Question

I have a master table, say tbl, which has three columns. Say I create a table that only shows the first record that is unique with respect to the combinations of col1 and col2. The SQL for such a table is defined as below:

CREATE TABLE pseudo_view AS SELECT col1, col2, col3 FROM tbl GROUP BY col1 col2

Now I see all unique combinations of col1 and col2 in this new table. I update this table such that col3 for a given record is changed. I want this to apply back to the master table tbl by affecting all duplicate records.

I am aware that this is not possible with a standard view. However, is there a way to create a dependent table, that, when it is updated, the master table will be updated as defined by the select mechanism? I desire a solution such that this pseudo-view table can have its records updated and deleted freely while also affecting the parent table.

Answer 1

It would work with some triggers on the pseudo_view and tbl

For insert, update and delete on tbl the “view” needs to update accordingly. Normally that is easy. The only hard part is when you delete the last pair of col1 and col2.

For update and delete on pseudo_table you need to define what should change on the master.

In my example I’ve choosen to have min(3) as the function so when a row is updated that same row (the one that used to have the smallest col3) is update and a new (possibly the same) value for the psuedotable is calculated.

When a row is deleted all rows in tbl needs to go as they would otherwise have given a row in pseudo as well (and that was just deleted).

CREATE TABLE tbl (col1 INT, col2 INT, col3 INT);
INSERT INTO tbl VALUES (1, 1, 1), (1, 1, 2), (2, 2, 3), (2, 2, 4);

CREATE TABLE pseudo_view
  AS
    SELECT
      col1,
      col2,
      MIN(col3) AS col3
    FROM tbl
    GROUP BY col1, col2;

DELIMITER //

-- update the row that originally was the min() and make sure that the new row in pseudo is the new min()
CREATE TRIGGER upd_pseudo
BEFORE UPDATE ON
  pseudo_view FOR EACH ROW
  BEGIN
    UPDATE tbl
    SET col3 = NEW.col3
    WHERE col1 = OLD.col1
          AND col2 = OLD.col2
          AND col3 = OLD.col3;

    SET NEW.col3 := (SELECT
                       MIN(col3)
                     FROM tbl
                     WHERE col1 = OLD.col1
                           AND col2 = OLD.col2);

  END //

-- delete all rows in tbl that has this col1 and col2
CREATE TRIGGER del_pseudo
AFTER DELETE ON
  pseudo_view FOR EACH ROW
  BEGIN
    DELETE FROM tbl
    WHERE col1 = OLD.col1 AND col2 = OLD.col2;
  END //

-- update pseudo to make sure it's still true
CREATE TRIGGER upd_tbl
AFTER UPDATE ON
  tbl FOR EACH ROW
  BEGIN
    UPDATE pseudo_view
    SET col3 = (SELECT
                  min(col3)
                FROM tbl
                WHERE col1 = NEW.col1 AND col2 = NEW.col2)
    WHERE col1 = NEW.col1 AND col2 = NEW.col2;
  END//

-- update pseudo to make sure it's still true
CREATE TRIGGER ins_tbl
AFTER INSERT ON
  tbl FOR EACH ROW
  BEGIN
    REPLACE pseudo_view
      VALUES (NEW.col1, NEW.col2, (SELECT
                                     min(col3)
                                   FROM tbl
                                   WHERE col1 = NEW.col1 AND col2 = NEW.col2));
  END//

-- update pseudo to make sure it's still true
CREATE TRIGGER del_tbl
AFTER DELETE ON
  tbl FOR EACH ROW
  BEGIN
    -- todo: add special case when the delete row was the last of that col1/col2-pair
    REPLACE pseudo_view
      VALUES (NEW.col1, NEW.col2, (SELECT
                                     min(col3)
                                   FROM tbl
                                   WHERE col1 = NEW.col1 AND col2 = NEW.col2));
  END//