Oracle study notes 007-1 (one-way function: character functions, numerical functions, conversion functions, date functions)

Oracle on the one-way function

Function Introduction
Conventional one-way function
Commonly used characters function
Common numerical function
Common conversion functions
Date Functions

Multiple date formats
Common date inquiry

Function Introduction

Oracle SQL provides specific functions for performing a particular operation, these functions greatly enhance the function of the SQL language, the use of most programming languages, the process returns to the received parameter data items result, the function can accept zero or more input parameters, and returns an output.
In Oracle function it is mainly divided into two categories (This paper describes a single line in which four common function)

One-way function: a function for every record in the table in the application, a single line of a result, returns a result.
Aggregation function: function while the polymerization may operate on multiple rows of data, and returns a result. Such as the SUM (x)
returns the result set sum column x.

Conventional one-way function

Character functions: for string manipulation.
Digital functions: a digital calculation, a return number.
Transfer function: can convert one data type to another data type.
Date Functions: Date and time of processing.
Null correlation function: NULL-related functions of
Decoding and encoding related functions
Nested functions: a function as a parameter of another ambiguous return value

Commonly used characters function

function	Explanation	Show	result
ASCII(parameter)	Returns the ASCII code of the character parameter	SELECT ASCII(‘X’) FROM DUAL;	88
CONCAT(parameter1,parameter2)	Connection string parameter1 and parameter2	SELECT CONCAT(‘HELLO’,‘WORLD’) FROM DUAL;	HELLOWORLD
LENGTH(parameter)	Returns the length of the parameter	SELECT LENGTH ( 'Oracle Study Notes') FROM DUAL;	10
LOWER(parameter)	parameter converted to lowercase	SELECT LOWER(‘JavaSE’) FROM DUAL;	javase
UPPER(parameter)	parameter to uppercase	SELECT UPPER(‘JavaSE’) FROM DUAL;	JAVASE
LTRIM(parameter[,X])	X amputated the left of the parameter of the string, the default amputated spaces	SELECT LTRIM(’###Hello–World###’,’###’) FROM DUAL;	JAVASE
RTRIM(parameter[,X])	The right of the parameter string of truncated X, the default amputated spaces:	SELECT RTRIM(’###Hello–World###’,’###’) FROM DUAL;	JAVASE
TRIM([X FROM]parameter)	The truncated sides parameter string X, truncated default space	SELECT TRIM(’#‘FROM’#Hello–World#’) FROM DUAL;	Hello–World
REPLACE(parameter,old,new)	Find old in the parameter, and replaced with new	SELECT REPLACE(‘Hello–World’,‘World’,‘nihao’) FROM DUAL;	Hello–nihao
SUBSTR(parameter,start[,length])	Back substring parameter, starting from the start, the interception of length characters, the default length, to the end of the default	SELECT SUBSTR(‘Hello–World’,8,5) FROM DUAL;	World

Common numerical function

function	Explanation	Show	result
ABS(X)	The absolute value of X	SELECT ABS(-9) FROM DUAL;	9
ACOS(X)	X inverse cosine	SELECT ACOS(1) FROM DUAL;	0
COS(X)	Cosine	SELECT COS(1) FROM DUAL;	0.5403023058
CEIL(X)	Greater than or equal to the minimum X	SELECT CEIL(9.9) FROM DUAL	10
FLOOR(X)	Less than or equal to the maximum value of X	SELECT FLOOR(9.9) FROM DUAL;	9
LOG(X,Y)	X-Y is the number of bottom	SELECT LOG(100,10) FROM DUAL;	0.5
V (X, Y)	X divided by Y remainder	SELECT MOD(10,3) FROM DUAL;	1
POWER(X,Y)	X的Y次幂	SELECT POWER(2,5) FROM DUAL;	32
ROUND(X[,Y])	X在第Y位四舍五入	SELECT ROUND(3.14155926，2) FROM DUAL;	3.14
SQRT(X)	X的平方根	SELECT SQRT(9) FROM DUAL;	3
TRUNC(X[,Y])	X在第Y位截断	SELECT TRUNC(3.1415926，3) FROM DUAL;	3.141

常用转换函数

--自动类型转换
SELECT 1+'99' FROM DUAL;

--将字符串转化为时间日期格式
SELECT add_months('15-3月-2020',2) FROM DUAL;

--TO_NUMBER   TO_CHAR   TO_DATE
SELECT SYSDATE FROM DUAL;

--把日期函数转换成字符串：TO_CHAR()
SELECT SYSDATE,TO_CHAR(SYSDATE) FROM DUAL;

--将数字转化为字符串
SELECT 1433223,TO_CHAR(1433223) FROM DUAL;

-- 将字符串转换成日期类型，必须要指定格式TO_DATE
SELECT MONTHS_BETWEEN(SYSDATE,TO_DATE('2019-03-15','YYYY-MM-DD')) FROM DUAL;

--将字符串转化为数字：字符串中必须只有数字
SELECT '123' ,TO_NUMBER('3.1415926') FROM DULA;

日期函数

ADD_MONTHS(d,n)，在某一个日期 d 上，加上指定的月数 n，返回计算后的新日期，d 表示日期，n 表示要加的月数
LAST_DAY(d)，返回指定日期当月的最后一天
ROUND(d[,format])，返回一个以 format 为格式的四舍五入日期值， d 是日期， format 是格式

① 如果 format 为“YEAR”则舍入到某年的 1 月 1 日，即前半年舍去，后半年作为下一年

② 如果 format 为“MONTH”则舍入到某月的 1 日，即前月舍去，后半月作为下一月

③ 如果 format 为“DAY”则舍入到最近的周的周日，即上半周舍去，下半周作为下一周周日

多种日期格式

YYYY：四位表示的年份
YYY，YY，Y：年份的最后三位、两位或一位，缺省为当前世纪
IYYY:ISO标准的四位年份
MM：01~12的月份编号
MONTH：全拼字符集表示的月份，右边用空格填补
D：当周第几天
DD：当月第几天
DDD：当年第几天
HH，HH12：一天中的第几个小时，12进制表示法
HH24：一天中的第几个小时，取值为00~23
MI：一小时中的分钟
SS：一分钟中的秒

常用日期查询

SELECT TRUNC(SYSDATE,'YEAR') FROM DUAL;

SELECT TRUNC(SYSDATE,'MONTH') FROM DUAL;

SELECT TRUNC(SYSDATE,'DAY') FROM DUAL;

--查询当天日期
SELECT TRUNC(SYSDATE) FROM DUAL;

--查询当月的第一天
SELECT TRUNC(SYSDATE, 'MM') FROM DUAL;

--查询当年第一天
SELECT TRUNC(SYSDATE,'YY') FROM DUAL;

--返回当前时间
SELECT TRUNC(SYSDATE, 'HH') FROM DUAL;

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