Question

**下面给出的是更新查询：表之间的关系：

要订购的card_order-多对一。
订购给客户-一对一。

收费客户-一对多。设置该值时，即使when子句满足，case表达式也始终求值为表达式的else。请帮助

update card_orders cbi
    join orders ord on cbi.card_order_no = ord.order_no
    join customers cust on cust.id = ord.customerid 
    join fees fee on fee.customerid = cust.id 
            and active = 1
set cbi.fee1 =  
    CASE WHEN fee.id = 4 and fee.amount > 0 THEN fee.amount 
    ELSE 'NO FEE' END   
where cbi.card_order_no = 10250;

// fee.amount是mysql的十进制数据类型设置该值时，即使when子句满足，case表达式也始终求值为表达式的else。更新还为订单表中的某些列设置了值

更新查询将在存储过程中使用 **

Answer 1

使用最后一对多的客户收费服务，您就无法真正控制使用哪个收费记录（UPDATE只更新每个记录一次，因此多个匹配项不会分别更新；即使它们匹配这样，以后的匹配将覆盖以前的更改。

您可以加入费用，添加案件中的条件...

UPDATE card_orders AS cbi
    INNER JOIN orders AS ord ON cbi.card_order_no = ord.order_no
    INNER JOIN customers AS cust ON cust.id = ord.customerid 
    LEFT JOIN fees AS fee1 
            ON fee1.customerid = cust.id AND fee1.active = 1
            AND fee1.id = 4 AND fee1.amount > 0
    LEFT JOIN fees AS feeX 
            ON feeX.customerid = cust.id AND feeX.active = 1
            AND feeX.id = Y AND feeX.amount > 0 
            -- or whatever feeX's case conditions would have been
SET cbi.fee1 = CASE WHEN fee1.customerid IS NOT NULL THEN fee1.amount ELSE 'NO FEE' END
  , cbi.feeX = CASE WHEN feeX.customerid IS NOT NULL THEN feeX.amount ELSE 'NO FEE' END
WHERE cbi.card_order_no = 10250
;

此版本在语义上略有不同。您的原始版本只会更新具有至少一个费用条目链接的card_orders条目；此版本将包含没有任何费用链接的条目。

mysql-更新，多个表联接，从case表达式设置值

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