Positions, or re-positions, a cursor to a specific row, and then copies expression values from that row into variables accessible from within the stored procedure or application.
Syntax 1 [SP]FETCH [ cursor-position ] cursor-name INTO variable-list [ FOR UPDATE ]
Syntax 2 [ESQL]FETCH [ cursor-position ] cursor-name [ INTO { hostvar-list | variable-list } | USING [ SQL ] DESCRIPTOR sqlda-name ] [ PURGE ] [ BLOCK n ] [ FOR UPDATE ] [ ARRAY fetch-count ]
cursor-position :
NEXT | PRIOR | FIRST | LAST
| { ABSOLUTE | RELATIVE } row-count
row-count : number or hostvar
cursor-name : identifier or hostvar
hostvar-list : may contain indicator variables
variable-list : stored procedure variables
sqlda-name : identifier
fetch-count : integer or hostvar
ParametersINTO clause The INTO clause is optional. If it is not specified, the FETCH statement positions the cursor only. The hostvar-list is for embedded SQL use only.
cursor position An optional positional parameter allows the cursor to be moved before a row is fetched. If not specified, NEXT is assumed. If the fetch includes a positioning parameter and the position is outside the allowable cursor positions, the SQLE_NOTFOUND warning is issued and the SQLCOUNT field indicates the offset from a valid position.
The OPEN statement initially positions the cursor before the first row.
NEXT clause Next is the default positioning, and causes the cursor to be advanced one row before the row is fetched.
PRIOR clause Causes the cursor to be backed up one row before fetching.
RELATIVE clause RELATIVE positioning is used to move the cursor by a specified number of rows in either direction before fetching. A positive number indicates moving forward and a negative number indicates moving backward. So, a NEXT is equivalent to RELATIVE 1 and PRIOR is equivalent to RELATIVE -1. RELATIVE 0 retrieves the same row as the last fetch statement on this cursor.
ABSOLUTE clause The ABSOLUTE positioning parameter is used to go to a particular row. A zero indicates the position before the first row. See Using cursors in procedures and triggers.
A one (1) indicates the first row, and so on. Negative numbers are used to specify an absolute position from the end of the cursor. A negative one (-1) indicates the last row of the cursor.
LAST clause A short form for ABSOLUTE -1.
Inserts and some updates to DYNAMIC SCROLL cursors can cause problems with cursor positioning. The database server does not put inserted rows at a predictable position within a cursor unless there is an ORDER BY clause on the SELECT statement. Sometimes the inserted row does not appear until the cursor is closed and opened again.
This occurs if a temporary table had to be created to open the cursor. For a description, see Use work tables in query processing (use All-rows optimization goal).
The UPDATE statement may cause a row to move in the cursor. This will happen if the cursor has an ORDER BY that uses an existing index (a temporary table is not created).
BLOCK clause Rows may be fetched by the client application more than one at a time. This is referred to as block fetching, prefetching, or multi-row fetching. The first fetch causes several rows to be sent back from the database server. The client buffers these rows, and subsequent fetches are retrieved from these buffers without a new request to the database server.
The BLOCK clause is for use in embedded SQL only. It gives the client and server a hint about how many rows may be fetched by the application. The special value of 0 means the request is sent to the database server and a single row is returned (no row blocking). The BLOCK clause will reduce the number of rows included in the next prefetch to the BLOCK value. To increase the number of rows prefetched, use the PrefetchRows connection parameter.
If you do not specify a BLOCK clause, the value specified on OPEN is used. See OPEN statement [ESQL] [SP].
FETCH RELATIVE 0 always re-fetches the row.
If prefetch is disabled for the cursor, the BLOCK clause is ignored and rows are fetched one at a time. If ARRAY is also specified, then the number of rows specified by ARRAY are fetched.
PURGE clause The PURGE clause is for use in embedded SQL only. It causes the client to flush its buffers of all rows, and then send the fetch request to the database server. Note that this fetch request may return a block of rows.
