MySQL函数 如何对select查询应用多个条件

我有一个MySQL函数，可以检索每个客户的库存.该函数运行得很好，但最近我需要向函数添加第三个参数.

My current working function:

CREATE FUNCTION getStock (productId INT, customerId INT) RETURNS INT(11)
BEGIN 
    DECLARE qty INT(11) DEFAULT 0;
    
    IF (customerId = 0)
        THEN
            SELECT (......) INTO qty 
            WHERE product_id = productId;
        ELSE
            SELECT (...) INTO QTY 
            WHERE product_id = productId AND customer_id = customerId;
    ENDIF;
    
    RETURN qty;
END;

问题是，现在我需要重构该查询并使用三个参数.我想申请一个更好的条件，比如:

CREATE FUNCTION getStock (productId INT, forceCustomerId INT, ignoreCustomerId INT) RETURNS INT(11)
BEGIN                 
    DECLARE qty INT(11) DEFAULT 0;
    DECLARE query VARCHAR(255);

    // Something like this
    SET query = SELECT (...) INTO qty
        WHERE product_id = productId;

    IF (forceCustomerId IS NOT NULL and forceCustomerId != 0)
        SET query &= AND customer_id = forceCustomerId;

    IF (ignoreCustomerId IS NOT NULL and ignoreCustomerId != 0)
        SET query &= AND customer_id != ignoreCustomerId;
        
    RETURN qty;
END;

我怎么才能做到这样的事情呢？