如何在MySQL中用空值替换列

Question

One of my columns in the table contains 'NULL' values and I would like to replace them with certain values.

I looked at this post: Replace nulls values in sql using select statement? and followed the answer.

My sql statement:

Select ifnull(`option`, 'MCQ') as `option` 
from question_table 

This statement returns me the columns there are already with 'MCQ', but the 'NULL' values are not replaced yet.

Need some guidance to change this.

Answer 1

If you want to change the data, you need an update :

update question_table
    set option = 'MCQ'
    where option is null;

A select statement does not change the database.

Answer 2

If you want to update the table use Gordon's answer , but maybe you just want to return a replacement value for NULL in the SELECT , you can use COALESCE :

SELECT COALESCE(`option`, 'MCQ') as `option` 
FROM question_table

This selects MCQ for every option -value that is NULL .

Another way is using CASE :

SELECT CASE WHEN `option` IS NULL THEN 'MCQ' ELSE `option` END as `option` 
FROM question_table

Answer 3

'NULL' is a string, NULL is a special value that means "unknown/unavailable". They are totally different things.

MySQL function IFNULL() handles NULL values (they cannot be compared using the regular comparison operators). You can use the regular comparison operators ( = , <> ) to work with strings, even when they are 'NULL' .

If your query produces 'NULL' values in the result set it means the values in the database are not NULL but strings ( 'NULL' ).

In this case the query you need is:

SELECT IF(`option` = 'NULL', 'MCQ', `option`) AS `option` 
FROM question_table