msyql查询(一)

查询表内容

格式：select * from 库名.表名

命令：select * from address_area

条件查询

查看石家庄市的

命令：select * from address_area where cityname='石家庄市'

查看石家庄市的有多少个

命令：select count(id) from address_area where cityname='石家庄市'

双表联合查询

内连接：inner join

命令：select * from ex_kehubiao_90 a inner join ex_kehubiao_95 b on a.客户姓名=b.客户姓名

左连接：left join

命令：select a.客户姓名,a.客户编号,b.联系电话 from ex_kehubiao_90 a left join ex_kehubiao_95 b on a.客户姓名=b.客户姓名

右连接：right join

命令：select a.id,a.客户姓名,a.客户编号,b.联系电话 from ex_kehubiao_90 a right join ex_kehubiao_95 b on a.`客户编号`=b.`客户编号`

查看某物品数量

命令：select 名称,sum(数量) as "数量" from ex_gudingzichan_95 group by 名称

 

统计出来这两列有多少条

命令：select count(name),count(cityname) from address_area

 

将空值返回0用如下语句:select ifnull(字段名,0) from 表名;

命令：select  ifnull(数量,0) from ex_gudingzichan_95