表：XS,XK,CJ
left join 表1 on 表1.字段=表2.字段
group by 分组条件
order by 排序条件 asc正序(小到大)，desc倒序
having 跟条件类似where，不过需要在返回集字段中包含该字段
否则会报错：Unknown column 'c.CJ' in 'having clause'，未知的列，在having
例：
#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倒序大到小
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