1.检索数据

1.1 distinct

##用于检索出单一的行值
select distinct vend_id from products.

##distinct 不止作用于其后置字段，它会查出所有列组合情况下单一的列。
select distinct vend_id,vend_name from products.

mysql> select * from test;
+----+------+-------+
| id | name | photo |
+----+------+-------+
|  1 | tony | photo |
|  1 | tom  | photo |
|  2 | tony | photo |
|  2 | tom  | photo |
|  2 | tom  | photo |
+----+------+-------+
5 rows in set

mysql> select distinct id,name from test;
+----+------+
| id | name |
+----+------+
|  1 | tony |
|  1 | tom  |
|  2 | tony |
|  2 | tom  |
+----+------+
4 rows in set

mysql> select distinct id,photo from test;
+----+-------+
| id | photo |
+----+-------+
|  1 | photo |
|  2 | photo |
+----+-------+
2 rows in set

1.2 多个字段倒序

多个字段想使用倒序时，必须为每个字段单独使用倒序，否则不会生效。

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