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Documentation
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Interactive SQL tutorial, learn about: SQL Server, Oracle, MySQL, DB2, Mimer SQL, PostgreSQL and Access.
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DBAs are always encouraging developers to use bind variables, but when bind variables are used against columns containing skewed data they sometimes lead to less than optimum execution plans. This is because the optimizer peaks at the bind variable value during the hard parse of the statement, so the value of a bind variable when the statement is first presented to the server can affect every execution of the statement, regardless of the bind variable values.
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A complete A-Z listing of all Oracle PL/SQL commands and hyperlinks to each one.
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Oracle supports a number of built in operators that fall into basic categories—simple arithmetic operators, comparison operators, logical operators, and operators that are used in select statements. In Oracle 9, support for User Defined Operators was also added.
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Analytic Functions, which have been available since Oracle 8.1.6, are designed to address such problems as "Calculate a running total", "Find percentages within a group", "Top-N queries", "Compute a moving average" and many more. Most of these problems can be solved using standard PL/SQL, however the performance is often not what it should be. Analytic Functions add extensions to the SQL language that not only make these operations easier to code; they make them faster than could be achieved with pure SQL or PL/SQL. These extensions are currently under review by the ANSI SQL committee for inclusion in the SQL specification.
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Autonomous transactions allow you to leave the context of the calling transaction, perform an independant transaction, and return to the calling transaction without affecting it's state. The autonomous transaction has no link to the calling transaction, so only commited data can be shared by both transactions.
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In the second part of a two-part series, we wrap up our discussion of best practices for handling PL/SQL variables. This article is excerpted from chapter four of the book Oracle PL/SQL Best Practices, Second Edition, written by Steven Feuerstein.
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One of the key functions of a database is to ensure that multiple users can read and write to the database without overwriting each other's changes inadvertently or reading inconsistent data due to in-progress changes. Designing application code to take account of concurrency issues is essential.
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Constraints are declaractions of conditions about the database that must remain true. These include attributed-based, tuple-based, key, and referential integrity constraints. The system checks for the violation of the constraints on actions that may cause a violation, and aborts the action accordingly. Information on SQL constraints can be found in the textbook. The Oracle implementation of constraints differs from the SQL standard, as documented in Oracle 9i SQL versus Standard SQL. Triggers are a special PL/SQL construct similar to procedures. However, a procedure is executed explicitly from another block via a procedure call, while a trigger is executed implicitly whenever the triggering event happens. The triggering event is either a INSERT, DELETE, or UPDATE command. The timing can be either BEFORE or AFTER.
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Use the CREATE TRIGGER statement to create and enable a database trigger, which is:
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A stored PL/SQL block associated with a table, a schema, or the database or
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An anonymous PL/SQL block or a call to a procedure implemented in PL/SQL or Java
Oracle Database automatically executes a trigger when specified conditions occur.
When you create a trigger, the database enables it automatically. You can subsequently disable and enable a trigger with the DISABLE and ENABLE clause of the ALTER TRIGGER or ALTER TABLE statement.
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