How SQL intercepts specified characters from strings (three functions of LEFT, MID, RIGHT)

Table of Contents

1.LEFT function

2.MID function

3.RIGHT function

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1.LEFT function

(1) Grammar: left(text,len) is left(string text, length)

Query Statementselect [column(s),] left(text,len)[from table]

Parameter description:

text: string, which can be directly quoted as a string or a field;

len: The length of the string to be intercepted, which is a positive integer. If it is 0 or a negative number, it returns a null value.

Description: The query enclosed in "[]" indicates optional.If you need to enter the from statement to specify the target table when querying with other fields, you also need to enter the from statement to specify the target table when text is a field. If you only use left to obtain a specified string to intercept, you can directly omit the from statement, but this usageGenerally only for debugging and not often used in practice.

（2）Usage: Intercept the characters of the specified length on the left.

(3) Example

Example 1: Intercept "hello" from "hello world".

select left('hello world',5)

There is a student table student, student ID, name, gender, native_place, nationality, date of birth born, age.as shown in the table below.

student

ID

name

sex

native_place

nation

born

age

1001

Zhang San

Male

Guangdong Province

Chinese

2000/3/16

22

1002

Chen Yi

Female

Guangdong

strong

1998/3/15

24

1003

Wang Wu

Male

Tibet

Hidden

2002/1/6

20

1004

Chen Hong

Female

Guangdong Province

Li

2001/1/17

21

1005

Li Si

Male

Northeast Province

Chinese

1999/3/16

23

1006

-

Female

Guangdong Province

Li

5/6/1998

24

1008

Chen Xiaoxiao

Female

Guangdong

strong

1997/5/9

25

Example 2: Remove the apostrophe from the place of origin, and intercept the first two characters.Obtain student ID, name, gender and age at the same time.

select ID,name,sex,left(native_place,2),agefrom student

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2.MID function

(1) Grammar and usage: there are two forms

#①Two parametersmid(text,start) #Get characters from an ordinal of the specified string#②Three parametersmid(text,start,len) #Get characters of the specified length from a certain position in the specified string

Parameter description:

text: string, which can be directly quoted as a string or a field;

start: The string position to start intercepting, which can be a positive or negative integer.

len: The length of the string to be intercepted, which is a positive integer. If it is 0 or a negative number, it returns a null value.

Note: The MID function is synonymous with SUBSTRING(), and the usage is the same.

(2) Instance

Example: Intercept the string "world" from "hello world".

When start is a positive number, the position of the start character "w" is 7. Note that the empty character between "hello" and "world" is counted as one character.

#start is a positive number#two two parametersselect mid('hello world',7) ?orselect mid('hello world'from 7)#Three parametersselect mid('hello world',7,5)orselect mid('hello world'from 7 for 5)

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3.RIGHT function

(1) Syntax: right(text,len) is right(string text, length)

Query Statementselect [column(s),] right(text,len)[from table]

Parameter description:

text: string, which can be directly quoted as a string or a field;

len: The length of the string to be intercepted, which is a positive integer. If it is 0 or a negative number, it returns a null value.

Description: The query enclosed in "[]" indicates optional.If you need to enter the from statement to specify the target table when querying with other fields, you also need to enter the from statement to specify the target table when text is a field. If you only use left to obtain a specified string to intercept, you can directly omit the from statement, but this usageGenerally only for debugging and not often used in practice.

（2）Usage: Intercept the characters of the specified length on the right.

(3) Example

Example 1: Intercept "world" from "hello world".

select right('hello world',5)

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The usage of SQL's LEFT, MID and RIGHT functions is very similar to EXCEL's LEFT, MID and RIGHT functions. Please refer to: