Question

我想根据plt_distinct表中的运输方式来计算labels表中GPS点的数量：

 CREATE TABLE `plt_distinct` (
  `directory` varchar(10) NOT NULL DEFAULT '',
  `latitude` double NOT NULL DEFAULT '0',
  `longitude` double NOT NULL DEFAULT '0',
  `flag` int(11) DEFAULT NULL,
  `altitude` double NOT NULL DEFAULT '0',
  `passeddate` varchar(255) DEFAULT NULL,
  `gpsdate` date ,
  `gpstime` time ,
  `gpsdatetime` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`directory`,`latitude`,`longitude`,`gpsdate`,`gpstime`,`altitude`)
);

CREATE TABLE `labels` (
  `directory` varchar(10) NOT NULL DEFAULT '',
  `starttime` datetime ,  
  `endtime` datetime ,
  `transportationmode` varchar(10) NOT NULL DEFAULT '',
  PRIMARY KEY (`directory`,`starttime`,`endtime`,`transportationmode`)
);

INSERT INTO labels (directory, starttime, endtime, transportationmode) 
VALUES (010, '2007-06-26 11:32:29', '2007-06-26 11:40:29','bus'), 
(010, '2008-03-28 14:52:54', '2008-03-28 15:59:59', 'train'),
(010, '2008-03-29 01:27:50', '2008-03-29 15:59:59', 'train'),
(010, '2008-03-31 16:00:08', '2008-03-31 16:09:01', 'taxi'),
(010, '2008-04-01 00:48:32', '2008-04-01 00:59:23', 'taxi');

INSERT INTO plt_distinct (directory, latitude, longitude, flag, altitude, 
passeddate,gpsdate, gpstime, gpsdatetime)
VALUES (010, 18.254638, 109.500127, 0, 26, 39347.6212037037, '2007-09-22', 
'14:54:32', '2007-09-22 14:54:32'),
(010, 18.254665, 109.500092, 0, 36, 39347.6212268519, '2007-09-22', 
'14:54:34', '2007-09-22 14:54:34'),
(010, 18.254925, 109.499998, 0, 44, 39347.6213078704, '2007-09-22', 
'14:54:41', '2007-09-22 14:54:41'),
(010, 18.255035, 109.499972, 0, 38, 39347.6213310185, '2007-09-22',
'14:54:43', '2007-09-22 14:54:43');

然后我写：

SELECT COUNT(*), CASE( WHEN transportationmode='bus' THEN 'bus', 
                       WHEN transportationmode='car' OR transportationmode='taxi' THEN 'car', 
                       WHEN transportationmode='train' THEN 'train') 
              END AS mode 
FROM plt_distinct JOIN labels USING(directory);

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHEN transportationmode='bus' THEN 'bus', WHEN transportationmode='car' OR trans' at line 1

编辑

我用示例数据生成db-fiddle here。

Answer 1

您只需要修复一些语法并使用group by即可。。。 MySQL允许您使用列别名：

SELECT COUNT(*),
       (CASE WHEN l.transportationmode IN ('bus', 'train') THEN l.transportationmode
             WHEN l.transportationmode IN ('car', 'taxi') THEN 'car' 
        END) AS mode 
FROM plt_distinct d JOIN 
     labels l
     USING (directory)
GROUP BY mode;

Here是db <>小提琴。

注意：

您的CASE带有多余的逗号和括号，可以对此进行修复。
尽管您使用CASE的基本逻辑，但IN比将= / OR链接在一起更简单-而且通常具有更好的性能。
这简化了CASE个条件的数量。
所有列引用都是限定的，这意味着该表是名称的一部分。

Answer 2

也许您需要这样的东西：

SELECT mode, COUNT(*)
FROM (
    SELECT 
        CASE transportationmode
            WHEN 'taxi' THEN 'car'
            ELSE transportationmode
        END AS mode 
    FROM plt_distinct 
    JOIN labels USING(directory)
) t1
GROUP BY mode;

MySQL COUNT CASE（计算某列中值的总出现次数）

2 个答案: