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It can Encapsulate the code we often use , Call directly when necessary . This is both Improved code efficiency , also Improved maintainability . stay SQL We can also Use functions to perform function operations on the retrieved data . Use these functions , Can greatly Improve the efficiency of user management of database .

Classification of functions

From the perspective of function definition , We can divide the function into Built in functions and Custom function . stay SQL In language , It also includes built-in functions and custom functions .

Built in function is a general function built in the system , The custom function is written according to our own needs

Two kinds of SQL function

One line function

Manipulate data objects
Accept parameters and return a result
Change only one line
Each line returns a result
Can be nested
The parameter can be a column or a value

Two 、 Numerical function

1. Basic functions

SELECT ABS(-123),ABS(32),SIGN(43),PI(),CEIL(32.32),CEILING(-32.32),FLOOR(32.32)
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;

Single line functions can be nested

SELECT TRUNCATE(ROUND(123.456,2),0)
FROM DUAL;

2. Angle and radian exchange function

SELECT RADIANS(30),RADIANS(60),RADIANS(90),
DEGREES(2*PI()),DEGREES(RADIANS(90))
FROM DUAL;

3. Trigonometric functions

SELECT SIN(RADIANS(30)),DEGREES(ASIN(1)),TAN(RADIANS(45)),DEGREES(ATAN(1))
FROM DUAL;

4. Exponents and logarithms

① Index

SELECT POW(2,5),POW(2,-1),POW(4,-2),EXP(2)
FROM DUAL;

② logarithm

SELECT LN(EXP(2))
FROM DUAL;

5. Conversion between bases

SELECT BIN(10),HEX(10),OCT(10),CONV(10,10,8)
FROM DUAL;

3、 ... and 、 String function

Be careful ：MySQL in , The position of the string is from 1 At the beginning .

1. test 1 ：ASCII CHAR_LENGTH LENGTH

SELECT ASCII('Abcvbn'),CHAR_LENGTH('HELLO'),CHAR_LENGTH(' Hello '),LENGTH('HELLO'),LENGTH(' Hello ')
FROM DUAL;

2. test 2：CONCAT CONCAT_WS

SELECT CONCAT(e.last_name,' welcome ',' to ',l.city)
FROM employees e JOIN locations l;

SELECT CONCAT_WS('-','HELLO','WORLD','HELLO','XI\'AN')
FROM DUAL;

3. test 3 ：INSERT,REPLACE

# The string HELLOWORLD From 2 Position start ,3 Substrings of characters long are replaced with strings aaaaa

SELECT INSERT('HELLOWORLD',2,3,'aaaaa')
FROM DUAL;

SELECT REPLACE('hello','ll','mmm')
FROM DUAL;

Four 、 Date and time functions

1. Get date 、 Time

SELECT CURDATE(),CURRENT_DATE(),CURTIME(),NOW(),SYSDATE(),
UTC_DATE(),UTC_TIME()
FROM DUAL;

2. Date and time stamp conversion

①UNIX_TIMESTAMP()

With UNIX Returns the current time in the form of a timestamp .

SELECT UNIX_TIMESTAMP()
FROM DUAL;

②UNIX_TIMESTAMP

Time date With UNIX Return in the form of a timestamp .

SELECT UNIX_TIMESTAMP('2022-2-22 22:22:22')
FROM DUAL;

③FROM_UNIXTIME

take UNIX The time of the timestamp is converted to the time of the normal format

SELECT FROM_UNIXTIME(1644726483)
FROM DUAL;

3. Get month 、 week 、 Weeks 、 Functions such as days

SELECT YEAR(CURDATE()),MONTH(CURDATE()),DAY(CURDATE()),
HOUR(CURTIME()),MINUTE(NOW()),SECOND(SYSDATE())
FROM DUAL;

SELECT MONTHNAME('2022-2-22'),DAYNAME('2022-2-22'),WEEKDAY('2022-2-22'),
QUARTER(CURDATE()),WEEK(CURDATE()),DAYOFYEAR(NOW()),
DAYOFMONTH(NOW()),DAYOFWEEK(NOW())
FROM DUAL;

4. Operation function of date

EXTRACT(type FROM date) Function type The value and meaning of ：

SELECT EXTRACT(SECOND FROM NOW()),EXTRACT(DAY FROM NOW()),
EXTRACT(HOUR_MINUTE FROM NOW()),#1244:12:44
EXTRACT(QUARTER FROM NOW()) # Get Quarterly 
FROM DUAL;

5. Time and second conversion function

SELECT TIME_TO_SEC(CURTIME()),
SEC_TO_TIME(46066)
FROM DUAL;

6. A function that calculates the date and time

The first ① Group

SELECT NOW(),DATE_ADD(NOW(),INTERVAL 1 YEAR)# Add one year to the current time 
FROM DUAL;

