2.1 基本函数
举例：
SELECT ABS(-123),ABS(32),SIGN(-23),SIGN(43),PI(),CEIL(32.32),CEILING(-43.23),FLOOR(32.32), FLOOR(-43.23),MOD(12,5) FROM DUAL;
SELECT RAND(),RAND(),RAND(10),RAND(10),RAND(-1),RAND(-1) FROM DUAL;
SELECT ROUND(12.33),ROUND(12.343,2),ROUND(12.324,-1),TRUNCATE(12.66,1),TRUNCATE(12.66,-1) FROM DUAL;
2.2 角度与弧度互换函数
SELECT RADIANS(30),RADIANS(60),RADIANS(90),DEGREES(2*PI()),DEGREES(RADIANS(90)) FROM DUAL;
2.3 三角函数
举例：
ATAN2(M,N)函数返回两个参数的反正切值。 与ATAN(X)函数相比，ATAN2(M,N)需要两个参数，例如有两个点point(x1,y1)和point(x2,y2)，使用ATAN(X)函数计算反正切值为ATAN((y2-y1)/(x2-x1))，使用ATAN2(M,N)计算反正切值则为ATAN2(y2-y1,x2-x1)。由使用方式可以看出，当x2-x1等于0时，ATAN(X)函数会报错，而ATAN2(M,N)函数则仍然可以计算。
ATAN2(M,N)函数的使用示例如下：
SELECT SIN(RADIANS(30)),DEGREES(ASIN(1)),TAN(RADIANS(45)),DEGREES(ATAN(1)),DEGREES(ATAN2(1,1)) FROM DUAL;
2.4 指数与对数
mysql> SELECT POW(2,5),POWER(2,4),EXP(2),LN(10),LOG10(10),LOG2(4) -> FROM DUAL; +----------+------------+------------------+-------------------+-----------+---------+ | POW(2,5) | POWER(2,4) | EXP(2) | LN(10) | LOG10(10) | LOG2(4) | +----------+------------+------------------+-------------------+-----------+---------+ | 32 | 16 | 7.38905609893065 | 2.302585092994046 | 1 | 2 | +----------+------------+------------------+-------------------+-----------+---------+ 1 row in set (0.00 sec)
2.5 进制间的转换
mysql> SELECT BIN(10),HEX(10),OCT(10),CONV(10,2,8) -> FROM DUAL; +---------+---------+---------+--------------+ | BIN(10) | HEX(10) | OCT(10) | CONV(10,2,8) | +---------+---------+---------+--------------+ | 1010 | A | 12 | 2 | +---------+---------+---------+--------------+
三、 字符串函数
注意：MySQL中，字符串的位置是从1开始的。
举例：
mysql> SELECT FIELD('mm','hello','msm','amma'),FIND_IN_SET('mm','hello,mm,amma') -> FROM DUAL; +----------------------------------+-----------------------------------+ | FIELD('mm','hello','msm','amma') | FIND_IN_SET('mm','hello,mm,amma') | +----------------------------------+-----------------------------------+ | 0 | 2 | +----------------------------------+-----------------------------------+ 1 row in set (0.00 sec)
mysql> SELECT NULLIF('mysql','mysql'),NULLIF('mysql', ''); +-------------------------+---------------------+ | NULLIF('mysql','mysql') | NULLIF('mysql', '') | +-------------------------+---------------------+ | NULL | mysql | +-------------------------+---------------------+ 1 row in set (0.00 sec)
