如果语句为 1，则运行特定的 SQL 语句

Question

我试图学习 MySQL 存储过程，但我陷入了 If..Then 问题。 我正在尝试弄清楚如何显示列对 IF 结果的影响。

这是我的代码，希望你明白我想要做什么，我的问题是在“...”和“...”之间：

CREATE DEFINER=`root`@`localhost` PROCEDURE `sp_helloworld`()
BEGIN

    SELECT
        ...
        IF pt.is_part_payment = 1 THEN
            pp.due_date AS due_date,
            pp.amount AS amount
        ELSE
            o.due_date AS due_date,
            o.amount AS amount
        END IF;
        ...
    FROM
        orders o
        INNER JOIN payment_types pt ON pt.id = o.payment_type_id 
        INNER JOIN part_payments pp ON pp.order_id = o.order_id
    WHERE
        o.active = 1
END

Answer 1

如您所见，您不能在 SQL 查询中间放置 if 语句。但是，您可以使用 case 表达式，尽管您必须为要单独查询的每个字段执行此操作：

SELECT
    ...
    CASE pt.is_part_payment WHEN 1 THEN pp.due_date ELSE o.due_date END
        AS due_date,
    CASE pt.is_part_payment WHEN 1 THEN pp.amount ELSE o.amount END AS amount,
...
FROM
    orders o
    INNER JOIN payment_types pt ON pt.id = o.payment_type_id 
    INNER JOIN part_payments pp ON pp.order_id = o.order_id
WHERE
    o.active = 1