Study notes 22/1/11

Function learning

1.trim() function

（1）trim()
grammar 1：trim([LEADING | TRAILING | BOTH [ str1] from] str2)

effect ： Delete the space around the string

Parameters :
leading | trailing | both from : These prefixes specify the deletion part ,leading from The prefix is to delete the previous part ,trailing from Is to delete the following part ,both from Is to delete both before and after ;
str1： Characters to be deleted , Do not write default delete space
str2： Target string

notes ：trim The specified characters can be deleted , But only a single character can be specified , Cannot specify string ;

eg： Delete the corresponding string ‘ WUHUQIFEI ’ Spaces at both ends

SELECT TRIM(' WUHUQIFEI ')  "TRIM EG"
FROM DUAL;

eg： Delete the corresponding string ‘ AAAAWUHUQIFEIAAAA’ The one in front of the middle A

SELECT TRIM(leading 'A' FROM 'AAAAWUHUQIFEIAAAA')  "TRIM EG"
FROM DUAL;

（2）ltrim（） And rtrim（） function
grammar ：ltrim | rtrim（str1 , str2）

effect ： Delete str1 The left or right exists in the character set str2 The characters in

Parameters ：
str1： Target string
str2： Character set

Return value ： Returns a string after deletion （ Character type ）

eg： Delete string ’xyxxDWEYExyyx’ The one on the right in the middle x And y

select rtrim('xyxxDWEYExyyx','xy') "e.g." from dual;

notes ： Pay attention to ltrim（） Function and rtrim The second argument to the function is the character set , It is a character set regardless of order ;

2.ROUND() Function and trunc() function

（1） When used with numeric data types ：
a.round（） function
grammar ：round（num1 [,num2]）

effect ： Yes num1 according to num2 To round ,num2 If it is not written, it will be rounded to decimal by default

Parameters ：
num1： Target parameters
num2： How many digits to round
When it is greater than zero , Count from the right of the decimal point
When it's zero , Round to an integer
When it's less than zero , Count from the left of the decimal point
Do not write default to zero

Return value ： Returns the number after rounding （ Numeric type ）

b.trunc（） function
grammar ：trunc（num1 , num2 ）

effect ： Yes num1 Intercept the specified num2 The number of

Parameters ：
num1： Target parameters
num2： Intercepts the specified location
If it's a positive number , Intercept from the decimal point （ Such as 1.5555 3 Is intercepted as 1.555）
If it is zero or not written , Is truncated to an integer
If negative , Then intercept the positive part （ Such as 1136.8418 -2 The result is 1100）

Return value ： Return the intercepted data （number type ）

（2） When used with date types ：
a.round（） Use of functions
grammar ：round（date,’ Format ’）

effect ： Round off in the following format date date

Parameters ：
date： Date of operation
Format ： The format is shown in the note below , Different formats correspond to different latest target dates

Return value ： Returns the most recent date （date type ）

eg： If “YEAR” Round to the year's 1 month 1 Japan , The first half of the year , The second half of the year is the next year

SELECT ROUND(TO_DATE('20210719','YYYYMMDD'),'YEAR')   second half of a year 
       ,ROUND(TO_DATE('20210425','YYYYMMDD'),'YEAR')  The first half of the year 
FROM DUAL;

notes ： The keyword correspondence used to define the format in the following single quotation marks ：
‘YEAR’– Returns the most recent year , In the first half of the year, it returns to January 1 at the beginning of this year , The second half of the year returns to January 1 of the following year ;
‘MONTH’– Returns the most recent month , The first half of the month returns to the first day of the month , The latter half of the month returns to January 1 of the next month ;
‘DDD’– Return to the last day
‘DAY’– Return to the most recent Sunday
‘W’– Returns the most recent and operation date The first day of the month has the same number of weeks
‘IW’– Return to the most recent Monday

SELECT ROUND(TO_DATE('20210719','YYYYMMDD'),'YEAR')   year 
       ,ROUND(TO_DATE('20210425','YYYYMMDD'),'MONTH')  month 
			 ,ROUND(TO_DATE('20210425','YYYYMMDD'),'Q')  quarter 
			 ,ROUND(TO_DATE('20210425','YYYYMMDD'),'DD')   Japan  --DDD It is also the abbreviation of day  
       ,ROUND(TO_DATE('20210425','YYYYMMDD'),'DDD')  Japan 
			 ,ROUND(TO_DATE('20210425','YYYYMMDD'),'DAY')  Sunday 
			 ,ROUND(TO_DATE('20210425','YYYYMMDD'),'W')  Thursday 
			 ,ROUND(TO_DATE('20210425','YYYYMMDD'),'IW')  Monday 
FROM DUAL;

b.trunc（） function
grammar ：trunc（date,‘ Format ’）

effect ： Intercept the date

Parameters ：
date： Operated date data
Format ： Format and round Basic generality of
When the year is intercepted , Intercepted to January 1 of this year
When the quarter is intercepted , Intercept to the first day of this quarter
When the month is intercepted , Intercept to the first day of this month

