群Concat查询问题

Question

我知道这可能是不可能回答，但我想弄清楚为什么查询返回0结果。

SELECT SQL_CALC_FOUND_ROWS 
    records.instance_id 
    ,instances.patient_id 
    ,form_1_data.field_1 AS field_0 
    ,form_1_data.field_2 AS field_1 
    ,records.field_240 AS field_2 
    ,**GROUP_CONCAT(events.start_time SEPARATOR '<br />')** AS field_3 
FROM form_4_data AS records 
LEFT JOIN instances ON instances.id= records.instance_id 
LEFT JOIN events ON records.instance_id = events.instance_id 
LEFT JOIN form_3_data ON records.field_111= form_3_data.instance_id 
LEFT JOIN form_1_data ON form_3_data.field_94= form_1_data.instance_id 
WHERE instances.active=1 
ORDER BY records.instance_id DESC 
LIMIT 25,25 

返回0结果。

但是，

SELECT SQL_CALC_FOUND_ROWS 
    records.instance_id 
    ,instances.patient_id 
    ,form_1_data.field_1 AS field_0 
    ,form_1_data.field_2 AS field_1 
    ,records.field_240 AS field_2 
    ,**events.start_time AS field_3** 
FROM form_4_data AS records 
LEFT JOIN instances ON instances.id= records.instance_id 
LEFT JOIN events ON records.instance_id = events.instance_id 
LEFT JOIN form_3_data ON records.field_111= form_3_data.instance_id 
LEFT JOIN form_1_data ON form_3_data.field_94= form_1_data.instance_id 
WHERE instances.active = 1 
ORDER BY records.instance_id DESC 
LIMIT 25,25 

回报：

instance_id patient_id field_0 field_1 field_2 field_3 
83   40   Jack  Nichols NULL  2011-04-11 20:00:00 
60   8   Olive Miller Split  NULL 
58   32   Mark  Green NPSG  2011-03-05 20:00:00 
58   32   Mark  Green NPSG  2011-04-09 20:00:00 
58   32   Mark  Green NPSG  2011-04-02 20:00:00 
58   32   Mark  Green NPSG  2011-04-09 20:00:00 
57   32   Mark  Green MSLT  NULL 
56   43   Jeremy Stevens NPSG  NULL 
31   32   Mark  Green MWT  2011-04-14 20:00:00 
28   29   Jerry Jones NULL  2011-04-01 20:00:00 
28   29   Jerry Jones NULL  2011-04-02 20:00:00 
28   29   Jerry Jones NULL  2011-03-30 20:00:00 
28   29   Jerry Jones NULL  2011-04-01 20:00:00 
28   29   Jerry Jones NULL  2011-04-14 20:00:00 
11   8   Olive Miller Split  2011-03-11 20:00:00 
11   8   Olive Miller Split  2011-03-30 20:00:00 
11   8   Olive Miller Split  2011-03-22 20:00:00 
11   8   Olive Miller Split  2011-04-06 20:00:00 
11   8   Olive Miller Split  2011-04-16 20:00:00 
11   8   Olive Miller Split  2011-03-31 20:00:00 
11   8   Olive Miller Split  2011-04-02 20:00:00 
11   8   Olive Miller Split  2011-04-03 20:00:00 
9   8   Olive Miller NPSG  2011-04-22 20:00:00 
9   8   Olive Miller NPSG  2011-03-29 20:00:00 
9   8   Olive Miller NPSG  2011-03-29 20:00:00 

Answer 1

group_concat不起作用，因为在你的查询中没有group by条款。
添加一个group by它会工作。

SELECT SQL_CALC_FOUND_ROWS 
    records.instance_id 
    ,instances.patient_id 
    ,form_1_data.field_1 AS field_0 
    ,form_1_data.field_2 AS field_1 
    ,records.field_240 AS field_2 
GROUP_CONCAT(events.start_time SEPARATOR '<br />') AS field_3 
FROM form_4_data AS records 
LEFT JOIN instances ON instances.id= records.instance_id 
LEFT JOIN events ON records.instance_id = events.instance_id 
LEFT JOIN form_3_data ON records.field_111= form_3_data.instance_id 
LEFT JOIN form_1_data ON form_3_data.field_94= form_1_data.instance_id 
WHERE instances.active=1 
GROUP BY records.instance_id /*<<-- addition */ 
ORDER BY records.instance_id DESC 
LIMIT 25,25 

Answer 2

如果你想只是一个新的行添加到您的日期，那么GROUP_CONCAT是不是你真正需要的。它是一个与GROUP BY一起使用的聚合函数，用于连接来自组合块中不同行的字符串。

如果我们有一个表周：

week weekday 
1 Monday 
1 Tuesday 
1 Friday 
2 Monday 
2 Sunday 

查询

SELECT week, GROUP_CONCAT(weekday SEPARATOR '<br />') as days 
FROM weeks 
GROUP BY week 

会给你：

week days 
1 Monday<br />Tuesday<br />Friday 
2 Monday<br />Sunday 

我相信你正在寻找的是一个简单的CONCAT：

SELECT week, CONCAT(weekday,'<br />') as day 
FROM weeks 

会给你：

week day 
1 Monday<br /> 
1 Tuesday<br /> 
1 Friday<br /> 
2 Monday<br /> 
2 Sunday<br />