SQL Note 1,left Join,group by,having

Table: XS,XK,CJ

Left JOIN table 1 on table 1. field = Table 2. Fields

GROUP BY group conditions

ORDER BY sort conditional ASC Positive order (small to large), DESC reverse

You have a condition like where, but you need to include the field in the Return set field

Otherwise error: Unknown column ' C.CJ ' in ' have clause ', unknown columns, having

Cases:

#查询有不及格的成绩的学生姓名
SELECT
S.XSMC, C.CJ
From CJ C
Left JOIN xs s on S.XSDM=C.XSDM
Left JOIN xk k on K.XKDM=C.XKDM
GROUP by C.XSDM
Having C.CJ <60

#查询所有学生所有学科的成绩 <=60
SELECT
S.xsmc,k.xkmc,c.cj
From XS S,xk K,CJ c
WHERE S.XSDM = C.XSDM
and K.XKDM = C.XKDM
and C.CJ <= 60

#查询学生总成绩
#ASC正序小到大, Desc is large to small in reverse
SELECT S.XSMC, SUM (C.CJ)
From XS S,xk K,CJ c

WHERE S.XSDM = C.XSDM
and K.XKDM = C.XKDM
GROUP by C.XSDM
ORDER by
SUM (C.CJ) ASC

SQL Note 1,left Join,group by,having