Question

我有一张coupons表和一张名为coupon_uses的第二张表。 coupons具有max_uses整数属性，用于限制优惠券的使用次数。每次使用优惠券时，都会创建coupon_uses条记录。

------------   ---------------
| coupons  |   | coupon_uses |
|----------|   |-------------|
| id       |   | coupon_id   |
| code     |   | ...         |
| max_uses |   ---------------
-----------

我如何构建一个查询，根据特定优惠券有多少相应的优惠券？我可以过滤优惠券表？例如，我希望使用的所有优惠券的次数少于其max_uses属性允许的次数。

Answer 1

这应该可以满足您的需求;

SELECT  c.max_uses ,
    c.id ,
    c.code ,
    COUNT(coupon_id) AS 'uses'
FROM    coupon_uses cu
    LEFT JOIN coupons c ON c.ID = cu.ID
GROUP BY c.max_uses ,
    c.id ,
    c.code    
  Having c.max_uses > count(coupon_id)

Answer 2

---  The coupons which are already used to its max limit
SELECT  count(cu.*), cp.id, cp.max_uses 
FROM coupons cp
LEFT JOIN coupon_uses cu ON cp.id = cu.coupon_id
GROUP BY cp.id, cp.max_uses
HAVING  count(cu.*) =  cp.max_uses

--  The coupons which are remaining 
SELECT  count(cu.*), cp.id, cp.max_uses , cp.max_uses - count(cu.*) AS remaing_usage
FROM coupons cp
LEFT JOIN coupon_uses cu ON cp.id = cu.coupon_id
GROUP BY cp.id, cp.max_uses
HAVING  count(cu.*) <  cp.max_uses

Answer 3

具有0次使用的优惠券在左连接中将获得1的计数。因此，我使用UNION来计算使用过的优惠券，而不是单独使用。

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    __block Boolean self_terminate = YES;

    _backgroundTask = [[UIApplication sharedApplication] beginBackgroundTaskWithName:@"DummyTask" expirationHandler:^{
        NSLog(@"Background task expired by iOS");
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            _backgroundTask = UIBackgroundTaskInvalid;
        }
    }];

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        NSLog(@"Background task started");

        while (true) {
            NSLog(@"background time remaining: %8.2f", [UIApplication sharedApplication].backgroundTimeRemaining);
            [NSThread sleepForTimeInterval:1];
        }

    });
}

如何在另一个表中按计数按组筛选一个表

3 个答案: