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What is the meaning of the OVER clause in Oracle?
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Basic usage of Oracle LISTAGG() function For example, the following query returns a comma-separated list of employees for each job title. In this example, the LISTAGG() function concatenates the first names of employees who have the same job title.
The plus sign is Oracle syntax for an outer join. There isn't a minus operator for joins. An outer join means return all rows from one table. Also return the rows from the outer joined where there's a match on the join key. If there's no matching row, return null.
A windowing clause is a set of parameters or keywords that defines the group (or window) of rows within a particular partition that will be evaluated for analytic function computation. The query in Listing 1 uses a windowing clause by default, because it uses an ORDER BY clause.
To delete duplicate records in Oracle, start by making sure the records are actually duplicates by entering the Standard Query Language, or SQL. After entering “SQL,” search for what you want to delete, like “delete from names where name = 'Alan. '” Then, enter “commit” for this command to take effect.
The OVER clause specifies the partitioning, ordering and window "over which" the analytic function operates.
Example #1: calculate a moving average
AVG(amt) OVER (ORDER BY date ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) date amt avg_amt ===== ==== ======= 1-Jan 10.0 10.5 2-Jan 11.0 17.0 3-Jan 30.0 17.0 4-Jan 10.0 18.0 5-Jan 14.0 12.0 It operates over a moving window (3 rows wide) over the rows, ordered by date.
Example #2: calculate a running balance
SUM(amt) OVER (ORDER BY date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) date amt sum_amt ===== ==== ======= 1-Jan 10.0 10.0 2-Jan 11.0 21.0 3-Jan 30.0 51.0 4-Jan 10.0 61.0 5-Jan 14.0 75.0 It operates over a window that includes the current row and all prior rows.
Note: for an aggregate with an OVER clause specifying a sort ORDER, the default window is UNBOUNDED PRECEDING to CURRENT ROW, so the above expression may be simplified to, with the same result:
SUM(amt) OVER (ORDER BY date) Example #3: calculate the maximum within each group
MAX(amt) OVER (PARTITION BY dept) dept amt max_amt ==== ==== ======= ACCT 5.0 7.0 ACCT 7.0 7.0 ACCT 6.0 7.0 MRKT 10.0 11.0 MRKT 11.0 11.0 SLES 2.0 2.0 It operates over a window that includes all rows for a particular dept.
SQL Fiddle: http://sqlfiddle.com/#!4/9eecb7d/122
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