Oracle built-in functions

1. Classification of built-in functions

• Built in functions ：Oracle Self contained function , Just call it , No need to define
• Oracle The built-in functions are divided into the following categories ：

1. Conversion function ; Convert data types
2. Date function ;
3. Character functions ;
4. Number function ;
5. Set function ;

2. Conversion function

2.1 to_char();

• effect ： Convert a non character type to a character type ;

-- Call the current system date 
select sysdate from dual;

-- Date type to character type 
-- Convert the current system time to string type , The time format is ：xxxx-xx-xx xx：xx：xx
select to_char(sysdate,'yyyy-mm-dd hh:mi:ss') from dual;

-- Convert the current system time to string type , Only the date of the year is displayed , The time format is ：xxxx-xx-xx
select to_char(sysdate,'yyyy-mm-dd')from dual;

-- Convert the current system time to string type , And the display is 24 hourly , The time format is ：xxxx year xx month xx Japan  xx:xx:xx
select to_char(sysdate,'yyyy" year "-mm" month "-dd" Japan " hh24:mi:ss') from dual

2.2 to_date();

• effect : Convert the time with large character type to date type ;

-- Put the character type time  '2021-08-18'  Convert to date type , The format is ：xxxx/xx/xx;
select to_date('2021-08-18','yyyy/mm/dd') from dual

-- Put the character type time  '2021-08-18 11:33:44'  Convert to date type , The format is ：xxxx/xx/xx xx:xx:xx;
select to_date('2021-08-18 11:33:44','yyyy/mm/dd hh:mi:ss') from dual

2.3 to_number();

• effect ： Convert numbers of character type to numeric type ;

select 1+to_number('1') from dual;

3. Date function

3.1 add_months()

• effect ： Add a specified number of months to the specified date , Find the current date ;
• Be careful ： This can only work on date types ;

-- Find the current system time 100 The date after months ;
select add_months(sysdate,100) from dual;

-- seek 2020 year 8 month 8 Japan ,100 The date after months ;
select add_months(to_date('2020-8-8','yyyy-mm-dd'),-2) from dual;

-- seek 2020 year 8 month 8 Japan ,100 The date three months ago ;
select add_months(to_date('2020-08-08','yyyy-mm-dd'),-100)from dual;

3.2 extract()

• effect ： Take the month, year and day in the date separately ;
• Be careful ： It can only work with date type ;

-- Take the year of the current system time 
select extract(year from sysdate)from dual;

-- Take the month of the current system 
select extract(month from sysdate)from dual;

-- Take the current system time and date 
select extract(day from sysdate)from dual;

-- seek emp table 2 Number of employees in the month 
select count(*)from emp where extract(month from hiredate)=2;

-- seek emp The number of employees in each month in the table 
select extract(month from hiredate),count(*)from emp group by extract(month from hiredate);

3.3 months_between()

• effect ： Find the month of the difference between two times ;
• Be careful ： Can only act on date types ;

--  seek 2020-2-2 The month that is different from the current system time ;
select months_between(sysdate,to_date('2017-02-02', 'yyyy/mm/dd')) from dual;

3.4 Last_day()

• effect ： Ask for the last day of this month ;

-- Find the last day of the current month ;
select last_day(sysdate) from dual;

-- seek 2021 The last day of the year ;
select last_day(to_date('2021-12', 'yyyy-mm'))from dual;

3.5 Next_day()

• effect ： Find the date of the next specified week ;
• Be careful ： Can only act on date types ;

-- Find the date of next Saturday ;
select next_day(sysdate,' Saturday '）from dual;

4. Character functions

select upper('abcdAsdfa') from dual;
-- Capitalize all ;

select lower('abcdAsdfa') from dual;
-- All to lowercase ;

select trim('abcd','a') from dual;
-- Remove the left character ( The first character is a Then remove , On the contrary, do not remove );

select trim ('acccsdfdsccccbcd','c') from dual:
-- Remove the character on the right ( The first character is c Then remove , On the contrary, do not remove );

select initcap('adad') from dual;
-- Convert initials to uppercase ;

select substr('abcdefghi',3,4) from dual;
-- Cut from the third place , section 4 individual ;

select instr('abcdefg', 'c') from dual;
-- Judge c The location of , Subscript from 1 Start ;

select concat ('aa', 'bb') from dual;
-- This means splicing , Less use , Because we can use || Instead of ;

select ascii('a') from dual;
-- seek a Of ic code ;

select char(77) from dual;
-- take ic The code is converted into the corresponding value character ;

select length ('abcd Zhang ') from dual;
-- Find the number of characters , Not the number of bytes ;

select (pad ('abde' ,9,'x') from dual;
-- padding-left , Start from the left x’ fill , Until the length of bytes becomes 9 until ;

select rpad ('abde' ,9,'x') from dual;
-- Right fill , Start from the edge x fill , Until the length of bytes becomes 9 until ;

select decode('c', 'a',1,'b',2,'c',3) from dual;
-- The result is 3, It means to judge several results , If the preceding symbol is a It outputs 1, If it is b It outputs 2, If it is c It outputs 3;

5. Mathematical functions

select ceil (57.12) from dual;
-- Rounding up ;

select floor (57.98) from dual;
-- Don't round ;

select round (54.99) from dual;
-- The result is 55, rounding ;

select round (54.59,1) from dual;
-- Round to the nearest decimal point 1 digit ;

select power (3,2) from dual;
--3 Of 2 Power 

select mod (6,7) from dual;
-- Remainder / model ;

select sqrt (4) from dual;
-- Square root ;

select sign (0) from dual;
-- If it is a positive number, the result is 1, If it is negative, the result is -1, If it is 0, The results for 0;

6. Comprehensive case

If the salary is less than 3500 It outputs “ Work hard ”, If it is equal to 3500 It outputs “ It is just fine ”, If it is greater than 3500 It outputs “ To pay taxes ”

select ename, decode(sign(sal-3500),1,' To pay taxes ',0,' It is just fine ',-1,' Work hard ')from emp;

7. Aggregate functions

7. Aggregate functions .

• sum, max, min, avg, count.