在数据库查询的时候，我们都是通过 select 字段来进行，它在查询中也有许多小的细节需要我们知道注意。
- select 查询关联属性文章列表：
- distinct 剔除查询结果重复行
- where 查询条件与运算符
- group by 分组
- having 条件筛选语句
- order by 排序
- limit 数据分页
优先看下面的简单使用:
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| test |
+----------------+
mysql> desc test;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
mysql> select * from test;
+----+------+------+
| id | name | age |
+----+------+------+
| 1 | dzm | 20 |
| 2 | xyq | 20 |
+----+------+------+
mysql> select id, name from test;
+----+------+
| id | name |
+----+------+
| 1 | dzm |
| 2 | xyq |
+----+------+
mysql> select id as a1, name as a2 from test;
+----+------+
| a1 | a2 |
+----+------+
| 1 | dzm |
| 2 | xyq |
+----+------+
现在上面这个数据库里面只有一个 test 表，但是我依然可以通过命令行查询一个不存在的表，表名用数字代替：
mysql> select 1;
+---+
| 1 |
+---+
| 1 |
+---+
// 这行则说明如果没字段名它会拿当前的属性作为字段名
mysql> select 2, now();
+---+---------------------+
| 2 | now() |
+---+---------------------+
| 2 | 2020-09-25 16:28:50 |
+---+---------------------+
// 这行则说明查询语句可以计算的
mysql> select 1+2;
+-----+
| 1+2 |
+-----+
| 3 |
+-----+
这行则说明很多函数都是可以在查询中使用的 round() ....
mysql> select 3+4*5, 6+round(5.7);
+-------+--------------+
| 3+4*5 | 6+round(5.7) |
+-------+--------------+
| 23 | 12 |
+-------+--------------+
查询字段可以使用别名
mysql> select 1 as a1;
+----+
| a1 |
+----+
| 1 |
+----+
mysql> select 1 as a1, 3+4*5 as a2, 6+round(5.7) as a3;
+----+----+----+
| a1 | a2 | a3 |
+----+----+----+
| 1 | 23 | 12 |
+----+----+----+
