fetch
FETCH() SQL Commands FETCH()
NAME
FETCH - retrieve rows from a query using a cursor
SYNOPSIS
FETCH [ direction { FROM | IN } ] cursorname
where 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 by FETCH. The cur-
sor position can be before the first row of the query result, on any
particular row of the result, or after the last row of the result.
When created, a cursor is positioned before the first row. After
fetching some rows, the cursor is positioned on the row most recently
retrieved. If FETCH runs off the end of the available rows then the
cursor is left positioned after the last row, or before the first row
if fetching backward. FETCH ALL or FETCH BACKWARD ALL will always
leave the cursor positioned after the last row or before the first
row.
The forms NEXT, PRIOR, FIRST, LAST, ABSOLUTE, RELATIVE fetch a single
row after moving the cursor appropriately. If there is no such row, an
empty result is returned, and the cursor is left positioned before the
first row or after the last row as appropriate.
The forms using FORWARD and BACKWARD retrieve the indicated number of
rows moving in the forward or backward direction, leaving the cursor
positioned on the last-returned row (or after/before all rows, if the
count exceeds the number of rows available).
RELATIVE 0, FORWARD 0, and BACKWARD 0 all request fetching the current
row without moving the cursor, that is, re-fetching the most recently
fetched row. This will succeed unless the cursor is positioned before
the first row or after the last row; in which case, no row is
returned.
PARAMETERS
direction
direction defines the fetch direction and number of rows to
fetch. It can be one of the following:
NEXT Fetch the next row. This is the default if direction is
omitted.
PRIOR Fetch the prior row.
FIRST Fetch the first row of the query (same as ABSOLUTE 1).
LAST Fetch the last row of the query (same as ABSOLUTE -1).
ABSOLUTE count
Fetch the count’th row of the query, or the
abs(count)’th row from the end if count is negative.
Position before first row or after last row if count is
out of range; in particular, ABSOLUTE 0 positions before
the first row.
RELATIVE count
Fetch the count’th succeeding row, or the abs(count)’th
prior row if count is negative. RELATIVE 0 re-fetches
the current row, if any.
count Fetch the next count rows (same as FORWARD count).
ALL Fetch all remaining rows (same as FORWARD ALL).
FORWARD
Fetch the next row (same as NEXT).
FORWARD count
Fetch the next count rows. FORWARD 0 re-fetches the
current row.
FORWARD ALL
Fetch all remaining rows.
BACKWARD
Fetch the prior row (same as PRIOR).
BACKWARD count
Fetch the prior count rows (scanning backwards). BACK-
WARD 0 re-fetches the current row.
BACKWARD ALL
Fetch all prior rows (scanning backwards).
count count is a possibly-signed integer constant, determining the
location or number of rows to fetch. For FORWARD and BACKWARD
cases, specifying a negative count is equivalent to changing
the sense of FORWARD and BACKWARD.
cursorname
An open cursor’s name.
OUTPUTS
On successful completion, a FETCH command returns a command tag of the
form
FETCH count
The count is the number of rows fetched (possibly zero). Note that in
psql, the command tag will not actually be displayed, since psql dis-
plays the fetched rows instead.
NOTES
The cursor should be declared with the SCROLL option if one intends to
use any variants of FETCH other than FETCH NEXT or FETCH FORWARD with
a positive count. For simple queries PostgreSQL will allow backwards
fetch from cursors not declared with SCROLL, but this behavior is best
not relied on. If the cursor is declared with NO SCROLL, no backward
fetches are allowed.
ABSOLUTE fetches are not any faster than navigating to the desired row
with a relative move: the underlying implementation must traverse all
the intermediate rows anyway. Negative absolute fetches are even
worse: the query must be read to the end to find the last row, and
then traversed backward from there. However, rewinding to the start of
the query (as with FETCH ABSOLUTE 0) is fast.
Updating data via a cursor is currently not supported by PostgreSQL.
DECLARE [declare(7)] is used to define a cursor. Use MOVE [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 in embedded SQL only. This
variant of FETCH described here returns the data as if it were a
SELECT result rather than placing it in host variables. Other than
this point, FETCH is fully upward-compatible with the SQL standard.
The FETCH forms involving FORWARD and BACKWARD, as well as the forms
FETCH count and FETCH ALL, in which FORWARD is implicit, are Post-
greSQL extensions.
The SQL standard allows only FROM preceding the cursor name; the
option to use IN is an extension.
SQL - Language Statements 2008-01-03 FETCH()
