我的数据库中有几个表，并且使用左外连接来连接表并执行查询。问题是，并非所有的匹配记录都被返回，第一个记录总是因某种原因而从结果中被跳过。
以下是SQL Fiddle，
的链接架构+测试数据：
mysql> select * from main;
+----------+------------+----------------+---------------+-------------+-------------+----------------+----------------+-----------------+------------+------------+----------------+-----------------+---------------+--------------------------+--------------+
| username | date | water_quantity | water_chilled | smoked_what | smoke_count | sleep_duration | study_duration | screen_duration | loud_level | heat_level | humidity_level | stress_physical | stress_mental | stress_notes | menstruation |
+----------+------------+----------------+---------------+-------------+-------------+----------------+----------------+-----------------+------------+------------+----------------+-----------------+---------------+--------------------------+--------------+
| test123 | 2012-09-16 | 1 | no | cigarettes | 20 | 480 | 0 | 420 | 2 | 7 | 7 | 6 | 4 | Roamed a lot on the bike | no |
| test123 | 2012-09-13 | 2 | no | cigarettes | 12 | 300 | 0 | 0 | 1 | 1 | 1 | 6 | 3 | met friends | no |
+----------+------------+----------------+---------------+-------------+-------------+----------------+----------------+-----------------+------------+------------+----------------+-----------------+---------------+--------------------------+--------------+
2 rows in set (0.00 sec)
mysql> select * from food;
+----------+------------+--------------+
| username | date | food |
+----------+------------+--------------+
| test123 | 2012-09-16 | rice |
| test123 | 2012-09-16 | pizza |
| test123 | 2012-09-16 | french fries |
| test123 | 2012-09-16 | burger |
| test123 | 2012-09-13 | naan |
| test123 | 2012-09-13 | fried rice |
| test123 | 2012-09-13 | lemon juice |
+----------+------------+--------------+
7 rows in set (0.00 sec)
mysql> select * from alcohol;
+----------+------------+--------------+------------------+
| username | date | alcohol_type | alcohol_quantity |
+----------+------------+--------------+------------------+
| test123 | 2012-09-16 | beer | 0 |
| test123 | 2012-09-16 | beer_ale | 0 |
| test123 | 2012-09-16 | absinthe | 0 |
| test123 | 2012-09-13 | rum | 0 |
| test123 | 2012-09-13 | tequila | 0 |
+----------+------------+--------------+------------------+
5 rows in set (0.00 sec)
mysql> select * from headache;
+----------+------------+-----------+----------+---------------------+
| username | date | intensity | duration | notes |
+----------+------------+-----------+----------+---------------------+
| test123 | 2012-09-16 | 6 | 12 | something something |
+----------+------------+-----------+----------+---------------------+
1 row in set (0.00 sec)
mysql> select * from headache_areas;
+----------+------------+-----------------+
| username | date | area |
+----------+------------+-----------------+
| test123 | 2012-09-16 | left_temple |
| test123 | 2012-09-16 | right_temple |
| test123 | 2012-09-16 | behind_left_ear |
+----------+------------+-----------------+
3 rows in set (0.00 sec)
mysql> select * from headache_symptoms;
+----------+------------+-----------+
| username | date | symptoms |
+----------+------------+-----------+
| test123 | 2012-09-16 | aura |
| test123 | 2012-09-16 | vertigo |
| test123 | 2012-09-16 | dizziness |
+----------+------------+-----------+
3 rows in set (0.00 sec)
mysql> select * from alcohol;
+----------+------------+--------------+------------------+
| username | date | alcohol_type | alcohol_quantity |
+----------+------------+--------------+------------------+
| test123 | 2012-09-16 | beer | 0 |
| test123 | 2012-09-16 | beer_ale | 0 |
| test123 | 2012-09-16 | absinthe | 0 |
| test123 | 2012-09-13 | rum | 0 |
| test123 | 2012-09-13 | tequila | 0 |
+----------+------------+--------------+------------------+
5 rows in set (0.00 sec)
mysql> select * from drugs;
+----------+------------+----------+
| username | date | drug |
+----------+------------+----------+
| test | 2012-08-21 | crocin |
| test | 2012-08-21 | azithral |
| test | 2012-08-21 | crocin |
