mysql - 不满足条件时 SELECT 语句应返回 0

Question

如果不满足条件，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'，则预期输出为: b.ber = 0 的结果如下所示:
接收数据
416342016
433004544
...
就我而言，如果它只返回一个条目就足够了:
接收数据
0

该语句嵌入到计算一些吞吐量的较大语句中。

是否可以不执行该语句两次？

Answer 1

这里我们将其更改为 LEFT OUTER JOIN。我们将把 B.ber = '0' 条件移到 FROM 子句中，因为使用外连接，这将给我们带来与 WHERE 子句中的条件不同的结果。

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，它将充当内连接。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 表的所有列的结果为 NULL。因此，这些记录的 B.trblksize 将为 NULL，并且它们的总和将为 NULL。