Mysql可以通过运算符来对表中数据进行运算，比如通过出生日期求年龄等
运算符包括四类，分别是：算数运算符、比较运算符、逻辑运算符和位运算符
算数运算符
加、减、乘运算
mysql> select a,a+5,a*2 from t1;
+------+------+------+
| a | a+5 | a*2 |
+------+------+------+
| 24 | 29 | 48 |
+------+------+------+
row in set (0.00 sec)
这里的原值为24,后面也可以使用混合运算，只需要注意优先级即可
除法和取模运算
mysql> select a,a/3,a div 3,a%5,mod(a,5) from t1;
+------+--------+---------+------+----------+
| a | a/3 | a div 3 | a%5 | mod(a,5) |
+------+--------+---------+------+----------+
| 24 | 8.0000 | 8 | 4 | 4 |
+------+--------+---------+------+----------+
row in set (0.00 sec)
此处 / 和 div 代表整除，% 和 mod 代表取模
要注意的是，如果被除数为0,那么计算结果是NULL
比较运算符
数值比较
mysql> select a,a=24,a<12,a>40,a>=24,a<=24,a!=24,a<>24,a<=>24 from t1;
+------+------+------+------+-------+-------+-------+-------+--------+
| a | a=24 | a<12 | a>40 | a>=24 | a<=24 | a!=24 | a<>24 | a<=>24 |
+------+------+------+------+-------+-------+-------+-------+--------+
| 24 | 1 | 0 | 0 | 1 | 1 | 0 | 0 | 1 |
+------+------+------+------+-------+-------+-------+-------+--------+
row in set (0.00 sec)
这里的1代表真，0代表假，需要说明的是<>和<=>
<>代表不等于，等同于!=
<=>代表等于，等同于=
此外，等于和不等于不仅可以比较数值，还能比较字符串
字符串比较
mysql> select a,a='24','ha'<>'ha','xa'='xa','b'!='b' from t1;
+------+--------+------------+-----------+----------+
| a | a='24' | 'ha'<>'ha' | 'xa'='xa' | 'b'!='b' |
+------+--------+------------+-----------+----------+
| 24 | 1 | 0 | 1 | 0 |
+------+--------+------------+-----------+----------+
row in set (0.00 sec)
is null 和is not null
mysql> select a,a is null, a is not null from t1;
+------+-----------+---------------+
| a | a is null | a is not null |
+------+-----------+---------------+
| 24 | 0 | 1 |
+------+-----------+---------------+
row in set (0.00 sec)
这里可以判断是否为空，NULL也可以跟NULL比较
between and和not between and
mysql> select a,a between 15 and 30,a not between 15 and 30 from t1;
+------+---------------------+-------------------------+
| a | a between 15 and 30 | a not between 15 and 30 |
+------+---------------------+-------------------------+
| 24 | 1 | 0 |
+------+---------------------+-------------------------+
row in set (0.00 sec)
between and 和not between and可以判断数值是否在某一区间内
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