从mysql中的两个表中获取数据

Question

我有两个表tblteacher和tblattendance共有tshortname字段。现在我想显示来自tblteacher的tshortname和状态为'yes'如果tblattendance中的特定日期我在tblattendance中也有tshortname否则状态应为'no'< / p>

select distinct(tblteacher.teachername),tblteacher.tshortname,
if(strcmp(tblteacher.tshortname,tblattendance.tshortname) and tblattendance.attdate='2013-07-01','no','yes') as status 
from tblteacher,tblclass,tblattendance

这是我执行上述查询时显示的数据

显示日期 - '2013-07-01'的数据对于某些老师来说是正确的。我没有得到问题所在。请帮我解决这个问题

更新

select distinct(tblteacher.tshortname),if (tblteacher.tshortname not in(SELECT  
distinct(t.tshortname) from tblteacher t left join tblattendance a on t.tshortname=a.tshortname 
where a.attdate='2013-07-03'),'no','yes') as status from tblteacher;

现在我总是根据就勤表中的日期获得42行和正确的状态

Answer 1

你在这里有一个表之间的经典笛卡尔连接。

您需要告诉数据库如何匹配各种表中的数据，如下所示：

select distinct 
    (tblteacher.teachername),tblteacher.tshortname,
    if(strcmp(tblteacher.tshortname,tblattendance.tshortname) and tblattendance.attdate='2013-07-01','no','yes') as status 
from 
    tblteacher
        join tblclass
            on tblteacher.tshortname=tblattendance.tshortname
        join tblattendance
            on tblteacher.tshortname=tblattendance.teacherShortName

在你的情况下，strcpm将处理前两个中的连接，但会将它与第三个表中的所有行匹配 - 这是你出错的地方。

编辑：根据您的评论，这可能会更好地让您按预期获得所有行。

select distinct 
    (tblteacher.teachername),tblteacher.tshortname,
    if(strcmp(tblteacher.tshortname,tblattendance.tshortname) and tblattendance.attdate='2013-07-01','no','yes') as status 
from 
    tblteacher
        left outer join tblclass
            on tblteacher.tshortname=tblattendance.tshortname
        left outer join tblattendance
            on tblteacher.tshortname=tblattendance.teacherShortName

Answer 2

您需要离开加入考勤表，然后

select if(tblattendance.tshortname is not null and attdate = '2013-07-01', 'yes', 'no') as status

修改：完整查询

SELECT DISTINCT
    t.tshortname,
    teachername, 
    IF(attdate = '2013-07-01','yes','no') AS status 
FROM tblteacher AS t
LEFT JOIN tblattendance AS a
       ON t.tshortname = a.tshortname

Answer 3

只是一个猜测......

 SELECT DISTINCT t.tshortname
      , CASE WHEN a.shortname IS NULL THEN 'no' ELSE 'yes' END status
   FROM tblteacher t 
   LEFT
   JOIN tblattendance a 
     ON t.tshortname = a.tshortname 
    AND a.attdate = '2013-07-03';