String function of MySQL function

Table of contents

1.concat: combine two or more strings into one string

2.length and char_length functions: get the length of the string in bytes and characters

3.left: returns the left part of the string with the specified length

4.right: return the right part of the string with the specified length

5.replace: Search and replace substrings in strings.

6.substring: Extract a substring from a position with a specific length.

7.trim: Remove unwanted characters from a string.

8. find_in_set: Find a string in a comma-separated list of strings

9.format: Format numbers with specific regional settings, rounded to decimal places.

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1.concat: combine two or more strings into one string

MySQL CONCAT()functions take one or more string arguments and concatenate them into a single string. CONCAT()Function requires at least one argument, otherwise an error is raised

Convert all arguments to string types before concatenating. If any argument is NULL, CONCAT()the function returns NULLa value.

If you want to use connectors, you need to splice connectors between the connected items.

查询语句：select CONCAT(supplier_id,'-',supplier_name) from bms_bills_memo where id = 4
结果：6-供应商4

CONCAT_WS()Function concatenates two or more string values ​​with predefined delimiters.

The first parameter is the delimiter, followed by the parameters to be spliced

Returns null only if the delimiter is null, if the item is null, no concatenation

查询语句：
select CONCAT_WS('-',supplier_id,supplier_name) from bms_bills_memo where id = 4

select CONCAT_WS('-',null,supplier_name) from bms_bills_memo where id = 4

select CONCAT_WS('-',null,null) from bms_bills_memo where id = 4
结果：
6-供应商4
供应商4
空

2.length and char_length functions: get the length of the string in bytes and characters

length: Get the length of the string in bytes

char_length: Get the length of the string in characters

Byte is a unit of measurement, which indicates the amount of data. It is a unit of measurement used by computer information technology to measure storage capacity. Usually, one byte is equal to eight bits.

Character (Character) Letters, numbers, words and symbols used in computers, such as 'A', 'B', '$', '&', etc.

Generally, in the English state, a letter or character occupies one byte, and a Chinese character is represented by two bytes.

• In ASCII code, an English letter (not case sensitive) is one byte, and a Chinese character is two bytes.
• In UTF-8 encoding, an English character is one byte, and a Chinese character is three bytes.
• In Unicode encoding, one English is one byte, and one Chinese is two bytes.
• Symbol: English punctuation is one byte, Chinese punctuation is two bytes. For example: English period . occupies 1 byte, Chinese period . Occupies 2 bytes in size.
• In UTF-16 encoding, the storage of an English alphabet character or a Chinese character requires 2 bytes (the storage of some Chinese characters in the Unicode extension area requires 4 bytes).
• In UTF-32 encoding, the storage of any character in the world requires 4 bytes.

查询语句：
select LENGTH(supplier_name) from bms_bills_memo  where id = 4

select CHAR_LENGTH(supplier_name) from bms_bills_memo  where id = 4
结果：
10
4
表中数据
supplier_name：供应商4

In conjunction with

SELECT postid,
       title,
       IF(CHAR_LENGTH(excerpt) > 20,
          CONCAT(LEFT(excerpt,20), '...'),
          excerpt) summary
FROM posts;

We use IFa function to check if the length of the excerpt column is greater than 20, and use the CONCAT statement to concatenate excerptthe value of the column with an ellipsis ( ), otherwise just get the entire excerpt ( ) content...excerpt

3.left: returns the left part of the string with the specified length

查询语句：
select LEFT(client_name,4) from bms_bills_memo where id = 4
结果：
青岛自贸
表中数据
client_name：青岛自贸新零售体验中心有限公司

LEFT()The function accepts two parameters:

• stris the string from which to extract the substring.
• lengthis a positive integer specifying the number of characters to return from the left.

LEFT()The function returns strthe leftmost length characters in a string. Returns the value if strthe or lengthargument is .NULLNULL

If lengthtrue 0or negative, LEFTthe function returns an empty string. If lengthgreater than strthe length of the string, LEFTthe function returns the entire strstring.

4.right: return the right part of the string with the specified length

查询语句：
select RIGHT(client_name,4) from bms_bills_memo where id = 4
结果：
有限公司
表中数据：
client_name：青岛自贸新零售体验中心有限公司

5.replace: Search and replace substrings in strings.

Takes three arguments, it replaces the string stringinold_stringnew_string

Note that when searching for text to replace, MySQL uses case-sensitive matching to perform the search for the string to replace

does not support regular expressions

查询语句：
select REPLACE(client_name,'青岛','威海') from bms_bills_memo where id = 4
结果：
威海自贸新零售体验中心有限公司

查询语句：
select REPLACE(bill_of_lading_no,'td','ab') from bms_bills_memo where id = 4
结果：
TD10004

结果没有发生变化，因为区分大小写

6.substring: Extract a substring from a position with a specific length.

SUBSTRING(string,position);
SUBSTRING(string FROM position);

There are two parameters:

• stringThe argument is the string whose substring is to be extracted.
• positionThe parameter is an integer specifying the starting character of the substring, which positioncan be a positive or negative integer.

If positionpositive, SUBSTRINGthe function extracts the substring from the beginning of the string.

If positionthe argument is zero, SUBSTRINGthe function returns an empty string.

