mysql 索引的概念和操做
經常使用的索引分爲下面幾種:
key:普通索引
unique key:惟一索引
primary key:主鍵索引
full text:全文索引
實例操做：
建立表t8添加索引:
mysql> create table t8 ( id int, name char(10), email char(20),key name(name),unique key(email) );
Query OK, 0 rows affected (0.01 sec)
mysql> desc t8;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(10) | YES | MUL | NULL | |
| email | char(20) | YES | UNI | NULL | |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.00 sec)
插入數據:
mysql> insert into t8 values ("1","lisi","lisi@email.com")
-> ;
Query OK, 1 row affected (0.01 sec)
查詢數據:
mysql> select * from t8;
+------+------+----------------+
| id | name | email |
+------+------+----------------+
| 1 | lisi | lisi@email.com |
+------+------+----------------+
1 row in set (0.00 sec)
#unique key惟一性,lisi@email.com郵箱已經存在將不容許相同.
mysql> insert into t8 values ("2","lisi","lisi@email.com")
-> ;
ERROR 1062 (23000): Duplicate entry 'lisi@email.com' for key 'email'
#建立表t9，主鍵key。
mysql> create table t9 ( id int, name char(10), email char(20),primary key (id),key id (name),unique key(email) );
Query OK, 0 rows affected (0.04 sec)
mysql> desc t9;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | NO | PRI | 0 | |
| name | char(10) | YES | MUL | NULL | |
| email | char(20) | YES | UNI | NULL | |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.00 sec)
##索引的長度:建索引時，能夠只索引列的前一部分的內容,好比前10個字符.
如 key id(email(10))
#索引email長度爲10個字符，並非只容許表中email列插入長度爲10個字符,而是說索引引用時只引用emial列前面10個字符.
mysql> create table t10 ( id int, name char(10), email char(20),primary key (id),key id (email(10)),unique key(email) );
Query OK, 0 rows affected (0.02 sec)
mysql> desc t10;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | NO | PRI | 0 | |
| name | char(10) | YES | | NULL | |
| email | char(20) | YES | UNI | NULL | |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.00 sec)
#插入超過10個字符長度的email，發現也是能夠的.
mysql> insert into t10 values (1,"lisi","sqweqeqeq@email.com");
Query OK, 1 row affected (0.01 sec)
多列索引:把2個或者多列的值，當作一個總體，而後創建索引.
mysql> create table t11 (
-> xing char(2),
-> ming char(10),
-> key xm(xing,ming)
-> );
Query OK, 0 rows affected (0.02 sec)
mysql> desc t11;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| xing | char(2) | YES | MUL | NULL | |
| ming | char(10) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> insert into t11 values ("李","世民");
Query OK, 1 row affected (0.00 sec)
mysql> select * from t11;
+------+--------+
| xing | ming |
+------+--------+
| 李 | 世民 |
+------+--------+
1 row in set (0.00 sec)
mysql> show index from t11 \G;
*************************** 1. row ***************************
Table: t11
Non_unique: 1
Key_name: xm
Seq_in_index: 1
Column_name: xing
Collation: A
Cardinality: 1
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: t11
Non_unique: 1
Key_name: xm
Seq_in_index: 2
Column_name: ming
Collation: A
Cardinality: 1
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
2 rows in set (0.00 sec)
ERROR:
No query specified
#多列索引的查詢。
mysql> select * from t11 where xing="李" and ming="世民";
+------+--------+
| xing | ming |
+------+--------+
| 李 | 世民 |
+------+--------+
1 row in set (0.01 sec)
查看查詢所能使用的索引：explain
mysql> explain select * from t11 where xing="李" and ming="世民";
+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+
| 1 | SIMPLE | t11 | ref | xm | xm | 38 | const,const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+
1 row in set (0.00 sec)
mysql> explain select * from t11 where xing="李";
+----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
| 1 | SIMPLE | t11 | ref | xm | xm | 7 | const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
mysql> explain select * from t11 where ming="世民";
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| 1 | SIMPLE | t11 | index | NULL | xm | 38 | NULL | 1 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
1 row in set (0.00 sec)
mysql> explain select * from t11 where ming="世民" and xing="李";
+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+
| 1 | SIMPLE | t11 | ref | xm | xm | 38 | const,const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+
1 row in set (0.00 sec)
冗餘索引:在某個列上，可能存在多個索引.好比xm(xing,ming) m(ming).目的在於加快查詢速度.
mysql> create table t12 ( xing char(2), ming char(10), key xm(xing,ming),key m(ming) );
Query OK, 0 rows affected (0.02 sec)
mysql> show index from t12;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t12 | 1 | xm | 1 | xing | A | 0 | NULL | NULL | YES | BTREE | | |
| t12 | 1 | xm | 2 | ming | A | 0 | NULL | NULL | YES | BTREE | | |
| t12 | 1 | m | 1 | ming | A | 0 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
mysql> show index from t12 \G;
*************************** 1. row ***************************
Table: t12
Non_unique: 1
Key_name: xm
Seq_in_index: 1
Column_name: xing
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: t12
Non_unique: 1
Key_name: xm
Seq_in_index: 2
Column_name: ming
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 3. row ***************************
Table: t12
Non_unique: 1
Key_name: m
Seq_in_index: 1
Column_name: ming
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
3 rows in set (0.00 sec)
ERROR:
No query specified
#刪除索引.
語法：
alter table 表名 drop index 索引名
drop index 索引名 on 表名
#刪除索引m
mysql> alter table t12 drop index m;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> drop index xm on t12;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show index from t12 \G;
Empty set (0.00 sec)
ERROR:
No query specified
添加索引
alter table 表名 add [index/unique] 索引名;
mysql> alter table t12 add index xm(xing,ming);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table t12 add unique m(ming);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table t12;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t12 | CREATE TABLE `t12` (
`xing` char(2) DEFAULT NULL,
`ming` char(10) DEFAULT NULL,
UNIQUE KEY `m` (`ming`),
KEY `xm` (`xing`,`ming`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
添加主鍵索引
mysql> alter table t12 add primary key(xing);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
刪除主鍵索引
mysql> alter table t12 drop primary key;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
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