Functions in MySQL statements

Mysql Functions in statements

MySQL The functions in are mainly divided into the following four categories ： String function 、 Numerical function 、 Date function 、 Flow function .

1 String function

MySQL There are many built-in string functions , The common ones are as follows ：

give an example

-- concat :  String splicing 
select concat('Hello' , ' MySQL');
-- lower :  All lowercase 
select lower('Hello');
-- upper :  Capitalize all 
select upper('Hello');
-- substring :  Intercept substring 
select substring('Hello MySQL',1,5);
-- lpad :  padding-left   Due to changes in business requirements , Employee's job number , Uniform for 5 digit , Not enough at present 5 All digits are filled in the front 0.
update emp set workno = lpad(workno, 5, '0');

2 Numerical function

Common numerical functions are as follows ：

give an example

-- ceil： Rounding up 
select ceil(1.1);
-- round： rounding 
select round(2.344,2);
-- rand： Get random numbers 
select rand();
--  Through the functions of the database , Generate a six digit random verification code . The random number can be obtained by rand() function , But the random number obtained is in 0-1 Between , So you can multiply it by 1000000, Then discard the decimal part , If the length is not enough 6 position , repair 0
select lpad(round(rand()*1000000 , 0), 6, '0');

3 Date function

Common date functions are as follows ：

give an example

-- curdate： The current date , fit date Type data 
select curdate();
-- curtime： current time 
select curtime();
-- now： Current date and time , fit datetime type 
select now();
insert into t_user(create_time) values (now())
-- YEAR , MONTH , DAY： The current year 、 month 、 Japan , It can be used to intercept time 
select YEAR(now()); select MONTH(now()); select DAY(now());
-- date_add： Increase the specified time interval 
select date_add(now(), INTERVAL 70 YEAR );
-- datediff： Get the number of days between the two dates , Can cooperate with now() perhaps curdate To get the time difference 
select datediff('2021-10-01', '2021-12-01');
--  Query the enrollment days of all employees , And sort them in reverse order according to the number of days of employment .
select name, datediff(curdate(), entrydate) as 'entrydays' from emp order by entrydays desc;

4 Flow function

Flow function is also a kind of commonly used function , Can be in SQL Statement to implement conditional filtering , So as to improve the efficiency of statements .

give an example

ifnull The first value is null, The value is the second

-- if
select if(false, 'Ok', 'Error');
-- ifnull  The first value is null, The value is the second 
select ifnull('Ok','Default');
select ifnull('','Default');
select ifnull(null,'Default');
-- case when then else end  It is often used to judge the grade 
--  demand :  Inquire about emp Name and working address of the employee on the form  ( Beijing / Shanghai  ---->  first-tier cities  ,  other  ---->  Second-tier cities )
select 
name, 
( case workaddress when ' Beijing ' then ' first-tier cities ' when ' Shanghai ' then ' first-tier cities ' else ' Second-tier cities ' end ) as ' Work address '
from emp;

select
id, name, 
(case when math >= 85 then ' good ' when math >=60 then ' pass ' else ' fail, ' end ) ' Math grades ',
(case when english >= 85 then ' good ' when english >=60 then ' pass ' else ' fail, ' end ) ' English grade ',
(case when chinese >= 85 then ' good ' when chinese >=60 then ' pass ' else ' fail, ' end ) ' Chinese grades ' 
from score;

5 Other functions

Add some functions used by the landlord in the actual development

• GROUP_CONCAT(str)： Grouping string splicing , And Use together in groups

Case study ： Query enterprise information and enterprise classification information , Among them, the relationship between enterprise classification information and enterprise is many to many , Query according to the common joint table , We will find that one piece of enterprise information corresponds to multiple enterprise classifications , Multiple records will appear . If you want to classify different enterprises of the same enterprise into one record , You can consider using it GROUP_CONCAT, This function will splice multiple pieces of data onto one piece of data （ String segmentation is used by default ）

select 
t_enterprise.*,
GROUP_CONCAT(DISTINCT  dic_enterprise_category.name) as enterpriseCategoryNames,
dic_operation_status.name as operationStatusName,
dic_region.name as regionName
from t_enterprise
	left join ref_enterprise_category
        on t_enterprise.id = ref_enterprise_category.enterprise_id 
	left join dic_enterprise_category
        on dic_enterprise_category.id = ref_enterprise_category.category_id
    left join dic_operation_status
        on t_enterprise.operate_status_id = dic_operation_status.id
	left join dic_region
        on t_enterprise.region_id = dic_region.id
GROUP BY id				

• FIELD(str,str1,str2,str3)： among str Is the field name , It can be done to SQL The result set of the query Sort in the specified order , Field str Press str1,str2,str3 Returns the query result set in the order of

Case study ： The result of the query will be pushed to the status （0: normal 1: Youtui 2: Off the shelf ） Push by priority , normal , Off shelf sorting , If the status is the same, it will be updated in descending order

select  Query data  from  The query table  where  Query criteria  order by FIELD(push_status,1,0,2)