Looping Over Result Sets in MySQL
This article explores the concept of looping over result sets in MySQL, a fundamental operation in database programming. We aim to emulate the functionality of the following PHP code:
$result = mysql_query("SELECT something FROM somewhere WHERE some stuff");
while ($row = mysql_fetch_assoc($result)) {
// check values of certain fields, decide to perform more queries, or not
// tack it all into the returning result set
}
Cursor-Based Approach
MySQL provides cursors for iterating over result sets. However, it lacks capabilities like FOR EACH ROW syntax for trigger implementations. Despite this, it is possible to create a cursor-based loop using the following approach:
SET @S = 1;
LOOP
SELECT * FROM somewhere WHERE some_conditions LIMIT @S, 1
-- IF NO RESULTS THEN
LEAVE
-- DO SOMETHING
SET @S = @S + 1;
END LOOP
Stored Procedure with Cursor
A more structured way to implement this logic is through a stored procedure that employs a cursor. Here is an example:
CREATE PROCEDURE GetFilteredData()
BEGIN
DECLARE bDone INT;
DECLARE var1 CHAR(16); -- or approriate type
DECLARE var2 INT;
DECLARE var3 VARCHAR(50);
DECLARE curs CURSOR FOR SELECT something FROM somewhere WHERE some stuff;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET bDone = 1;
DROP TEMPORARY TABLE IF EXISTS tblResults;
CREATE TEMPORARY TABLE IF NOT EXISTS tblResults (
--Fld1 type,
--Fld2 type,
--...
);
OPEN curs;
SET bDone = 0;
REPEAT
FETCH curs INTO var1, var2, var3;
IF whatever_filtering_desired
-- here for whatever_transformation_may_be_desired
INSERT INTO tblResults VALUES (var1, var2, var3);
END IF;
UNTIL bDone END REPEAT;
CLOSE curs;
SELECT * FROM tblResults;
END
Considerations
While cursors can be helpful for implementing complex business rules, they come with certain drawbacks. Therefore, it is recommended to consider alternative methods, such as expressing the desired transformations and filters in a single, declarative SQL query.
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