Introduction and advanced level of MySQL (9)

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1、 Built in functions

Function name Example The functionality
Concat concat(s1,s2…sn) Pass ⼊ Parameter connection assembly of ⼀ A string .
Length length(‘hello world’) The result is 11 Return string ⻓ degree
Upper upper(‘abcd’) Return to ABCD Convert the string to ⼤ Write
Ltrim Ltrim(‘ abc’) Return to ’abc’ Remove the space to the left of the string
Rtrim Rtrim(‘abc ’) Return to ’abc’ Remove the space to the right of the string
Replace Replace(‘abcccd’,‘c’,‘x’) Return to ’abxxxd’ take abcccd in c Replace with x
Locate Locate(‘a’,‘helloaaa’) return 6 return ⼦ strand a In string helloaaa The first ⼀ There are four positions , Returns if it does not exist 0
Substring substring(‘Johnson’,5,3) Return to ‘son’ From 5 I'm starting to intercept ⻓ Degree is 3 String
Now now() Returns the current time of the system Returns the current time of the system
TIMESTAMPDIFF TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)
example ：
SELECT TIMESTAMPDIFF
(MONTH,‘2009-09-01’,‘2009-10-01’);
return 1
return ⽇ Period or ⽇ Integer difference between period time expressions
unit It can be ： MICROSECOND
(microseconds), SECOND, MINUTE,
HOUR, DAY, WEEK, MONTH,
QUARTER, YEAR.
DateAdd DATE_ADD(‘1998-01-02’, interval 1
YEAR)
return 1999-01-02
Want to ⽇ Add numbers to the specified part of the period , among ：YEAR
It means year ,month Express ⽉,day Table is ⽇
Dayofweek dayofweek('2008-08-08‘) return 6
dayofweek：（1 = Sunday, 2 = Monday,
…, 7 = Saturday）
return ⼀ Mid week position
DateDiff select datediff(‘2008-08-08’, ‘2008-08-
01’); The result returned to 7
Two ⽇ Period minus date1 - date2, Return days
Date date(‘2008-09-10 07:15:30’);
return 2008-09-10
Returns a... As a string ⽇ Period part .
Abs Abs(-1) Return to ”1” Find the absolute value
Ceiling Ceiling(24.1) The result returned to 25 ⼤ On 24.1 The most ⼩ Integers
Floor Floor(24.1) The result returned to 24 ⼩ On 24.1 The most ⼤ Integers
Power Power(2,3) The result returned to “8” Calculation 2 Of 3 Time ⽅
Round Round(68.32,1) The result returned to 68.30 return ⼀ A digital , House ⼊ To the specified ⻓ Degree or accuracy
Sign Sign(123) The result returned to 1 Returns the symbol of a numeric value , Positive and negative zeros are returned respectively 1,
-1, 0
Sqrt Sqrt(16) The result returned to “4” Kaiping ⽅
Convert convert(1234,char(4))
The result returned to ”1234”
Data type conversion function , take 1234 Numeric type to
Replace with char type
Length length(‘ Come on China ’) The result returned to 12 Returns the number of bytes of any data type
ifnull(expr,value) select ifnull(city,‘ Unknown ’) from studentInfo If the value of the expression ⾮ empty , Returns the
value , Otherwise return to value
select user,host from mysql.user Returns the name of the computer and returns the current ⽤ The name of the computer on which the user logs in
SELECT Current_User Returns the current ⽤ The login name of the user returns the current ⽤ Household

2、SQL actual combat

--  function ——js function java The methods are very similar , There are receiving parameters and return values , A tool for writing is used to make fixed 
 Content 
1
2 --  Intercepting string   Determine whether it is null  —— Functions are tools that come with the database itself 
34 --  The function uses the format ：
5 --  Writing directly is actually a demonstration. Just get the value directly 
6 select CONCAT('a','b','c') as  Name 1;
7 --  Source table , Operate on the column value of each row and return 
8 select CONCAT(studentid,'_',studentname) from studentinfo
9 where CONCAT(studentid,'_',studentname)='2011001002_ Ma Yu '
101 -- SUBSTRING(str,pos) The first target string   The second is the location of the interception   Location slave 1 Start 
12 select SUBSTRING('abcdefg',2);
13 --  The first is the goal   The second is the starting position   The third length 
14 select SUBSTRING('abcdeft',2,3);
15 -- SUBSTR(str,pos)
16 select substr('abcdeft',2,3);
17 --  Operation table ： Check all people surnamed Zhang 
18 select SUBSTR(studentname,1,1),a.* FROM studentinfo a
19 where SUBSTR(studentname,1,1)=' Horse '
201 --  Get system time 
22 select now()
234 --  Date plus 
25 --  Back to 4 Months 
26 select DATE_ADD('2020-3-24',INTERVAL -4 month)
27 --  Year plus 2 year 
28 select DATE_ADD('2020-3-24',INTERVAL 2 year)
29 --  Add 40 God 
30 select DATE_ADD('2020-3-24',INTERVAL 40 day)
312 --  Determine whether it is null -- If there is a value, it shows , Not displayed as none 
33 select IFNULL(Birthday,' nothing '),a.* from studentinfo a
345 --  Integer 
36 --  Down to the whole 
37 select floor(23.6666),floor(23.333)
38 --  Up to the whole 
39 select ceil(23.6666),ceil(23.333)
40 --  rounding 
41 select ROUND(23.6666,2),round(23.333,2)