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Set operations in SQL is a type of operations which allow the results of multiple queries to be combined into a single result set. [ 1 ] Set operators in SQL include UNION , INTERSECT , and EXCEPT , which mathematically correspond to the concepts of union , intersection and set difference .
Set operation may have one of the following meanings. Any operation with sets; Set operation (Boolean), Boolean set operations in the algebra of sets; Set operations (SQL), type of operation in SQL; Fuzzy set operations, a generalization of crisp sets for fuzzy sets
SQL includes operators and functions for calculating values on stored values. SQL allows the use of expressions in the select list to project data, as in the following example, which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.
The concatenation operator is . (dot). Array elements are accessed and set with square brackets in both associative arrays and indexed arrays. Curly brackets can be used to access array elements, but not to assign. PHP has three types of comment syntax: /* */ which serves as block comments, and // as well as # which are used for inline comments ...
In the most SQL dialects the double dash (--) can be used instead. Esoteric languages Many esoteric programming languages follow the convention that any text not executed by the instruction pointer (e.g., Befunge ) or otherwise assigned a meaning (e.g., Brainfuck ), is considered a "comment".
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The relational algebra uses set union, set difference, and Cartesian product from set theory, and adds additional constraints to these operators to create new ones.. For set union and set difference, the two relations involved must be union-compatible—that is, the two relations must have the same set of attributes.
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. Many graphical front-ends for databases use the ideas from QBE today.