotels that are close to the beach may also be expensive. In this case, the skyline operator would only present those hotels that are not worse than any other hotel in both price and distance to the beac ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SQL Syntax
The syntax of the SQL programming language is defined and maintained by ISO/IEC SC 32 as part of ''ISO/IEC 9075''. This standard is not freely available. Despite the existence of the standard, SQL code is not completely portable among different database systems without adjustments. Language elements , width=500, caption=A chart showing several of the SQL language elements that compose a single statement. This adds one to the population of the USA in the country table. The SQL language is subdivided into several language elements, including: * ''Keywords'' are words that are defined in the SQL language. They are either reserved (e.g. , and ), or non-reserved (e.g. , and ). List of SQL reserved words. * ''Identifiers'' are names on database objects, like tables, columns and schemas. An identifier may not be equal to a reserved keyword, unless it is a delimited identifier. Delimited identifiers means identifiers enclosed in double quotation marks. They can contain characte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Like (SQL)
In English, the word ''like'' has a very flexible range of uses, ranging from conventional to non-standard. It can be used as a noun, verb, adverb, adjective, preposition, particle, conjunction, hedge, filler, quotative, and semi-suffix. Uses Comparisons ''Like'' is one of the words in the English language that can introduce a simile (a stylistic device comparing two dissimilar ideas). It can be used as a preposition, as in "He runs ''like'' a cheetah"; it can also be used as a suffix, as in "She acts very child-''like''. It can also be used in non-simile comparisons such as, "She has a dog ''like'' ours". As a conjunction ''Like'' is often used in place of the subordinating conjunction ''as'', or ''as if''. Examples: * They look ''like'' they have been having fun. * They look ''as if'' they have been having fun. Many people became aware of the two options in 1954, when a famous ad campaign for Winston cigarettes introduced the slogan " Winston tastes good—like a ciga ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
