LENGTH : to get number of bytes in a string

We can get number of bytes present against each data can be found by using LENGTH command.
SELECT LENGTH('Welcome');
The output is 7

Student Table

Let us find out the length of name present in our student table.
SELECT id, name, length(name) FROM `student`
We will get a list of id , name and length like this
1 John Deo  8
2 Max Ruin 8
3 Arnold 6
We can also use length in numeric field like this
SELECT id, name, length(name),length(mark) FROM `student`
Output will be 2 for marks less than 100 and more than 9 ( two digits)

Now let us try to find some blank data using length query
SELECT id, name FROM `student` where length(mark) <=1
This will display records where Mark is single digit or not entered.

What is the difference between length and char_length ?

Length returns string measured in bytes, but char_length returns number of characters. All chars does not occupy single byte particularly Unicode where chars are encoded in two bytes. Here is an example.
SELECT LENGTH( _utf8 '' ) , CHAR_LENGTH( _utf8 '' )
Output is here
2,1

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