SQL键,MUL与PRI对比UNI

Question

MySQL中的MUL,PRI和UNI有什么区别？

我正在使用以下命令处理MySQL查询:

desc mytable; 

其中一个字段显示为MUL键,其他字段显示为UNI或PRI.

我知道如果一个密钥是PRI,那么每个表只能有一个记录与该密钥相关联.如果密钥是MUL,这是否意味着可能存在多个关联记录？

这是mytable的回应.

+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| courseid  | int(11) | YES  | MUL | NULL    |       | 
| dept      | char(3) | YES  |     | NULL    |       | 
| coursenum | char(4) | YES  |     | NULL    |       | 
+-----------+---------+------+-----+---------+-------+

Answer 1

DESCRIBE <table>; 

这实际上是一个快捷方式:

SHOW COLUMNS FROM <table>;

无论如何,"Key"属性有三个可能的值:

1. PRI
2. UNI
3. MUL

PRI和UNI的含义非常清楚:

• PRI =>主键
• UNI =>唯一键

第三种可能性,MUL(你问过)基本上是一个既不是主键也不是唯一键的索引.名称来自"多个",因为允许多个相同值的出现.直接来自MySQL文档:

如果PRI是UNI,则该列是非唯一索引的第一列,其中在列中允许多次出现给定值.

还有一个最后的警告:

如果键值不止一个适用于表的给定列,键显示一个具有最高优先级,顺序MUL,PRI,UNI.

总的来说,MySQL文档非常好.如有疑问,请查看!

Answer 2

这意味着该字段是(一部分)非唯一索引.你可以发行

show create table <table>;

要查看有关表结构的更多信息.

Answer 3

关于MySQL中的MUL,PRI和UNI是什么？

从MySQL 5.7文档:

• 如果Key是PRI,则该列是PRIMARY KEY或者是多列PRIMARY KEY中的列之一.
• 如果Key是UNI,则该列是UNIQUE索引的第一列.(UNIQUE索引允许多个NULL值,但您可以通过检查Null字段来判断该列是否允许NULL.)
• 如果Key为MUL,则该列是非唯一索引的第一列,其中在列中允许多次出现给定值.

实例

对照组,这个例子既没有PRI,也没有MUL,也没有UNI:

mysql> create table penguins (foo INT);
Query OK, 0 rows affected (0.01 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

一列的表和一列上的索引具有MUL:

mysql> create table penguins (foo INT, index(foo));
Query OK, 0 rows affected (0.01 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | YES  | MUL | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

具有作为主键的列的表具有PRI

mysql> create table penguins (foo INT primary key);
Query OK, 0 rows affected (0.02 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

具有唯一键的列的表具有UNI:

mysql> create table penguins (foo INT unique);
Query OK, 0 rows affected (0.01 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | YES  | UNI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

包含foo和bar的索引的表在foo上只有MUL:

mysql> create table penguins (foo INT, bar INT, index(foo, bar));
Query OK, 0 rows affected (0.01 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | YES  | MUL | NULL    |       |
| bar   | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

在两列上具有两个单独索引的表具有每个列的MUL

mysql> create table penguins (foo INT, bar int, index(foo), index(bar));
Query OK, 0 rows affected (0.01 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | YES  | MUL | NULL    |       |
| bar   | int(11) | YES  | MUL | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

索引跨越三列的表在第一列上具有MUL:

mysql> create table penguins (foo INT, 
       bar INT, 
       baz INT, 
       INDEX name (foo, bar, baz));
Query OK, 0 rows affected (0.01 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| foo   | int(11) | YES  | MUL | NULL    |       |
| bar   | int(11) | YES  |     | NULL    |       |
| baz   | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)

具有引用另一个表的主键的外键的表是MUL

mysql> create table penguins(id int primary key);
Query OK, 0 rows affected (0.01 sec)

mysql> create table skipper(id int, foreign key(id) references penguins(id));
Query OK, 0 rows affected (0.01 sec)

mysql> desc skipper;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  | MUL | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

坚持在你的新皮层,并将表盘设置为"冰沙".

Answer 4

对于Mul来说,这对我来说也是有用的文档 - http://grokbase.com/t/mysql/mysql/9987k2ew41/key-field-mul-newbie-question

"MUL表示密钥允许多行具有相同的值.也就是说,它不是UNIque密钥."

例如,假设您有两个模型,Post和Comment.Post与Comment有很多关系.然后,对于Comment表,有一个MUL键(Post id)是有道理的,因为许多注释可以归属于同一个Post.