SQL Server about like operator (including the problem of field data automatically filling in spaces)

SQL Can be used in like Keyword combined with wildcard for fuzzy query . But there are some small problems in the data type of the field .SQL The supported wildcards are ：%、[ ]、[ ^ ]、_, among [ ] In Microsoft Access and SQL Sever Support collection .

1、 The percent character （%）

create table Person(
ID Varchar(20),
name Varchar(20),
age Varchar(10),
sex Varchar(2)
)
  
insert into Person values ('0001',' Delireba ','20','m')
insert into Person values ('0002',' Yang Mi ','20','m')
insert into Person values ('0003',' Andy ','20','m')
insert into Person values ('0004','angalbaby','20','m')

For example, you need to query name Information about the person surnamed Yang

select ID,name,age,sex
from Person
where name like ' Yang %'  --% Can match any character , Include 0 individual 
-- Query contains baby People's information 
select ID,name,age,sex
from Person
where name like '%baby%'

Note in particular that the field type is nchar（n） The data of , If you define name The fields are nchar（20） Type, that is, the length of this field is 20, If the filled text is not filled, include Access A lot of it DBMS Will use spaces to supplement the contents of the field . For example, I want to inquire name For information about people ending in a power, use the following statement ：

select ID,name,age,sex
from Person
where name like '% power '

Will not be retrieved , The wildcard should be changed to ’% power %’.
This is because nchar（n） The type is fixed length , Storage Unicode character , The data length is insufficient n Fill in the half width space in English .

And here's a rule ：
When unicode data （nchar or nvarchar） And like When used together , Trailing spaces make sense . But right and wrong unicode data , Trailing spaces are meaningless . That's why char（n） The type is also a fixed length but does not match spaces .

in the light of SQL Of varchar For a detailed description of string data types, please refer to
https://blog.csdn.net/m0_46412065/article/details/105027680

2、 Underline _ wildcard

And % similar , But it only matches a single character . Of course, you can use multiple underscores to match multiple characters . For example, if you want to query the two character surname Yang

select * from 
Person
where name like ' Yang _'

You also need to pay attention to underline matching nchar（n） The problem with type data . If name The data type of the field is nchar（10）, Query with the above statement will not match any data .
3、[ ] and [^]
such as

select * from 
Person 
where age like '[0-9]0'

Will match age Field to 0~9 Any beginning between , With 0 Ending character

select * from 
Person 
where id like '%[^2-4]'

Will not match 2~4 Any length string at the end