man fetch howto postgresql8.4

man fetch howto postgresql8.4

时间:2015-07-10 12:44来源:网络整理 作者:KKWL 点击:
MAN page from Mandriva2010 postgresql8.4-8.4.7-0.1mdv2010.0.i586.rpm FETCHSection: SQL Commands (7) Updated: 2011-01-27 NAMEFETCH - retrieve rows from a query using a cursor SYNOPSIS FETCH [ direction { FROM | IN } ] cursornamewhere directi

MAN page from Mandriva 2010 postgresql8.4-8.4.7-0.1mdv2010.0.i586.rpm

FETCHSection: SQL Commands (7)
Updated: 2011-01-27
 NAMEFETCH - retrieve rows from a query using a cursor


FETCH [ direction { FROM | IN } ] cursornamewhere direction can be empty or one of: NEXT PRIOR FIRST LAST ABSOLUTE count RELATIVE count count ALL FORWARD FORWARD count FORWARD ALL BACKWARD BACKWARD count BACKWARD ALL DESCRIPTION

FETCH retrieves rows using a previously-created cursor.

A cursor has an associated position, which is used byFETCH. The cursor position can be before the first row of thequery result, on any particular row of the result, or after the last rowof the result. When created, a cursor is positioned before the first row.After fetching some rows, the cursor is positioned on the row most recentlyretrieved. If FETCH runs off the end of the available rowsthen the cursor is left positioned after the last row, or before the firstrow if fetching backward. FETCH ALL or FETCH BACKWARDALL will always leave the cursor positioned after the last row or beforethe first row.

The forms NEXT, PRIOR, FIRST,LAST, ABSOLUTE, RELATIVE fetcha single row after moving the cursor appropriately. If there is nosuch row, an empty result is returned, and the cursor is leftpositioned before the first row or after the last row asappropriate.

The forms using FORWARD and BACKWARDretrieve the indicated number of rows moving in the forward orbackward direction, leaving the cursor positioned on thelast-returned row (or after/before all rows, if the count exceeds the number of rowsavailable).

RELATIVE 0, FORWARD 0, andBACKWARD 0 all request fetching the current row withoutmoving the cursor, that is, re-fetching the most recently fetchedrow. This will succeed unless the cursor is positioned before thefirst row or after the last row; in which case, no row is returned.

Note:This page describes usage of cursors at the SQL command level.If you are trying to use cursors inside a PL/pgSQLfunction, the rules are different ---see in the documentation.

 PARAMETERSdirectiondirection definesthe fetch direction and number of rows to fetch. It can be oneof the following:NEXTFetch the next row. This is the default if direction is omitted.PRIORFetch the prior row.FIRSTFetch the first row of the query (same as ABSOLUTE 1).LASTFetch the last row of the query (same as ABSOLUTE -1).ABSOLUTE countFetch the count'th row of the query,or the abs(count)'th row fromthe end if count is negative. Positionbefore first row or after last row if count is out of range; inparticular, ABSOLUTE 0 positions beforethe first row.RELATIVE countFetch the count'th succeeding row, orthe abs(count)'th priorrow if count isnegative. RELATIVE 0 re-fetches thecurrent row, if any.countFetch the next count rows (same asFORWARD count).ALLFetch all remaining rows (same as FORWARD ALL).FORWARDFetch the next row (same as NEXT).FORWARD countFetch the next count rows.FORWARD 0 re-fetches the current row.FORWARD ALLFetch all remaining rows.BACKWARDFetch the prior row (same as PRIOR).BACKWARD countFetch the prior count rows (scanningbackwards). BACKWARD 0 re-fetches thecurrent row.BACKWARD ALLFetch all prior rows (scanning backwards).

countcount is apossibly-signed integer constant, determining the location ornumber of rows to fetch. For FORWARD andBACKWARD cases, specifying a negative count is equivalent to changingthe sense of FORWARD and BACKWARD.cursornameAn open cursor's name. OUTPUTS

On successful completion, a FETCH command returns a commandtag of the form

FETCH countThe count is the numberof rows fetched (possibly zero). Note that inpsql, the command tag will not actually bedisplayed, since psql displays the fetchedrows instead. NOTES

The cursor should be declared with the SCROLLoption if one intends to use any variants of FETCHother than FETCH NEXT or FETCH FORWARD witha positive count. For simple queriesPostgreSQL will allow backwards fetchfrom cursors not declared with SCROLL, but thisbehavior is best not relied on. If the cursor is declared withNO SCROLL, no backward fetches are allowed.

ABSOLUTE fetches are not any faster thannavigating to the desired row with a relative move: the underlyingimplementation must traverse all the intermediate rows anyway.Negative absolute fetches are even worse: the query must be read tothe end to find the last row, and then traversed backward fromthere. However, rewinding to the start of the query (as withFETCH ABSOLUTE 0) is fast.

DECLARE [declare(7)]is used to define a cursor. UseMOVE [move(7)]to change cursor position without retrieving data. EXAMPLES

The following example traverses a table using a cursor:

BEGIN WORK;-- Set up a cursor:DECLARE liahona SCROLL CURSOR FOR SELECT * FROM films;-- Fetch the first 5 rows in the cursor liahona:FETCH FORWARD 5 FROM 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 the previous row:FETCH PRIOR FROM liahona; code | title | did | date_prod | kind | len-------+---------+-----+------------+--------+------- P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08-- Close the cursor and end the transaction:CLOSE liahona;COMMIT WORK; COMPATIBILITY

The SQL standard defines FETCH for use inembedded SQL only. The variant of FETCHdescribed here returns the data as if it were aSELECT result rather than placing it in hostvariables. Other than this point, FETCH isfully upward-compatible with the SQL standard.

The FETCH forms involvingFORWARD and BACKWARD, as wellas the forms FETCH count and FETCHALL, in which FORWARD is implicit, arePostgreSQL extensions.

The SQL standard allows only FROM preceding the cursorname; the option to use IN is an extension. SEE ALSOCLOSE [close(7)], DECLARE [declare(7)], MOVE [move(7)]

 IndexThis document was created byman2html,using the manual pages.