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In computing, aliasing describes a situation in which a data location in memory can be accessed through different symbolic names in the program. Thus, modifying the data through one name implicitly modifies the values associated with all aliased names, which may not be expected by the programmer. As a result, aliasing makes it particularly difficult to understand, analyze and optimize programs. Aliasing analysers intend to make and compute useful information for understanding aliasing in programs.
aliasis shell command that defines a word that the shell replaces with associated text before interpreting a command line. It is often used to enhance productivity by abbreviating a command or for including commonly-used arguments with a command. The command is available in Unix shells, AmigaDOS, 4DOS/4NT, FreeDOS, KolibriOS, PowerShell, ReactOS, EFI shell, and IBM i. Aliasing functionality in MS-DOS and Command Prompt is provided by theDOSKEYcommand.Since aliases are defined only for a shell session, regularly-used aliases are often defined in a session startup shell script such as
.bashrc. Thealiascommands may either be written in the config script directly or sourced from a separate file.Aliases were introduced in the C shell to survive in descendant shells such as tcsh and bash. As these aliases were limited to one line they were useful for creating relatively simple shortcut commands, but not more complex constructs. Older versions of the Bourne shell did not offer aliases, but did provide functions, which are more powerful than the csh alias. Eventually, the csh alias was implemented in the bash and ksh shells. With shells that support both functions and aliases but no parameterized inline shell scripts, the use of functions wherever possible is recommended. None-the-less, aliases are necessary where chained aliases are required.
An alias is a feature of SQL that is supported by most, if not all, relational database management systems (RDBMSs). Aliases provide users with the ability to reduce the amount of code required for a query, and to make queries simpler to understand. In addition, aliasing is required when doing self joins (i.e. joining a table with itself.)
In SQL, you can alias tables and columns. A table alias is called a correlation name, according to the SQL standard. A programmer can use an alias to temporarily assign another name to a table or column for the duration of the current SELECT query. Assigning an alias does not actually rename the column or table. This is often useful when either tables or their columns have very long or complex names. An alias name could be anything, but usually it is kept short. For example, it might be common to use a table alias such as "pi" for a table named "price_information".
The general syntax of an alias is
SELECT * FROM table_name [AS] alias_name. Note that the AS keyword is completely optional and is usually kept for readability purposes. Here is some sample data that the queries below will be referencing:Using a table alias:
We can also write the same query like this (Note that the AS clause is omitted this time):
A column alias is similar:
In the returned result sets, the data shown above would be returned, with the only exception being "DepartmentID" would show up as "Id", and "DepartmentName" would show up as "Name".
Also, if only one table is being selected and the query is not using table joins, it is permissible to omit the table name or table alias from the column name in the SELECT statement. Example as follows:
Some systems, such as Postgres and Presto, support specifying column aliases together with table aliases. E.g.
would produce the same result set as before. In this syntax it is permissible to omit aliases for some column names. In the example, an alias was provided for DepartmentId, but omitted for DepartmentName. Columns with unspecified aliases will be left unaliased. This syntax is often used with expressions that do not produce useful table and column names, such as VALUES and UNNEST. As an example, one may conveniently test the above SQL statements without creating an actual Departments table by using expressions such as
In computer programming, a declaration is a language construct specifying identifier properties: it declares a word's (identifier's) meaning. Declarations are most commonly used for functions, variables, constants, and classes, but can also be used for other entities such as enumerations and type definitions. Beyond the name (the identifier itself) and the kind of entity (function, variable, etc.), declarations typically specify the data type (for variables and constants), or the type signature (for functions); types may also include dimensions, such as for arrays. A declaration is used to announce the existence of the entity to the compiler; this is important in those strongly typed languages that require functions, variables, and constants, and their types to be specified with a declaration before use, and is used in forward declaration. The term "declaration" is frequently contrasted with the term "definition", but meaning and usage varies significantly between languages; see below.
Declarations are particularly prominent in languages in the ALGOL tradition, including the BCPL family, most prominently C and C++, and also Pascal. Java uses the term "declaration", though Java does not require separate declarations and definitions.
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