在MySQL中什么是MUL, PRI和UNI ?
从MySQL 5.7文档:
如果“Key”为“PRI”，则该列为“PRIMARY Key”或多列“PRIMARY Key”中的一列。
如果Key为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字段来判断列是否允许NULL。)
如果Key为MUL，则该列是非唯一索引的第一列，在该列中允许多次出现给定值。
生活的例子
对照组，本例中既没有PRI, 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)
一个只有一列并且在这一列上有索引的表有一个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)
具有主键列的表具有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)
具有唯一键列的表有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)
一个索引覆盖了foo和bar的表只有在foo上有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)
在两个列上有两个独立索引的表每个列都有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)
一个索引横跨三列的表在第一列上有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)
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