Be careful ：

Talk about interception

3.replace() Function and translate() function

grammar ：replace（ Field , str1 , str2）
effect ： Put the... Contained in the field str1 Replace the part of with str2 character

4. Fill function

（1）lpad（） function

（2）rpad（） function

5.ASCII() Function and CHR() function

（1）ASCII（） function
grammar ：ASCII（str）

effect ： Read str Of ASCII Code value

Parameters ：
str： Target character or string

Return value ： Returns the of the objective function ASCII Code value （ Integer types ）

eg：ASCII(x) Return character x Of ASCII code .

SELECT ASCII('C') FROM DUAL;
SELECT ASCII('a') FROM DUAL;

eg： When str When it's a string , The first letter is returned ASCII code

SELECT ASCII('AB') FROM DUAL;
SELECT ASCII('CBA') FROM DUAL;

（2）CHR（） function
grammar ：CHR（int）

effect ： Output ASCII The code value is equal to int The characters of

Parameters ：
int： An integer

Return value ： return ASCII Code for int A character of （ Character type ）

eg： Output ASCII The code value is equal to 65 The characters of

SELECT CHR(65) FROM DUAL;

notes ：ASCII The code table has only 128 position , When int Greater than 127 when ,
Due to the expansion of character coding system , At this time, the corresponding display is GBK code （《 Code extension specification for Chinese characters 》）, So it won't overflow , It shows GBK Some characters in the code ;

eg： Show GBK Code for 777 The characters of ;

SELECT CHR(777) FROM DUAL;

6.instr() function

grammar ：instr（ Field name ,str1[,pos] [,nth]）

effect ： Find... In the corresponding field str1 String location

Parameters ：
Field name ： Target field
str1： Specified to match characters or strings
pos： Where to start matching , Do not write default is 1, When pos When it is equal to a negative number, it is from the penultimate pos Bits count from back to front
nth： Refers to the number of times the specified matching string needs to appear

Return value ： Returns the position of the matched character or string

eg： stay "helloworld" The last of 1(d) No. 1 position starts , Look back for the second time “l” The location of

select instr('helloworld','l',-1,2) from dual;

7.substr() function

grammar ：substr（str,pos[,len ]）

effect ： stay str In the from pos Characters start to be intercepted backwards len A character long string ;

Parameters ：
str： Intercepted string
pos： Where to start intercepting , When it's negative , Number from last string , Such as -3 Is to intercept from the penultimate position
len： How long is the interception , Do not write default intercept to the end

Return value ： Return from str The string intercepted in the middle （ Character type ）

notes ：substr（） When the function intercepts a string, it always intercepts it from front to back , Even in the middle pos The argument is negative , It is also intercepted from front to back ;
Another way ,substr（） The execution of a function is , Get the target string first str, And then according to pos Starting number （ Negative numbers are counted from the back to the front ）, After the reservation, start from pos Start from front to back ;

eg： Find out that the first letter of the name is A The employees'

SELECT *
FROM EMP E
WHERE SUBSTR(E.ENAME,1,1) = 'A' ;

eg： Use one sql Statement to extract the city information of each person

CREATE TABLE zone (
                  name VARCHAR2(20),
                  address VARCHAR2(50)
                  );
 
 INSERT INTO zone VALUES(' Zhang San ',' hubei - yichang - Five peaks ');
 INSERT INTO zone VALUES(' Li Si ',' Inner Mongolia - Hohhot, - Qingshuihe ');
 INSERT INTO zone VALUES(' Xiao Ming ',' guangdong - Shenzhen - Bao'an ');

COMMIT;


SELECT A.NAME,
       SUBSTR(SUBSTR(A.ADDRESS,INSTR(A.ADDRESS,'-')+1),1,
			 INSTR(SUBSTR(A.ADDRESS,INSTR(A.ADDRESS,'-')+1),'-')-1) CITY
FROM ZONE A;

Subquery review