| test | 2012-08-21 | azithral |
| test | 2012-08-21 | crocin |
| test | 2012-08-21 | azithral |
| test123 | 2012-09-13 | ching |
| test123 | 2012-09-13 | chong |
| test123 | 2012-09-13 | blah1 |
| test123 | 2012-09-13 | blurg2 |
+----------+------------+----------+
10 rows in set (0.00 sec)
查询我尝试了结果：
mysql> SELECT m.*,
-> GROUP_CONCAT(DISTINCT f.food SEPARATOR ',') AS food,
-> GROUP_CONCAT(DISTINCT a.alcohol_type SEPARATOR ',') AS alcohol,
-> a.alcohol_quantity,
-> GROUP_CONCAT(DISTINCT d.drug SEPARATOR ',') AS drug,
-> h.intensity AS headache_intensity,
-> h.duration AS headache_duration,
-> GROUP_CONCAT(DISTINCT ha.area) AS headache_areas,
-> GROUP_CONCAT(DISTINCT hs.symptoms) AS headache_symptoms,
-> h.notes AS headache_notes
-> FROM main AS m
-> LEFT OUTER JOIN food AS f ON f.username = m.username AND f.date = m.date
-> LEFT OUTER JOIN headache AS h ON h.username = m.username AND h.date = m.date
-> LEFT OUTER JOIN headache_symptoms AS hs ON hs.username = m.username AND hs.date = m.date
-> LEFT OUTER JOIN headache_areas AS ha ON ha.username = m.username AND ha.date = m.date
-> LEFT OUTER JOIN drugs AS d ON d.username = m.username AND d.date = m.date
-> LEFT OUTER JOIN alcohol AS a ON a.username = m.username AND a.date = m.date
-> ;
+----------+------------+----------------+---------------+-------------+-------------+----------------+----------------+-----------------+------------+------------+----------------+-----------------+---------------+--------------------------+--------------+------------------------------------------------------------+------------------------------------+------------------+--------------------------+--------------------+-------------------+------------------------------------------+------------------------+---------------------+
| username | date | water_quantity | water_chilled | smoked_what | smoke_count | sleep_duration | study_duration | screen_duration | loud_level | heat_level | humidity_level | stress_physical | stress_mental | stress_notes | menstruation | food | alcohol | alcohol_quantity | drug | headache_intensity | headache_duration | headache_areas | headache_symptoms | headache_notes |
+----------+------------+----------------+---------------+-------------+-------------+----------------+----------------+-----------------+------------+------------+----------------+-----------------+---------------+--------------------------+--------------+------------------------------------------------------------+------------------------------------+------------------+--------------------------+--------------------+-------------------+------------------------------------------+------------------------+---------------------+
| test123 | 2012-09-16 | 1 | no | cigarettes | 20 | 480 | 0 | 420 | 2 | 7 | 7 | 6 | 4 | Roamed a lot on the bike | no | rice,pizza,french fries,burger,naan,fried rice,lemon juice | beer,beer_ale,absinthe,rum,tequila | 0 | ching,chong,blah1,blurg2 | 6 | 12 | left_temple,right_temple,behind_left_ear | aura,vertigo,dizziness | something something |
+----------+------------+----------------+---------------+-------------+-------------+----------------+----------------+-----------------+------------+------------+----------------+-----------------+---------------+--------------------------+--------------+------------------------------------------------------------+------------------------------------+------------------+--------------------------+--------------------+-------------------+------------------------------------------+------------------------+---------------------+
1 row in set (0.00 sec)
它没有显示表中的其他记录。有人可以帮我解决这个问题吗？提前谢谢。
答案 0 :(得分：3)
它完全输出你“要求它”做的事情（使用group concat）：
USERNAME |...| FOOD | ALCOHOL ...
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
test123 |...| rice,pizza,french fries,burger,naan,fried rice,lemon juice | beer,beer_ale,absinthe,rum,tequila ...
当您使用group concat时，它会将grouped列的所有值连接到一个输出字段中。
<强>更新
我想我知道你想要实现的目标，因为它在表main的两个记录中都是相同的用户名，它将显示为一列，为了解决你需要找到第一个不相同的字段对于这两个记录 - date。因此，您只需将GROUP BY m.username, date添加到end of your query。