FOR UPDATE clause The FOR UPDATE clause indicates that the fetched row will subsequently be updated with an UPDATE WHERE CURRENT OF CURSOR statement. This clause causes the database server to put an intent lock on the row. The lock is held until the end of the current transaction. See How locking works and the FOR UPDATE clause of the SELECT statement.
ARRAY clause The ARRAY clause is for use in embedded SQL only. It allows so-called wide fetches, which retrieve more than one row at a time, and which may improve performance.
To use wide fetches in embedded SQL, include the fetch statement in your code as follows:
EXEC SQL FETCH ... ARRAY nnn |
where ARRAY nnn is the last item of the FETCH statement. The fetch count nnn can be a host variable. The SQLDA must contain nnn * (columns per row) variables. The first row is placed in SQLDA variables 0 to (columns per row)-1, and so on.
For a detailed example of using wide fetches, see Fetching more than one row at a time.
RemarksThe FETCH statement retrieves one row from the named cursor. The cursor must have been previously opened.
Embedded SQL use A DECLARE CURSOR statement must appear before the FETCH statement in the C source code, and the OPEN statement must be executed before the FETCH statement. If a host variable is being used for the cursor name, the DECLARE statement actually generates code and must be executed before the FETCH statement.
The server returns in SQLCOUNT the number of records fetched, and always returns a SQLCOUNT greater than zero unless there is an error or warning.
If the SQLSTATE_NOTFOUND warning is returned on the fetch, the sqlerrd[2] field of the SQLCA (SQLCOUNT) contains the number of rows by which the attempted fetch exceeded the allowable cursor positions. The value is 0 if the row was not found but the position is valid; for example, executing FETCH RELATIVE 1 when positioned on the last row of a cursor. The value is positive if the attempted fetch was beyond the end of the cursor, and negative if the attempted fetch was before the beginning of the cursor. The cursor is positioned on the last row if the attempted fetch was beyond the end of the cursor, and on the first row if the attempted fetch was before the beginning of the cursor.
After successful execution of the fetch statement, the sqlerrd[1] field of the SQLCA (SQLIOCOUNT) is incremented by the number of input/output operations required to perform the fetch. This field is actually incremented on every database statement.
Single row fetch One row from the result of the SELECT statement is put into the variables in the variable list. The correspondence is one-to-one from the select list to the host variable list.
Multi-row fetch One or more rows from the result of the SELECT statement are put into either the variables in variable-list or the program data areas described by sqlda-name. In either case, the correspondence is one-to-one from the select-list to either the hostvar-list or the sqlda-name descriptor array.
PermissionsThe cursor must be opened, and the user must have SELECT permission on the tables referenced in the declaration of the cursor.
Side effectsA FETCH statement may cause multiple rows to be retrieved from the server to the client if prefetching is enabled. See prefetch option.
See also Standards and compatibilitySQL/2008 With minor exceptions, Syntax 1 of the FETCH statement is a core feature of the SQL/2008 standard. Scrolling options other than NEXT constitute optional SQL language feature F431, "Read-only scrollable cursors". SQL Anywhere does not support the optional FROM clause of the FETCH statement as documented in the SQL/2008 standard.
Syntax 2 is a vendor extension.
The FOR UPDATE, PURGE, ARRAY, BLOCK, and USING [SQL] DESCRIPTOR clauses are vendor extensions.
ExampleThe following is an embedded SQL example:
EXEC SQL DECLARE cur_employee CURSOR FOR SELECT EmployeeID, Surname FROM Employees; EXEC SQL OPEN cur_employee; EXEC SQL FETCH cur_employee INTO :emp_number, :emp_name:indicator; |
The following is a procedure example:
BEGIN
DECLARE cur_employee CURSOR FOR
SELECT Surname
FROM Employees;
DECLARE name CHAR(40);
OPEN cur_employee;
lp: LOOP
FETCH NEXT cur_employee into name;
IF SQLCODE <> 0 THEN LEAVE lp END IF;
...
END LOOP;
CLOSE cur_employee;
END |
 |
Discuter à propos de cette page dans DocCommentXchange.
|
Copyright © 2010, iAnywhere Solutions, Inc. - SQL Anywhere 12.0.0 |