如何将PostgreSQL的“ erge_db”（又名upsert）功能转换为MySQL

小编典典

如何将PostgreSQL的“ erge_db”（又名upsert）功能转换为MySQL

sql

CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
    LOOP
        -- first try to update the key
        UPDATE db SET b = data WHERE a = key;
        IF found THEN
            RETURN;
        END IF;
        -- not there, so try to insert the key
        -- if someone else inserts the same key concurrently,
        -- we could get a unique-key failure
        BEGIN
            INSERT INTO db(a,b) VALUES (key, data);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
            -- Do nothing, and loop to try the UPDATE again.
        END;
    END LOOP;
END;
$$
LANGUAGE plpgsql;

SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');

可以在MySQL中将其表示为用户定义的函数吗？如果可以，该如何表达？会比MySQL的标准有优势INSERT...ON DUPLICATE KEY UPDATE吗？

注意：我专门在寻找用户定义的函数，而不是INSERT...ON DUPLICATE KEY UPDATE。

阅读 187

2021-04-22

共1个答案

小编典典

在MySQL 5.5.14上测试。

CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

DELIMITER //
CREATE PROCEDURE merge_db(k INT, data TEXT) 
BEGIN
    DECLARE done BOOLEAN;
    REPEAT
        BEGIN
            -- If there is a unique key constraint error then 
            -- someone made a concurrent insert. Reset the sentinel
            -- and try again.
            DECLARE ER_DUP_UNIQUE CONDITION FOR 23000;
            DECLARE CONTINUE HANDLER FOR ER_DUP_UNIQUE BEGIN
                SET done = FALSE;
            END;

            SET done = TRUE;
            SELECT COUNT(*) INTO @count FROM db WHERE a = k;
            -- Race condition here. If a concurrent INSERT is made after
            -- the SELECT but before the INSERT below we'll get a duplicate
            -- key error. But the handler above will take care of that.
            IF @count > 0 THEN 
                UPDATE db SET b = data WHERE a = k;
            ELSE 
                INSERT INTO db (a, b) VALUES (k, data);
            END IF;
        END;
    UNTIL done END REPEAT;
END//

DELIMITER ;

CALL merge_db(1, 'david');
CALL merge_db(1, 'dennis');

一些想法：

您不能先进行更新，然后再检查，@ROW_COUNT()因为它会返回实际更改的行数。如果该行已经具有您要更新的值，则该值为0。
另外，@ROW_COUNT()复制也不安全。
您可以使用REPLACE...INTO。
如果使用InnoDB或具有事务支持的表，则可以使用SELECT...FOR UPDATE（未测试）。

与仅使用相比，我认为此解决方案没有任何优势INSERT...ON DUPLICATE KEY UPDATE。

2021-04-22