View (SQL): Difference between revisions

Revision as of 01:37, 16 March 2020

Revision as of 18:59, 11 May 2020

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A view is equivalent to its source query. When queries are run against views, the query is modified. For example, if there exists a view named accounts_view with the content as follows:

<sourcesyntaxhighlight lang="sql">

accounts_view:

-------------

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JOIN accounts_table a

ON a.customer_id = c.customer_id

</syntaxhighlight>

</source>

then the application could run a simple query such as:

<sourcesyntaxhighlight lang="sql">

Simple query

------------

Line 52:

balance

FROM accounts_view

</syntaxhighlight>

</source>

The RDBMS then takes the simple query, replaces the equivalent view, then sends the following to the [[query optimizer]]:

<sourcesyntaxhighlight lang="sql">

Preprocessed query:

------------------

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FROM table_customers c JOIN accounts_table a

ON a.customer_id = c.customer_id )

</syntaxhighlight>

</source>

The optimizer then removes unnecessary fields and complexity (for example: it is not necessary to read the address, since the parent invocation does not make use of it) and then sends the query to the SQL engine for processing.