Functions can be called using the SQL standard syntax with FROMthe keywordSUBSTRING

查询语句:
select SUBSTRING(bill_of_lading_no,2) from bms_bills_memo where id = 4
结果：
D10004

查询语句：
select SUBSTRING(bill_of_lading_no,-2) from bms_bills_memo where id = 4
结果：
04

表中数据：
bill_of_lading_no:TD10004

If you want to specify the length of the substring to be extracted from a string, you can use SUBSTRINGa function of the form

SUBSTRING(string,position,length);
SUBSTRING(string FROM position FOR length);

查询语句：
select SUBSTRING(bill_of_lading_no,2,5) from bms_bills_memo where id = 4
结果：
D1000

查询语句：
select SUBSTRING(bill_of_lading_no,-7,5) from bms_bills_memo where id = 4
结果：
TD100

表中数据：bill_of_lading_no:TD10004

SUBSTR()function is SUBSTRING()a synonym for function, so they can be used interchangeably.

Usage of SUBSTRING_INDEX

Split according to a specific symbol and take out the corresponding value

查询语句：
select SUBSTRING_INDEX('qwe-etr-tyu','-',1)
select SUBSTRING_INDEX('qwe-etr-tyu','-',2)
select SUBSTRING_INDEX('qwe-etr-tyu','-',-1)
select SUBSTRING_INDEX('qwe-etr-tyu','-',-2)

结果：
qwe
qwe-etr
tyu
etr-tyu

7.trim: Remove unwanted characters from a string.

TRIM([{BOTH|LEADING|TRAILING} [removed_str]] FROM str);

You can use the LEADING, TRAILINGor BOTHoption to explicitly instruct TRIM()the function to remove leading, trailing, or both leading and trailing unnecessary characters from the string.

If you don't specify anything, TRIM()the function defaults to using BOTHoptions.

[removed_str]is the string to delete. By default it is a space. This means that if you do not specify a specific string, TRIM()the function only removes spaces.

stris removed_strthe string to remove subcharacters from.

TRIM()The function returns a string with unwanted characters removed.

查询语句：
//从字符串中除去前导和尾随空格
SELECT TRIM(' MySQL TRIM Function ');
//仅删除前导空格
SELECT TRIM(LEADING FROM '    MySQL TRIM Function   ');
//仅删除尾随空格
SELECT TRIM(TRAILING FROM '    MySQL TRIM Function   ');
//删除字符串末尾的换行符
-- 方式一
SELECT 
    TRIM(TRAILING '\n' FROM field_name)
FROM table_name;

-- 方式二
SELECT 
    TRIM(TRAILING '\r' FROM field_name)
FROM table_name;

-- 方式三
SELECT 
    TRIM(TRAILING '\r\n' FROM field_name)
FROM table_name;

If you want to remove only leading or trailing spaces, you can use other string functions: LTRIMandRTRIM

Use LTRIMa function to remove leading spaces from a string

SELECT LTRIM('MySQL LTRIM function'); 
Use RTRIM()a function to remove trailing spaces from a string

SELECT RTRIM('MySQL RTRIM function '); 

8. find_in_set: Find a string in a comma-separated list of strings

FIND_IN_SET(needle,haystack);

FIND_IN_SET()The function accepts two parameters:

• The first parameter needleis the string to look for.
• The second argument haystackis a comma-separated list of strings to search.

FIND_IN_SET()The function returns an integer or a NULLvalue depending on the value of the argument:

• If needleor haystackis NULL, the function returns NULLa value.
• Returns zero if needlenot haystackin, or the empty string.haystack
• If needlein haystack, returns a positive integer.

Note that this function will not work properly if you needleinclude a comma ( ). ，Also, if needleit is a constant string, and haystackit is a SETcolumn of type , MySQL will use bit arithmetic optimization.

查询语句：
select FIND_IN_SET('red','yellow,red,blue')
结果：
2

where you can use find_in_set directly

SELECT 
    name, belts
FROM
    divisions
WHERE
    FIND_IN_SET('red', belts);

SELECT 
    name, belts
FROM
    divisions
WHERE
    NOT FIND_IN_SET('black', belts);

The FIND_IN_SET function has the same functionality as the IN operator

column IN (x, y, z)The expression is FIND_IN_SET(column, 'x,y,z')the same as .

INOperators can accept any number of arguments, each separated by a comma. However, FIND_IN_SETthe function has only two parameters.

INOperators are used when you want to match a value against a list of values ​​in the database . And when you want to match a value against a comma-separated list of values ​​stored in the database, you can use FIND_IN_SETa function.

9.format: Format numbers with specific regional settings, rounded to decimal places.

FOMRAT(N,D,locale);

FORMATThe function formats the number N to the format, eg "#,###,###.##", rounded to Done decimal place. It returns a value as a string.

FORMATThe function accepts three parameters:

• Nis the number to format.
• Dis the number of decimal places to round off.
• localeis an optional parameter that determines the thousand separator and the grouping between separators. If localethe operator is omitted, MySQL will use it by default en_US. The following link provides all locale names supported by MySQL: http://dev.mysql.com/doc/refman/5.7/en/locale-support.html

查询语句：
select format(123456.1258,2)
select format(123456.1248,2)
结果：
123,456.13
123,456.12

Two functions can be combined: FORMATand CONCAT. FORMATThe function rounds the inventory value format to 2decimal places. And the concat function adds the RMB symbol ( ￥)  at the beginning of the inventory value string

SELECT 
    productname,
    CONCAT('￥',
            FORMAT(quantityInStock * buyPrice, 2)) stock_value
FROM
    products;