MySQL触发器更新动态列

Question

I have a table that i need to update every time something is inserted in another table.

table to be updated: counters on insert in sales . the problem is the column that will be updated in counters differs based on the insert in the table sales .

I have been looking around and searching since yesterday, but to no avail. i tried using the CAST(.. AS CHAR) function but to no avail.

this is what the statement should look like:

CREATE TRIGGER `sales_AFTER_INSERT` AFTER INSERT ON `sales` FOR EACH ROW
BEGIN
    UPDATE counters SET {NEW.sale_provider} = {NEW.sale_provider} + NEW.sale_quantity WHERE counters.cnt_sendername = NEW.sale_sendername;
END;

where the {New.sale_provider} is the name of the column to be updated.

counter table looks like:

| sendername | prov1 | prov2 | prov3 | ...

Where column names (prov1,prov2..) are the id of the providers

Is this possible? how can I do this? I am new to triggers and still lack the deeper understanding of SQL language and MySQL's capabilities.

Notes:

• There are 4 columns as a start since there are 4 providers, the counter table serves a quick access to limits instead of calculating it every time, this will be used quite a lot and recalculating it every time is very resources consuming.
• With every new provider added a new column will be added to the counter table, it should be updateable as well, thus some form of a variable is needed.

your help is most appreciated!

EDIT:

The point is that the updated column will be different every time, I know this is unconventional but i believe it is doable non the less. The new data inserted into sales will affect a column in counter , but which column it will effect is what needs to be determined.

After looking for a while i found something which can solve what i need, but still can't get it to run:

CREATE TRIGGER `sales_AFTER_INSERT` AFTER INSERT ON `sales` FOR EACH ROW
BEGIN
    SET colname= New.sale_provider;
    UPDATE counters SET colname = colname + NEW.sale_quantity WHERE counters.cnt_sendername = NEW.sale_sendername;
END;

but it tells me that colname is not a system variable? I have been searching for how to assign a variable and they all look like what i did, what is wrong?

Answer 1

You can do what you want with a single update, using a condition on each column:

CREATE TRIGGER `sales_AFTER_INSERT` AFTER INSERT ON `sales` FOR EACH ROW
BEGIN
    UPDATE counters c
        SET prov1 = c.prov1  + (case when NEW.sale_provider = 'prov1' THEN NEW.sale_quantity else 0 end),
            prov2 = c.prov2  + (case when NEW.sale_provider = 'prov2' THEN NEW.sale_quantity else 0 end),
            prov3 = c.prov3  + (case when NEW.sale_provider = 'prov3' THEN NEW.sale_quantity else 0 end),
            prov4 = c.prov4 + (case when NEW.sale_provider = 'prov4' THEN NEW.sale_quantity else 0 end)         
        WHERE c.cnt_sendername = NEW.sale_sendername;
END;

I would also recommend that you change the table to have one row per provider. That way, you don't have to change the trigger to add new providers:

CREATE TRIGGER `sales_AFTER_INSERT` AFTER INSERT ON `sales` FOR EACH ROW
BEGIN
    UPDATE counters c
        SET quantity = c.quantity  + NEW.sale_quantity
        WHERE c.cnt_sendername = NEW.sale_sendername AND
              c.provider = NEW.sale_provider
END;

This makes querying the table as a cross-tab a little bit harder, but that is not a big deal.

Both of these assume that counters is pre-initialized with all the rows it needs. This is probably not reasonable. So, you should create a unique index on sales_after_insert(cnt_sendername, provider) and use insert on duplicate key :

CREATE TRIGGER `sales_AFTER_INSERT` AFTER INSERT ON `sales` FOR EACH ROW
BEGIN
    INSERT INTO counters(cnt_sendername, provider, quantity)
         VALUES (NEW.sale_sendername, NEW.sale_provider, NEW.sale_quantity)
         ON DUPLICATE KEY UPDATE quantity = quantity + VALUES(quantity);
END;