MySQL查询的几种方式
今天我们继续是做MySQL的基础查询的命令，主要还是练习为主：
今天压根就没有时间进行学习，工作压力是越来越重了；
还是忙里抽时间出来一点事一点；加油吧~！
1、查找空值
is null #代表查找空值
#查找空值
MariaDB [linux]> select * from t1 where id not in (1,3,5,7,9);
+----+-----------+------+----------+
| id | name | job | isdelete |
+----+-----------+------+----------+
| 4 | zhaoliu | NULL | |
| 6 | xiaoqiang | lala | |
| 8 | xiaom | NULL | |
+----+-----------+------+----------+
3 rows in set (0.00 sec)
2、聚合
#查找id最大的信息
MariaDB [linux]> select max(id) from t1;
+---------+
| max(id) |
+---------+
| 8 |
+---------+
1 row in set (0.00 sec)
#查找id最小的信息
MariaDB [linux]> select min(id ) from t1;
+----------+
| min(id ) |
+----------+
| 1 |
+----------+
1 row in set (0.00 sec)
#统计一下信息
MariaDB [linux]> select count(*) from t1 where isdelete=0;
+----------+
| count(*) |
+----------+
| 6 |
+----------+
1 row in set (0.00 sec)
3、排序：
语法说明：
order by 字段 asc 根据“列”从小到大排序
order by 字段 desc 根据“列”从大到小排序
#根据id，从大到小进行排序
MariaDB [linux]> select * from t1 order by id desc;
+----+-----------+-----------+----------+
| id | name | job | isdelete |
+----+-----------+-----------+----------+
| 8 | xiaom | NULL | |
| 7 | mengmeng | dancer | |
| 6 | xiaoqiang | lala | |
| 5 | xiaoliang | docter | |
| 4 | zhaoliu | NULL | |
| 3 | wangwu | teacher | |
| 1 | zhangsan | kuaijishi | |
+----+-----------+-----------+----------+
7 rows in set (0.00 sec)
4、分组
举例说明
group by 字段
#根据名字分组，统计同名的个数：
MariaDB [linux]> select count(*),name from t1 group by name;
+----------+-----------+
| count(*) | name |
+----------+-----------+
| 1 | mengmeng |
| 1 | wangwu |
| 1 | xiaoliang |
| 1 | xiaom |
| 1 | xiaoqiang |
| 1 | zhangsan |
| 1 | zhaoliu |
+----------+-----------+
7 rows in set (0.00 sec)
5、限制
举例说明：
limit n; 显示前n行
#显示前两行
MariaDB [linux]> select * from t1 limit 2;
+----+----------+-----------+----------+
| id | name | job | isdelete |
+----+----------+-----------+----------+
| 1 | zhangsan | kuaijishi | |
| 3 | wangwu | teacher | |
+----+----------+-----------+----------+
2 rows in set (0.00 sec)
#显示从第2行开始后的2行；
MariaDB [linux]> select * from t1 limit 2,2;
+----+-----------+--------+----------+
| id | name | job | isdelete |
+----+-----------+--------+----------+
| 4 | zhaoliu | NULL | |
| 5 | xiaoliang | docter | |
+----+-----------+--------+----------+
2 rows in set (0.00 sec)
#显示从第3行开始后的4行
MariaDB [linux]> select * from t1 limit 4 offset 2;
+----+-----------+--------+----------+
| id | name | job | isdelete |
+----+-----------+--------+----------+
| 4 | zhaoliu | NULL | |
| 5 | xiaoliang | docter | |
| 6 | xiaoqiang | lala | |
| 7 | mengmeng | dancer | |
+----+-----------+--------+----------+
4 rows in set (0.00 sec)
分享环节：
hi，亲爱的朋友们：
- 感谢你们耐心完这个笔记，如果笔记中出现的一些软件包、资源找不到的可以直接留言&私聊，我看见了就回复；
- 资源免费共享；有需要滴滴，（仅仅是我有的）
我的坚持初衷：💕立志要成为一名架构师
- 不断地去坚持学，其中的各种各样的难度，不言而喻~！
- 坚持不是一件容易的事情，但它却是成功的关键。做起来吧~！
如果你也想要坚持：那么组团吧，咋们一块互相监督；一天一点分享也是进步；最怕就是孤军奋战！加油吧，追梦人~！