SELECT DATE_ADD(NOW(),INTERVAL 1 DAY) AS col1,DATE_ADD('2022-2-22 22:22:22',INTERVAL 1 SECOND) AS col2,
ADDDATE('2022-2-22 22:22:22',INTERVAL 1 SECOND) AS col3,
DATE_ADD('2022-2-22 22:22:22',INTERVAL '1_1' MINUTE_SECOND) AS col4,
DATE_ADD(NOW(), INTERVAL -1 YEAR) AS col5, # It could be negative 
DATE_ADD(NOW(), INTERVAL '1_1' YEAR_MONTH) AS col6 # Need single quotes 
FROM DUAL;

SELECT DATE_SUB('2022-2-22',INTERVAL 31 DAY) AS col1,
SUBDATE('2022-2-22',INTERVAL 31 DAY) AS col2,
DATE_SUB('2022-2-22 22:22:22',INTERVAL '1 1' DAY_HOUR) AS col3
FROM DUAL;

The first ② Group

SELECT
ADDTIME(NOW(),20),SUBTIME(NOW(),30),
SUBTIME(NOW(),'1:1:3'),
DATEDIFF(NOW(),'2022-2-22'),
TIMEDIFF(NOW(),'2022-2-22 22:22:22'),
FROM_DAYS(366),TO_DAYS('0000-12-25'),
LAST_DAY(NOW()),MAKEDATE(YEAR(NOW()),12),
MAKETIME(10,21,23),
PERIOD_ADD(20200101010101,10)
FROM DUAL;

7. Formatting and parsing of dates

format ： date ————> character string
Explain Analysis ： character string ————> date
At this point, we are talking about the explicit formatting and parsing of dates , What I have been exposed to before is the implicit formatting or parsing of dates
SELECT *
FROM employees
WHERE hire_date = '1993-01-13';
' ' The writing method in must be the same as the original date format to find , The date cannot be reversed

① Above non GET_FORMAT Function fmt Common format characters for parameters ：

②GET_FORMAT Function date_type and format_type The parameter values are as follows ：

（1） format

SELECT 
DATE_FORMAT(CURDATE(),'%Y-%M-%D') AS DATE1,
DATE_FORMAT(NOW(),'%Y-%m-%d') AS DATE2,
TIME_FORMAT(CURTIME(),'%H:%i:%S') AS DATE3,
TIME_FORMAT(CURTIME(),'%h:%i:%S') AS DATE4,
DATE_FORMAT(NOW(),'%Y-%M-%D %h:%i:%S %W %w %T %r') AS DATE5
FROM DUAL;

（2） analysis ： The reverse of formatting

SELECT STR_TO_DATE('2022-February-13th 03:11:05 Sunday 0 15:11:05',
'%Y-%M-%D %h:%i:%S %W %w') AS RESULT
FROM DUAL;

SELECT GET_FORMAT(DATE, 'USA')
FROM DUAL;

SELECT DATE_FORMAT(CURDATE(),GET_FORMAT(DATE, 'USA'))
FROM DUAL;

5、 ... and 、 Process control functions

Process processing functions can be based on different conditions , Perform different processes , Can be in SQL Statement to implement different conditional choices .

MySQL The process processing functions in mainly include IF()、IFNULL() and CASE() function .

# example 1
SELECT last_name,salary,IF(salary > 6000,' High wages ',' Low wages ') "details"
FROM employees;

# example 2
SELECT employee_id,salary, CASE WHEN salary>=15000 THEN ' High salaries '
WHEN salary>=10000 THEN ' potential share '
WHEN salary>=8000 THEN ' Prick silk '
ELSE ' grassroots ' END " describe "
FROM employees;

# example 3
SELECT oid,`status`, CASE `status` WHEN 1 THEN ' Unpaid '
WHEN 2 THEN ' Paid '
WHEN 3 THEN ' Shipped '
WHEN 4 THEN ' Confirm receipt '
ELSE ' Invalid order ' END
FROM t_order;

6、 ... and 、 Encryption and decryption functions

The encryption and decryption function is mainly used to encrypt and decrypt the data in the database , To prevent data from being stolen by others . These functions are very useful in ensuring database security .

You can see ENCODE(value,password_seed) Function and DECODE(value,password_seed) Functions are inverse functions to each other .

SELECT PASSWORD('MySQL')  #PASSWORD stay MySQL 8.0 To discard 
FROM DUAL; 

SELECT MD5('MySQL'),SHA('MySQL')
FROM DUAL;

notes ：PASSWORD & SHA Encryption is irreversible

7、 ... and 、 MySQL Information functions

MySQL There are some built-in functions that can be queried MySQL Functions of information , These functions are mainly used for Help database development or operation and maintenance personnel better maintain the database .

SELECT VERSION(),# Version number 
CONNECTION_ID(),# Connect ID
DATABASE(),SCHEMA(),USER(),CURRENT_USER(),
CHARSET(' China '),COLLATION(' China ')
FROM DUAL;