GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
GNU/Linux |
RedHat 6.2(Zoot) |
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fetch(l) |
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FETCH − Gets rows using a cursor
FETCH [
selector ] [ count ] { IN | FROM }
cursor
FETCH [ RELATIVE ] [ { [ # | ALL | NEXT | PRIOR ] } ]
FROM ] cursor
INPUTS
selector
selector defines the
fetch direction. It can be one the following:
FORWARD
fetch next row(s). This is the default if selector is omitted.
BACKWARD
fetch previous row(s).
RELATIVE
Noise word for SQL92 compatibility.
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count |
count determines how many rows to fetch. It can be one of the following: |
#
|
A signed integer that specify how many rows to fetch. Note that a negative integer is equivalent to changing the sense of FORWARD and BACKWARD. | |||
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ALL |
Retrieve all remaining rows. | ||
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NEXT |
Equivalent to specifying a count of 1. | ||
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PRIOR |
Equivalent to specifying a count of -1. | ||
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cursor |
An open cursor’s name.
OUTPUTS
FETCH returns the results of the query defined by the
specified cursor. The following messages will be returned if
the query fails:
NOTICE: PerformPortalFetch: portal
"cursor" not found
If cursor is not previously declared. The cursor must be declared within a transaction block.
NOTICE: FETCH/ABSOLUTE not supported, using RELATIVE
Postgres does not support absolute positioning of cursors.
ERROR: FETCH/RELATIVE at current position is not supported
SQL92 allows one to repetatively retrieve the cursor at its "current position" using the syntax
FETCH RELATIVE 0 FROM cursor
Postgres does not currently support this notion; in fact the value zero is reserved to indicate that all rows should be retrieved and is equivalent to specifying the ALL keyword. If the RELATIVE keyword has been used, the Postgres assumes that the user intended SQL92 behavior and returns this error message.
FETCH allows a user to retrieve rows using a cursor. The number of rows retrieved is specified by #. If the number of rows remaining in the cursor is less than #, then only those available are fetched. Substituting the keyword ALL in place of a number will cause all remaining rows in the cursor to be retrieved. Instances may be fetched in both FORWARD and BACKWARD directions. The default direction is FORWARD.
Tip: Negative numbers are allowed to be specified for the row count. A negative number is equivalent to reversing the sense of the FORWARD and BACKWARD keywords. For example, FORWARD -1 is the same as BACKWARD 1.
NOTES
Note that the FORWARD and BACKWARD keywords are Postgres
extensions. The SQL92 syntax is also supported, specified in
the second form of the command. See below for details on
compatibility issues.
Once all rows are fetched, every other fetch access returns no rows.
Updating data in a cursor is not supported by Postgres, because mapping cursor updates back to base tables is not generally possible, as is also the case with VIEW updates. Consequently, users must issue explicit UPDATE commands to replace data.
Cursors may only be used inside of transactions because the data that they store spans multiple user queries.
Use move(l) to change cursor position. declare(l) will define a cursor. Refer to begin(l), commit(l), and rollback(l) for further information about transactions.
The following examples traverses a table using a cursor.
--set up and
use a cursor:
--
BEGIN WORK;
DECLARE liahona CURSOR
FOR SELECT * FROM films;
--Fetch first 5
rows in the cursor liahona:
--
FETCH FORWARD 5 IN liahona;
code |title
|did| date_prod|kind |len
-----+-----------------------+---+----------+----------+------
BL101|The Third Man |101|1949-12-23|Drama | 01:44
BL102|The African Queen |101|1951-08-11|Romantic | 01:43
JL201|Une Femme est une Femme|102|1961-03-12|Romantic |
01:25
P_301|Vertigo |103|1958-11-14|Action | 02:08
P_302|Becket |103|1964-02-03|Drama | 02:28
--Fetch
previous row:
--
FETCH BACKWARD 1 IN liahona;
code |title
|did| date_prod|kind |len
-----+-----------------------+---+----------+----------+------
P_301|Vertigo |103|1958-11-14|Action | 02:08
-- close the
cursor and commit work:
--
CLOSE liahona;
COMMIT WORK;
SQL92
Note: The non-embedded use of cursors is a Postgres extension. The syntax and usage of cursors is being compared against the embedded form of cursors defined in SQL92.
SQL92 allows absolute positioning of the cursor for FETCH, and allows placing the results into explicit variables.
FETCH ABSOLUTE
#
FROM cursor
INTO :variable [, ...]
ABSOLUTE
The cursor should be positioned to the specified absolute row number. All row numbers in Postgres are relative numbers so this capability is not supported.
:variable
Target host variable(s).
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fetch(l) | |