I usually work with Oracle database and when creating stored procedures one can write cursors where the where clause can have a variable, value of which can be provided at run time.
How do you write something similar in mySQL
Something like
DECLARE myCursor cursor select col1 from table1 where col2 = &1;
OPEN myCursor ("NEW");
You may use user-defined variable and/or local variable (including procedure parameters), both assigned externally and calculated internally, in the cursor definition:
CREATE PROCEDURE test_proc( {parameters} )
BEGIN
DECLARE _id_ INT;
DECLARE cur CURSOR FOR
SELECT id FROM test WHERE val = {variable};
DECLARE EXIT HANDLER FOR NOT FOUND
BEGIN
CLOSE cur;
END;
OPEN cur;
LOOP
FETCH cur INTO _id_;
SELECT _id_;
END LOOP;
END
But you cannot to alter the parameter "on the fly" - after cursor is opened the changes in its parameters will be iglored (during the opening the cursor's text is fixed, the values instead of names are used in fixed text).
Hence when you need to alter dynamically assigned parameter then you must close and reopen the cursor.
DEMO fiddle
Related
I am trying to use a cursor in MySQL to call a stored procedure many times. I want to call it as many times as a value for my_id exists in some temporary table, and iterate through those ids and concatenate the results.
Anyway, I'm having trouble with this part of the process:
DECLARE curs CURSOR FOR
SELECT something FROM somewhere;
I don't want to select something from somewhere. I want something like
DECLARE curs CURSOR FOR
CALL storedproc(#an_id);
Can the DECLARE statement be used to call a stored procedure? Or does it have to be associated with a SELECT only? Googling around, I'm afraid that the latter is the case.
Using a cursor requires some standard boilerplate code to surround it.
Using a cursor to call a stored procedure for each set of values from the table requires essentially the same boilerplate. You SELECT the values you want to pass, from wherever you're getting them (which could be a temporary table, base table, or view, and can include calls to stored functions) and then call the procedure with those values.
I've written an syntactically valid example of that boilerplate code, below, with comments to explain what each component is doing. There are few things I dislike more than being asked to just do something "just because" -- so everything is (hopefully) explained.
You mentioned calling the procedure with multiple values, so this example uses 2.
Note that there events that happen her are in a specific order for a reason. Variables have to be declared first, cursors have to be declared before their continue handlers, and loops have to follow all of those things. This gives an impression that there's some fairly extreme inflexibility, here, but that's not really the case. You can reset the ordering by nesting additional code inside BEGIN ... END blocks within the procedure body; for example, if you needed a second cursor inside the loop, you'd just declare it inside the loop, inside another BEGIN ... END.
DELIMITER $$
DROP PROCEDURE IF EXISTS `my_proc` $$
CREATE PROCEDURE `my_proc`(arg1 INT) -- 1 input argument; you might not need one
BEGIN
-- from http://stackoverflow.com/questions/35858541/call-a-stored-procedure-from-the-declare-statement-when-using-cursors-in-mysql
-- declare the program variables where we'll hold the values we're sending into the procedure;
-- declare as many of them as there are input arguments to the second procedure,
-- with appropriate data types.
DECLARE val1 INT DEFAULT NULL;
DECLARE val2 INT DEFAULT NULL;
-- we need a boolean variable to tell us when the cursor is out of data
DECLARE done TINYINT DEFAULT FALSE;
-- declare a cursor to select the desired columns from the desired source table1
-- the input argument (which you might or might not need) is used in this example for row selection
DECLARE cursor1 -- cursor1 is an arbitrary label, an identifier for the cursor
CURSOR FOR
SELECT t1.c1,
t1.c2
FROM table1 t1
WHERE c3 = arg1;
-- this fancy spacing is of course not required; all of this could go on the same line.
-- a cursor that runs out of data throws an exception; we need to catch this.
-- when the NOT FOUND condition fires, "done" -- which defaults to FALSE -- will be set to true,
-- and since this is a CONTINUE handler, execution continues with the next statement.
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
-- open the cursor
OPEN cursor1;
my_loop: -- loops have to have an arbitrary label; it's used to leave the loop
LOOP
-- read the values from the next row that is available in the cursor
FETCH NEXT FROM cursor1 INTO val1, val2;
IF done THEN -- this will be true when we are out of rows to read, so we go to the statement after END LOOP.
LEAVE my_loop;
ELSE -- val1 and val2 will be the next values from c1 and c2 in table t1,
-- so now we call the procedure with them for this "row"
CALL the_other_procedure(val1,val2);
-- maybe do more stuff here
END IF;
END LOOP;
-- execution continues here when LEAVE my_loop is encountered;
-- you might have more things you want to do here
END $$
DELIMITER ;
Can the DECLARE statement be used to call a stored proc?
Not possible and documentation is pretty clear on that
Cursor DECLARE Syntax
This statement declares a cursor and associates it with a SELECT statement that retrieves the rows to be traversed by the cursor. To fetch the rows later, use a FETCH statement. The number of columns retrieved by the SELECT statement must match the number of output variables specified in the FETCH statement.
I have built a MySQL procedure.
i have to define v_table variable within procedure
Who can help me ?
