SELECT 语句在不满足条件时应返回 0

Question

如果不满足条件，我如何使以下 MYSQL 语句返回 0。

(SELECT Sum(B.trblksize) AS RxData
 FROM   referencecoexistence_ber_lte B,
        (SELECT C.*
         FROM   referencecoexistence C
         WHERE  `sw_ver` = '0.4'
                AND `lte_n_frames` = '50'
                AND `lte_rb` = '6'
                AND `lte_mcs` = '11'
                AND `lte_p_mw` = '6.000000e-03'
                AND `lte_vmsf_type` = 'BS'
                AND `lte_vmsf_subframes` = '8 9'
                AND `wlan_mcs` = '5'
                AND `wlan_p_mw` = '100'
                AND `channel` = 'A330'
                AND `lte_freq_hz` = '2403000000'
                AND `wlan_freq_hz` = '2412000000'
                AND `wlan_ieee` = '802.11n') AS TableX
 WHERE  TableX.id = B.id
        AND B.ber = '0'
 GROUP  BY lte_dist_m)  

结果是：空集（0.280 秒）如果条件包括 B.ber = '0'，则预期的 output 是：b.ber = 0 的结果如下所示：
接收数据
416342016
433004544
...
在我的情况下，如果它只返回一个条目就足够了：
接收数据
0

该语句嵌入在计算某些吞吐量的较大语句中。

是否可以不执行该语句两次？

Answer 1

在这里，我们将其更改为 LEFT OUTER JOIN。 我们将 B.ber = '0' 条件移动到 FROM 子句中，因为使用外连接，这将给我们带来与条件在 WHERE 子句中不同的结果。

SELECT Sum(B.trblksize) AS RxData
FROM (SELECT C.*
      FROM   referencecoexistence C
      WHERE  `sw_ver` = '0.4'
        AND `lte_n_frames` = '50'
        AND `lte_rb` = '6'
        AND `lte_mcs` = '11'
        AND `lte_p_mw` = '6.000000e-03'
        AND `lte_vmsf_type` = 'BS'
        AND `lte_vmsf_subframes` = '8 9'
        AND `wlan_mcs` = '5'
        AND `wlan_p_mw` = '100'
        AND `channel` = 'A330'
        AND `lte_freq_hz` = '2403000000'
        AND `wlan_freq_hz` = '2412000000'
        AND `wlan_ieee` = '802.11n') AS TableX
  LEFT OUTER JOIN referencecoexistence_ber_lte B
    ON TableX.id = B.id
       AND B.ber = '0'
 GROUP  BY lte_dist_m

现在当 B.ber 为零时，它将像一个内部连接。 但是，当 B.ber 不为零时，则连接中将缺少内连接结果，但外连接将包含 B 表的所有列的结果，但 TableX 表的所有列的 NULL。 所以这些记录的 B.trblksize 为 NULL，它们的总和为 NULL。

Answer 2

我通过简单地使用以下方法解决了这个问题

(SELECT SUM(IF(T_.BER =0,T_.TrBlkSize,0)) as RxData, Lte_Dist_m FROM 
    (ReferenceCoexistence WHERE <CONDITION LIST>) as X, 
    ReferenceCoexistence_BER_Lte as T_ 
WHERE X.ID = T_.ID GROUP BY Lte_Dist_m)