语法图

代码

log

log::= LOG "(" expr1 "," expr2 ")"

LOG函数计算expr2以expr1为底的对数。

本函数的规则如下：

• expr1和expr2均为YashanDB认可的通用表达式，其值须为数值型或可以转换为NUMBER类型的字符型（转换失败返回Invalid number错误）。对于其他类型，函数返回类型不支持。

• 根据对数的数学概念，expr1应该为除0和1以外的正数，expr2应该为任意一个正数，除此之外的其他情况函数处理规则受配置参数MATH_FUNC_RETURN_DECIMAL影响：

  • 若MATH_FUNC_RETURN_DECIMAL = FALSE（默认值），处理规则如下：

    expr1值（行） expr2值（列）	除1以外的正数	1	0	负数	Inf	-Inf	Nan	-Nan
    正数	对数	Inf	0	Nan	0	Nan	Nan	Nan
    1	0	Nan	0	Nan	0	Nan	Nan	Nan
    0	-Inf	-Inf	Nan	Nan	Nan	Nan	Nan	Nan
    负数	Nan	Nan	Nan	Nan	Nan	Nan	Nan	Nan
    Inf	Inf	Inf	Nan	Nan	Nan	Nan	Nan	Nan
    -Inf	Nan	Nan	Nan	Nan	Nan	Nan	Nan	Nan
    Nan	Nan	Nan	Nan	Nan	Nan	Nan	Nan	Nan
    -Nan	Nan	Nan	Nan	Nan	Nan	Nan	Nan	Nan

  • 若MATH_FUNC_RETURN_DECIMAL = TRUE，处理规则如下：

    expr1值（行） expr2值（列）	除1以外的正数	1	0、负数、Nan、Inf、-Inf
    正数	对数	0	函数返回错误
    1	0	函数返回错误	函数返回错误
    0、负数、Nan、Inf、-Inf	函数返回错误	函数返回错误	函数返回错误

• 当expr1或expr2的值为NULL时，函数返回NULL。

• 当expr1或expr2的值均为合法的非NULL值时，函数返回值类型同样受配置参数MATH_FUNC_RETURN_DECIMAL影响：

  • 若MATH_FUNC_RETURN_DECIMAL = TRUE且expr1或expr2的值不含FLOAT/DOUBLE类型，函数返回NUMBER类型。

  • 否则，函数返回DOUBLE类型。

示例1

SHOW PARAMETER MATH_FUNC_RETURN_DECIMAL

NAME                                           VALUE                                        
---------------------------------------------- --------------------------------
MATH_FUNC_RETURN_DECIMAL                    FALSE

SELECT LOG(2,4) res FROM DUAL;
        RES
-----------
   2.0E+000

SELECT LOG(b'10',b'100') res FROM DUAL;
        RES
-----------
   2.0E+000

SELECT LOG(1,4) res FROM DUAL;
        RES
-----------
        Inf

SELECT LOG(2,0) res FROM DUAL;
        RES
-----------
       -Inf

SELECT LOG(2,'nan') res FROM DUAL;
        RES
-----------
       Nan

SELECT LOG('inf',4) res FROM DUAL;
        RES
-----------
       Nan

DROP TABLE IF EXISTS number_fd;
CREATE TABLE number_fd(numberf FLOAT, numberd DOUBLE);
INSERT INTO number_fd VALUES(0,5.55);
INSERT INTO number_fd VALUES(1,5.55);
INSERT INTO number_fd VALUES(2,-5.55);
INSERT INTO number_fd VALUES(2,0);
INSERT INTO number_fd VALUES('2','Inf');
INSERT INTO number_fd VALUES('2','-Inf');
INSERT INTO number_fd VALUES('2','Nan');
INSERT INTO number_fd VALUES('2','-Nan');

SELECT LOG(numberf,numberd) res1, LOG(numberd,numberf) res2 FROM number_fd;
       RES1        RES2 
----------- ----------- 
          0        -Inf
        Inf           0
        Nan         Nan
       -Inf           0
        Inf           0
        Nan         Nan
        Nan         Nan
        Nan         Nan

示例2

SHOW PARAMETER MATH_FUNC_RETURN_DECIMAL

NAME                                           VALUE                                        
---------------------------------------------- --------------------------------
MATH_FUNC_RETURN_DECIMAL                    TRUE

SELECT LOG(2,4) res FROM DUAL;
        RES
-----------
          2

SELECT LOG(b'10',b'100') res FROM DUAL;
        RES
-----------
          2

SELECT LOG(2,1) res FROM DUAL;
        RES
-----------
          0

SELECT LOG(1,4) res FROM DUAL;
YAS-04426 the argument value is out of range

SELECT LOG(2,0) res FROM DUAL;
YAS-04426 the argument value is out of range

SELECT LOG(2,'nan') res FROM DUAL;
YAS-04426 the argument value is out of range

SELECT LOG('inf',4) res FROM DUAL;
YAS-04426 the argument value is out of range