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A UNION query could combine results from both tables. Note that UNION ALL does not guarantee the order of rows. Rows from the second operand may appear before, after, or mixed with rows from the first operand. In situations where a specific order is desired, ORDER BY must be used. Note that UNION ALL may be much faster than plain UNION.
Query rewriting is a typically automatic transformation that takes a set of database tables, views, and/or queries, usually indices, often gathered data and query statistics, and other metadata, and yields a set of different queries, which produce the same results but execute with better performance (for example, faster, or with lower memory use). [1]
An SQL UPDATE statement changes the data of one or more records in a table. Either all the rows can be updated, or a subset may be chosen using a condition. The UPDATE statement has the following form: [1] UPDATE table_name SET column_name = value [, column_name = value ...] [WHERE condition]
A join clause in the Structured Query Language combines columns from one or more tables into a new table. The operation corresponds to a join operation in relational algebra . Informally, a join stitches two tables and puts on the same row records with matching fields : INNER , LEFT OUTER , RIGHT OUTER , FULL OUTER and CROSS .
Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. A nested query is also known as a subquery. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases (all depending on implementation), the use of ...
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In the above table, the next query extracts for each row the values of a window with one preceding and one following row: SELECT LAG ( name , 1 ) OVER ( ORDER BY name ) "prev" , name , LEAD ( name , 1 ) OVER ( ORDER BY name ) "next" FROM people ORDER BY name
Command-query separation (CQS) is a principle of imperative computer programming. It was devised by Bertrand Meyer as part of his pioneering work on the Eiffel programming language . It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both.