1. ABS(x): 返回x的绝对值

mysql> select ABS(1), ABS(-1), ABS(0);
+--------+---------+--------+
| ABS(1) | ABS(-1) | ABS(0) |
+--------+---------+--------+
| 1 | 1 | 0 |
+--------+---------+--------+?

2. PI(): 返回圆周率

mysql> select PI();
+----------+
| PI() |
+----------+
| 3.141593 |
+----------+?

3. SQRT(x): 返回x的平方根，要求（x为非负数，返回NULL）
mysql> select SQRT(49), SQRT(0), SQRT(-49);
+----------+---------+-----------+
| SQRT(49) | SQRT(0) | SQRT(-49) |
+----------+---------+-----------+
| 7 | 0 | NULL |
+----------+---------+-----------+

4. MOD(x,y): 求余函数，返回x被y除后的余数；对于带有小数部分的数据值也起作用，它返回除法运算后的精确余数。

mysql> select MOD(31,8), MOD(21,-8), MOD(-7,2), MOD(-7,-2), MOD(45.5,6);
+-----------+------------+-----------+------------+-------------+
| MOD(31,8) | MOD(21,-8) | MOD(-7,2) | MOD(-7,-2) | MOD(45.5,6) |
+-----------+------------+-----------+------------+-------------+
| 7 | 5 | -1 | -1 | 3.5 |
+-----------+------------+-----------+------------+-------------+?

5. CEIL(X): 返回不小X的最小整数值，返回值转为一个BIGINT.

mysql> select CEIL(-3.35), CEIL(3.35);
+-------------+------------+
| CEIL(-3.35) | CEIL(3.35) |
+-------------+------------+
| -3 | 4 |
+-------------+------------+

6. CEILING(X): 同CEIL(X)

mysql> select CEILING(-3.35), CEILING(3.35);
+----------------+---------------+
| CEILING(-3.35) | CEILING(3.35) |
+----------------+---------------+
| -3 | 4 |
+----------------+---------------+?

7. FLOOR(X):返回不大于X的最大整数值，返回值转为一个BIGINT.

mysql> select FLOOR(-3.35), FLOOR(3.35);
+--------------+-------------+
| FLOOR(-3.35) | FLOOR(3.35) |
+--------------+-------------+
| -4 | 3 |
+--------------+-------------+

8. RAND()和RAND(X)

RAND(X) 返回一个随机浮点值，范围在０～１之间，X为整数，它被称作种子值，用来产生重复序列。即当Ｘ值相同时，产生的随机数也相同；

mysql> select RAND(10), RAND(10), RAND(2), RAND(-2);
+--------------------+--------------------+--------------------+--------------------+
| RAND(10) | RAND(10) | RAND(2) | RAND(-2) |
+--------------------+--------------------+--------------------+--------------------+
| 0.6570515219653505 | 0.6570515219653505 | 0.6555866465490187 | 0.6548542125661431 |
+--------------------+--------------------+--------------------+--------------------+

RAND(): 不带参数的RAND()每次产生不同０～１之间的随机数

mysql> SELECT RAND(), RAND(), RAND();
+--------------------+--------------------+---------------------+
| RAND() | RAND() | RAND() |
+--------------------+--------------------+---------------------+
| 0.6931893636409094 | 0.5147262984092592 | 0.49406343185721285 |
+--------------------+--------------------+---------------------+

9. ROUND(X)和ROUND(X,Y): 四舍五入函数，对Ｘ值按照Ｙ进行四舍五入，Ｙ可以省略，默认值为０;若Ｙ不为０，则保留小数点后面指定Ｙ位。