Question

我有一个主表，比如tbl，它有三列。假设我创建了一个表格，该表格仅显示与col1和col2组合相关的第一条记录。这种表的SQL定义如下：

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

现在，我在此新表中看到了col1和col2的所有独特组合。我更新此表，以便更改给定记录的col3。我希望通过影响所有重复记录将其应用回主表tbl。

我知道标准视图无法做到这一点。但是，有没有办法创建一个从属表，当它更新时，主表将按照选择机制的定义进行更新？我希望有一个解决方案，这个伪视图表可以自由更新和删除其记录，同时也影响父表。

Answer 1

它可以在pseudo_view和tbl

上使用一些触发器

对于tbl上的插入，更新和删除，“视图”需要相应地更新。通常这很容易。唯一困难的部分是当你删除最后一对col1和col2时。

对于pseudo_table的更新和删除，您需要定义主服务器上应该更改的内容。

在我的示例中，我选择将min（3）作为函数，因此当更新行时，同一行（曾经具有最小col3的行）更新并且新（可能是相同的）值对于psuedotable计算。

当一行被删除时，tbl中的所有行都需要去，否则它们也会伪造一行（并且刚被删除）。

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//

MySQL：从伪视图表更新主表

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