MySQL：從僞視圖表更新主表

Question

我有一個主表，比如說tbl，它有三列。假設我創建了一個表格，該表格僅顯示關於col1和col2的組合的唯一第一條記錄。這種表的SQL定義如下：

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

現在我看到的col1和col2所有獨特的組合，在這個新表。我更新了這個表格，以便對給定記錄的col3進行更改。我希望通過影響所有重複記錄，將其應用回主表tbl。

我知道這是不可能的標準視圖。然而，有沒有辦法創建一個從屬表，當它被更新時，主表將按照select機制的定義進行更新？我希望有一個解決方案，這個僞視圖表可以自由地更新和刪除其記錄，同時也影響父表。

Answer 1

它將與在pseudo_view一些觸發器和TBL

插入，更新工作，TBL「查看」刪除需要進行相應的更新。通常這很容易。唯一困難的部分是當你刪除最後一對col1和col2時。

對於pseudo_table上的更新和刪除，您需要定義在主服務器上應該更改的內容。

在我的例子中，我選擇min（3）作爲函數，所以當一行更新時，同一行（曾經擁有最小col3的那行）更新，並且一個新的（可能是相同的）計算可消費的價值。

當一行被刪除時，tbl中的所有行都需要去，因爲它們本來會在pseudo中給出一行（並且剛被刪除）。

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//