一、count(*)、count(1)、count(字段名)的区别
select count(*) from t_md_inst --153797 --包含字段为null 的记录
select count(1) from t_md_inst ---153797 --包含字段为null 的记录
select count(class_id) from t_md_inst --19883 --不包含class_id是null的记录
select count(distinct class_id) from t_md_inst --29 ---不显示class_id重复的记录
二、group by + having + order by
select quote_class, quoteed_type
from t_meta_quote
group by quote_class, quoteed_type
having quote_class != 'DDIndex'
order by quoteed_type desc, quote_class asc;
三、||连接符:连接字段和字符串
四、distinct两个字段的去重
五、伪列、别名、is null、like