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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.
The CASE expression, for example, enables SQL to perform conditional branching within queries, providing a mechanism to return different values based on evaluated conditions. This logic can be particularly useful for data transformation during retrieval, especially in SELECT statements.
A query includes a list of columns to include in the final result, normally immediately following the SELECT keyword. An asterisk ("*") can be used to specify that the query should return all columns of the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that include:
The first unnamed parameter, mandatory, is the category name to be counted. A second unnamed parameter, optional, may be used to count several subsets of that category, as described in mw:Help:Magic words#Statistics.
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Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped. if condition then-- statements elseif condition then-- more statements elseif condition then-- more statements;... else-- other statements; end if; For example, for a shop offering as much as a 30% discount for an item:
A query includes a list of columns to include in the final result, normally immediately following the SELECT keyword. An asterisk ("*") can be used to specify that the query should return all columns of all the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that include:
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