SQL 笔记1，left join，group by，having

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