Thanks
Paolo
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
open cur1;
igmLoop: loop
fetch cur1 into v_column,v_table;
IF done THEN
LEAVE igmLoop;
END IF;
update v_table set v_column=replace(v_column,'Ã ','a`');
end loop igmLoop;
close cur1;
end
Neither table names, nor column names can be dynamic within a stored procedure. You need to assemble the sql statement as a string and execute it using PREPARE, EXECUTE, DEALLICATE PREPARE statements. So, basically, you will create a prepared statement out of your query and execute it.
The linked documentation contains examples as well.
For example , is it ok to do below ?
DECLARE aId VARCHAR(20);
DECLARE cur1 CURSOR FOR SELECT id FROM new_records WHERE is_loaded = false;
read_loop: LOOP
FETCH cur1 INTO aId;
...
update new_records set is_loaded = True where id = aId ;
...
CLOSE cur1;
END
Cursors in MySQL are ASENSITIVE (13.6.6 Cursors).
An INSENSITIVE Cursor is a Cursor that effectively causes a separate
copy of its result Table to be created; the Cursor accesses that copy,
rather than the original result, so any changes made to the original
result by other methods won't be visible to this Cursor. A SENSITIVE
Cursor is a Cursor that works directly on its result Table: it makes
no copy, so other changes made to the result Table will be visible to
this Cursor. An ASENSITIVE Cursor may or may not make a copy of its
result Table; whether other changes to its result Table will be
visible is implementation-defined. The default is an ASENSITIVE
Cursor.
- From DECLARE CURSOR Statement -
SQL Fiddle demo
However, depending on what you need to do, there are other ways to update the table.
I have a mysql stored procedure made it recursively, it always returned 499 rows max.
my stored procedure is moving in a tree (not a binary tree) and check the nodes if they have children and so on until it reached the leaves.
I don't know how can I convert my code into non-recursive way, I just want to ask for tow points:
how can I make an infinite recursive in mysql(mysql server version is 5.5)?
if that can't happened, how can I change my cod into non-recursive way?
CREATE PROCEDURE `get_citations`(in _pub_id int(10),in _lvl int,citation_count int)
BEGIN
DECLARE done INT DEFAULT FALSE;
declare p_id,c_count int;
declare _counter int default 1;
DECLARE cur1 CURSOR FOR SELECT pat_publn_id,cited_count from temp.a_citations
where pub_parent=_pub_id ;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
insert into a_citations
(pat_publn_id ,
publn_nr ,
publn_kind,
publn_auth,
publn_date,
cited_pat_publn_id,
cited_count,
pub_lvl,
pub_parent)
(select p.pat_publn_id,p.publn_nr,p.publn_kind,p.publn_auth,p.publn_date,c.cited_pat_publn_id,
(select count(*) as cnt FROM patstat1304.tls212_citation c2 where c2.cited_pat_publn_id=c.pat_publn_id) as cited_count,_lvl as pub_lvl,_pub_id as pub_parent
from patstat1304.tls212_citation c,patstat1304.tls211_pat_publn p
where c.pat_publn_id=p.pat_publn_id and c.cited_pat_publn_id=_pub_id);
commit;
OPEN cur1;
read_loop: LOOP
fetch cur1 into p_id,c_count;
IF (c_count !=0) then
call get_citations( p_id,_lvl+1,c_count);
commit;
END if;
IF done THEN
LEAVE read_loop;
END IF;
set _counter=_counter+1;
if(_counter=citation_count) then
LEAVE read_loop;
end if;
end loop;
CLOSE cur1;
END
MySQL can not execute stored procedures with a very deep nesting.
Very soon, error ER_STACK_OVERRUN_NEED_MORE will appear.
Increasing the thread stack to go further will not work either.
To change the recursive call to a non recursive one, consider something like this:
1) Create a table named publications_to_process, with the publication and search level.
2) To start the search, insert the original publication in this table, with level 1.
3) In a loop, fetch one publication, examine the citations, and add the publications listed in the publications_to_process, incrementing the level.
4) As a bonus, for cases like:
Pub_1 --> Pub_2 --> Pub_3,
Pub_1 --> Pub_3
there is no need to add Pub_3 again to the search if it has been processed already.
In other words, the publications are more likely to be a directed graph that a tree.
5) Either make the table temporary, or consider adding a PROCESSLIST_ID column, so that different sessions (having a different CONNECTION_ID()) do not step on each other, when executing this search in parallel.
I have got this stored procedure:
DELIMITER //
DROP PROCEDURE IF EXISTS cursor_example//
CREATE PROCEDURE cursor_example()
BEGIN
DECLARE niche_id INT;
DECLARE niche_name VARCHAR(100);
DECLARE curl CURSOR FOR SELECT * FROM `niche`;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done=1;
OPEN curl;
my_loop:LOOP
FETCH curl INTO niche_id,niche_name;
IF done=1 THEN
LEAVE my_loop;
END IF;
END LOOP my_loop;
CLOSE curl;
END//
DELIMITER ;
I want to output everything that the curl fetches. So I thought to put SELECT statement inside or outside the loop..but that wouldnt give me the results that I want. How do I get back all the sql results from that cursor.
And what are the advantages of using cursors compare to other a simple unbound - SELECT statement. I mean I could get the results that I wanted by simply USING a select statement without writing all that cursor code?