如何将PostgreSQL "merge_db“(又称upsert)函数转换为MySQL

Question

直接从手册中获取，这里是canonical example of merge_db in PostgreSQL

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');

在MySQL中可以将其表示为用户定义的函数吗?如果可以，如何表示？与MySQL的标准INSERT...ON DUPLICATE KEY UPDATE相比有什么优势吗？

注意:我特别想找一个用户定义的函数，而不是INSERT...ON DUPLICATE KEY UPDATE。

Answer 1

已在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：

• 您不能先执行更新，然后再检查@ROW_COUNT()，因为它会返回实际更改的行数。如果行已经具有您尝试更新的值，则此值可能为0。
• 此外，@ROW_COUNT()不是复制安全的。
• 您可以使用REPLACE...INTO.
• If

InnoDB或具有事务支持的表，您可以使用SELECT...FOR UPDATE (未测试)。

与仅仅使用INSERT...ON DUPLICATE KEY UPDATE相比，我看不出这种解决方案有什么优势。