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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:
A table in a SQL database schema corresponds to a predicate variable; the contents of a table to a relation; key constraints, other constraints, and SQL queries correspond to predicates. However, SQL databases deviate from the relational model in many details, and Codd fiercely argued against deviations that compromise the original principles. [3]
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:
In situations where the number of unique values of a column is far less than the number of rows in the table, column-oriented storage allow significant savings in space through data compression. Columnar storage also allows fast execution of range queries (e.g., show all records where a particular column is between X and Y, or less than X.)
NoSQL systems are also sometimes called "Not only SQL" to emphasize that they may support SQL-like query languages or sit alongside SQL databases in polyglot-persistent architectures. [3] [4] Non-relational databases have existed since the late 1960s, but the name "NoSQL" was only coined in the early 2000s, [5] triggered by the needs of Web 2.0 ...
Queries (or goals) have the same syntax as the bodies of rules and are commonly written in the form: B 1 , ..., B n . In the simplest case of Horn clauses (or "definite" clauses), all of the A, B 1 , ..., B n are atomic formulae of the form p(t 1 ,..., t m ), where p is a predicate symbol naming a relation, like "motherhood", and the t i are ...
The SQL:2003 standard makes minor modifications to all parts of SQL:1999 (also known as SQL3), and officially introduces a few new features such as: [1] XML-related features ; Window functions; the sequence generator, which allows standardized sequences; two new column types: auto-generated values and identity-columns; the new MERGE statement
The example query above logically always returns zero rows because the comparison of the i column with Null always returns Unknown, even for those rows where i is Null. The Unknown result causes the SELECT statement to summarily discard every row. (However, in practice, some SQL tools will retrieve rows using a comparison with Null.)