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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 p 3,a%5,mod(a,5) from t1;
+------+--------+---------+------+----------+
| a    | a/3    | a p 3 | a%5  | mod(a,5) |
+------+--------+---------+------+----------+
|   24 | 8.0000 |       8 |    4 |        4 |
+------+--------+---------+------+----------+
 row in set (0.00 sec)
此处 / 和 p 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,a40,a>=24,a24,a24 from t1;
+------+------+------+------+-------+-------+-------+-------+--------+
| a    | a=24 | a40 | a>=24 | a24 | a24 |
+------+------+------+------+-------+-------+-------+-------+--------+
|   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
in
mysql> select a,a in(1,2,23),a in(24,12,22) from t1;
+------+--------------+----------------+
| a    | a in(1,2,23) | a in(24,12,22) |
+------+--------------+----------------+
|   24 |            0 |              1 |
+------+--------------+----------------+
 row in set (0.00 sec)
判断操作数是否在某一集合内
like
mysql> select s,s like 'beijing',s like 'b%g',s like 'bei____',s like '%jing' from t2;
+---------+------------------+--------------+------------------+----------------+
| s       | s like 'beijing' | s like 'b%g' | s like 'bei____' | s like '%jing' |
+---------+------------------+--------------+------------------+----------------+
| beijing |                1 |            1 |                1 |              1 |
+---------+------------------+--------------+------------------+----------------+
 row in set (0.00 sec)
ike可以用来匹配字符串，_代表单个字符，%代表多个字符
逻辑运算符
与运算
mysql> select 2&&2,2&&null,2 and 3,2 and 2;
+------+---------+---------+---------+
| 2&&2 | 2&&null | 2 and 3 | 2 and 2 |
+------+---------+---------+---------+
|    1 |    null |       1 |       1 |
+------+---------+---------+---------+
 row in set (0.00 sec)
这里&&和and意思一样
或运算
mysql> select 2||2,2||null,2 or 3,2 or 0;
+------+---------+--------+--------+
| 2||2 | 2||null | 2 or 3 | 2 or 0 |
+------+---------+--------+--------+
|    1 |       1 |      1 |      1 |
+------+---------+--------+--------+
 row in set (0.00 sec)
这里||和or的意思一样
非运算
mysql> select !1,!2,!null;
+----+----+-------+
| !1 | !2 | !null |
+----+----+-------+
|  0 |  0 |  null |
+----+----+-------+
 row in set (0.00 sec)
此外还有位运算，目前还没用到，等用到的时候再补上
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