MySQL运算符
算术运算符
mysql提供的运算符有+, -, *, /(DIV), %(MOD)
mysql> select * from test;
+----+------+-------+
| id | name | price |
+----+------+-------+
| 1 | tag1 | 12 |
| 2 | tag2 | 13 |
| 3 | tag3 | 14 |
| 4 | tag4 | 10 |
| 5 | tag6 | 16 |
+----+------+-------+
5 rows in set (0.00 sec)
mysql> select id,name,price*price from test;
+----+------+-------------+
| id | name | price*price |
+----+------+-------------+
| 1 | tag1 | 144 |
| 2 | tag2 | 169 |
| 3 | tag3 | 196 |
| 4 | tag4 | 100 |
| 5 | tag6 | 256 |
+----+------+-------------+
比较运算符
operator | operator |
= | Is not null |
> | Between |
< | in |
>= | not in |
<= | like |
!=(<>) | not like |
Is null | regexp |
注：针对字段使用正则表达式，如果满足条件那么返回1，否则返回0。between and确定一个范围，如果字段值在范围内，那么返回1，否则返回0.
使用例子：
mysql> select * from test where price in (13,14);
+----+------+-------+
| id | name | price |
+----+------+-------+
| 2 | tag2 | 13 |
| 3 | tag3 | 14 |
+----+------+-------+
2 rows in set (0.00 sec)
mysql> select * from test where price not in (13,14);
+----+------+-------+
| id | name | price |
+----+------+-------+
| 1 | tag1 | 12 |
| 4 | tag4 | 10 |
| 5 | tag6 | 16 |
+----+------+-------+
3 rows in set (0.00 sec)
mysql> select * from test where name like '%6';
+----+------+-------+
| id | name | price |
+----+------+-------+
| 5 | tag6 | 16 |
+----+------+-------+
1 row in set (0.00 sec)
mysql> select name, name regexp '^tag', name regexp '4$' from test;
+------+--------------------+------------------+
| name | name regexp '^tag' | name regexp '4$' |
+------+--------------------+------------------+
| tag1 | 1 | 0 |
| tag2 | 1 | 0 |
| tag3 | 1 | 0 |
| tag4 | 1 | 1 |
| tag6 | 1 | 0 |
+------+--------------------+------------------+
5 rows in set (0.00 sec)
mysql> select price, price between 14 and 16 qualified_price from test;
+-------+-----------------+
| price | qualified_price |
+-------+-----------------+
| 12 | 0 |
| 13 | 0 |
| 14 | 1 |
| 10 | 0 |
| 16 | 1 |
+-------+-----------------+
逻辑运算符
character | effect |
&&(AND) | 与 |
||(OR) | 或 |
!(NOT) | 非 |
XOR | 异或 |
与运算：任何一个数据为0返回0，所有数据不为0，不为NULL，返回1.
mysql> select price,price&&1 from test;
+-------+----------+
| price | price&&1 |
+-------+----------+
| 12 | 1 |
| 13 | 1 |
| 14 | 1 |
| 10 | 1 |
| 16 | 1 |
| 0 | 0 |
+-------+----------+
或运算：存在任何一个不为NULL，不为0的数据，返回1. 否则返回NULL（如果有数据中有NULL）或者0。
mysql> select price, price || 0 from test;
+-------+------------+
| price | price || 0 |
+-------+------------+
| 12 | 1 |
| 13 | 1 |
| 14 | 1 |
| 10 | 1 |
| 16 | 1 |
| 0 | 0 |
| NULL | NULL |
+-------+------------+
非运算：返回和数据相反的结果。不过需要注意的是NULL的操作数据结果依然是NULL。
mysql> select price, !price from test;
+-------+--------+
| price | !price |
+-------+--------+
| 12 | 0 |
| 13 | 0 |
| 14 | 0 |
| 10 | 0 |
| 16 | 0 |
| 0 | 1 |
| NULL | NULL |
+-------+--------+
异或运算：对于x1 XOR x2, 当x1,x2都为0或非0，那么返回0；一个是0，一个是非0，返回1；只要有一个是NULL，返回NULL。
mysql> select price, price XOR 1 from test;
+-------+-------------+
| price | price XOR 1 |
+-------+-------------+
| 12 | 0 |
| 13 | 0 |
| 14 | 0 |
| 10 | 0 |
| 16 | 0 |
| 0 | 1 |
| NULL | NULL |
+-------+-------------+
位运算符
&, |, ~, ^, <<, >>
针对二进制数进行的运算。
mysql> select price, price>>1,price<<1, price&1,price|1,~price from test;
+-------+----------+----------+---------+---------+----------------------+
| price | price>>1 | price<<1 | price&1 | price|1 | ~price |
+-------+----------+----------+---------+---------+----------------------+
| 12 | 6 | 24 | 0 | 13 | 18446744073709551603 |
| 13 | 6 | 26 | 1 | 13 | 18446744073709551602 |
| 14 | 7 | 28 | 0 | 15 | 18446744073709551601 |
| 10 | 5 | 20 | 0 | 11 | 18446744073709551605 |
| 16 | 8 | 32 | 0 | 17 | 18446744073709551599 |
| 0 | 0 | 0 | 0 | 1 | 18446744073709551615 |
| NULL | NULL | NULL | NULL | NULL | NULL |
+-------+----------+----------+---------+---------+----------------------+
MySQL查询优化
索引
索引可以快速定位表中的记录，提高查询的效率。
例如：
