Bagaimana untuk Cekap Gelung Melalui Set Keputusan dalam MySQL: Kursor atau Jadual Sementara?

Looping Over Result Sets in MySQL: Using Cursors and Temporary Tables

When writing stored procedures in MySQL, it is often necessary to loop over the results of a select query and perform additional operations or conditional branching based on the data. While cursors provide a straightforward way to iterate over result sets, they are not always the most efficient or practical solution.

In this specific scenario, the goal is to select data from several tables based on specific criteria and then make decisions based on the retrieved values. One approach would be to use a cursor to fetch rows one at a time and perform the necessary checks, but a more efficient alternative is to employ a cursor in conjunction with a temporary table.

Here's a modified version of the MySQL procedure that utilizes a cursor and a temporary table:

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

This procedure employs a cursor to fetch rows from the initial select query and then checks for a specific condition. If the condition is met, the values are inserted into a temporary table named tblResults. After iterating over all the rows, the procedure selects the filtered data from the temporary table.

While cursors can be useful in certain scenarios, it is important to consider their limitations. They can introduce performance bottlenecks and are generally more difficult to work with compared to declarative SQL. If possible, it is recommended to avoid using cursors and instead explore alternative solutions such as using window functions, common table expressions (CTEs), or stored functions.

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