连接表时出现1064错误

Question

SELECT School_info.district_id,count(School_info.no_of_class) ls 
from   School_info where     school_level IN('ls')  
JOIN
SELECT t.district_id,count(t.no_of_class) p from School_info as t     
where school_level IN('ps')
ON
( School_info.district_id = t.district_id)

我的错误在哪里，我没找到。当我运行上面的查询时，它显示为： -

MySQL说：文档

  

1064 - 您的SQL语法出错;检查与MySQL服务器版本对应的手册，以获得正确的语法   接近＆＃39; JOIN SELECT t.district_id，count（t.no_of_class）p from   School_info as t where sc＆＃39;在第2行

Answer 1

您的查询有很多语法错误，但无论如何都可以简化为：

select 
    district_id
    , sum(school_level = 'ls') ls 
    , sum(school_level = 'ps') p
from school_info 
where school_level in ('ls','ps') 
group by district_id

sum(school_level = 'p')使用MySQL对sum()内的表达式进行布尔评估的事实，如果为真则返回1。对所有1进行求和会得到与计算行相同的结果。

这将为您提供如下输出：

district_id ls p

其中ls和p是重要的。