mysql> select * from test;
+----+------+-------+
| id | name | price |
+----+------+-------+
| 1 | tag1 | 12 |
| 2 | tag2 | 13 |
| 3 | tag3 | 14 |
| 4 | tag4 | 10 |
| 5 | tag6 | 16 |
| 6 | dada | 0 |
| 7 | dada | NULL |
+----+------+-------+
7 rows in set (0.00 sec)
mysql> explain select * from test where price="NULL";
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 7 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.01 sec)
mysql> create index price_index on test(price);
Query OK, 0 rows affected (0.10 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from test where price="NULL";
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
| 1 | SIMPLE | test | ref | price_index | price_index | 5 | const | 1 | NULL |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
可以看出，访问的行数由7变成1.
删除索引：
mysql> drop index price_index on test;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
多列索引
在表的多个字段中创建一个索引。
mysql> select * from test;
+----+------+-------+
| id | name | price |
+----+------+-------+
| 7 | dada | NULL |
| 6 | dada | 0 |
| 1 | tag1 | 12 |
| 2 | tag2 | 13 |
| 3 | tag3 | 14 |
| 4 | tag4 | 10 |
| 5 | tag6 | 16 |
+----+------+-------+
7 rows in set (0.02 sec)
mysql> create index name_price_index on test(name,price);
mysql> desc select * from test where name="tag1" and price=12;
+----+-------------+-------+------+-----------------------------------+------------------+---------+-------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------------------------+------------------+---------+-------------+------+--------------------------+
| 1 | SIMPLE | test | ref | index_name_price,name_price_index | index_name_price | 37 | const,const | 1 | Using where; Using index |
+----+-------------+-------+------+-----------------------------------+------------------+---------+-------------+------+--------------------------+
通过desc可以看出，多列索引让检索的行数减小至1。
创建索引(已建好表的基础上)，总结列表：
类型 | 语法 |
normal | create index index_name on table_name(field); |
unique | create unique index index_name on table_name(field); |
fulltext | create fulltext index index_name on table_name(field); |
single-row | create index index_name on table_name(field(length)); |
multiple-row | create index index_name on table_name(field1,field2,filed3…); |
space | create spatial index index_name on table_name(field); |
删除索引语句：drop index index_name on table_name
可以通过show create语句查看新的表结构。
mysql> show create table test\G;
*************************** 1. row ***************************
Table: test
Create Table: CREATE TABLE `test` (
`id` smallint(6) NOT NULL,
`name` varchar(30) NOT NULL,
`price` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `name_price_index` (`name`,`price`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
分析查询语句
除了前面使用的explain，还有describe（desc）。
mysql> desc select * from test where name="tag1";
+----+-------------+-------+------+------------------+------------------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+------------------+------------------+---------+-------+------+--------------------------+
| 1 | SIMPLE | test | ref | index_name_price | index_name_price | 32 | const | 1 | Using where; Using index |
+----+-------------+-------+------+------------------+------------------+---------+-------+------+--------------------------+
高速缓存
MySQL将查询的结果放到高速缓存中，如果用户有相同的查询操作，高速缓存就发挥作用了。
查询是否开启高速缓存
mysql> show variables like '%query_cache';
+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| have_query_cache | YES |
+------------------+-------+
临时表
临时表存储于内存中，读写速率更快。临时表依赖于会话，如果处于不同的会话中，即使是同一用户也能使用同名临时表，不会冲突。当断开数据连接的时候，整个临时表也就消失了。
mysql> create temporary table tmp(id smallint, power int);
Query OK, 0 rows affected (0.04 sec)
mysql> insert into tmp value (12,34);
Query OK, 1 row affected (0.02 sec)
mysql> select * from tmp;
+------+-------+
| id | power |
+------+-------+
| 12 | 34 |
+------+-------+
