MySql Query Replace NULL with Empty String in Sele

问题:

How do you replace a NULL value in the select with an empty string? It doesnt look very professional to output "NULL" values.

This is very unusual and based on my syntax I would expect it to work. Hoping for an explanation why it doesnt.

select CASE prereq WHEN (prereq IS NULL) THEN " " ELSE prereq end from test;

Example of what the original table looks like, what I want, and what actual prints:

original     wanted      what actually prints
--------     ------      ---------------------
value1       value1      
NULL                     NULL
value2       value2      
NULL                     NULL

As you can see it does the opposite of what I want, hence I tried flipping the IS NULL to IS NOT NULL and of course that didnt fix it, also tried swapping the position of when case, which did not work.

Edit: It seems the 3 solutions given below all do the task. regards

select if(prereq IS NULL ," ",prereq ) from test
select IFNULL(prereq,"") from test
select coalesce(prereq, '') from test

回答1:

If you really must output every values including the NULL ones:

select IFNULL(prereq,"") from test

回答2:

SELECT COALESCE(prereq, '') FROM test

Coalesce will return the first non-null argument passed to it from left to right. If all arguemnts are null, it'll return null, but we're forcing an empty string there, so no null values will be returned.

Also note that the COALESCE operator is supported in standard SQL. This is not the case of IFNULL. So it is a good practice to get use the former. Additionally, bear in mind that COALESCE supports more than 2 parameters and it will iterate over them until a non-null coincidence is found.

回答3:

Try below ;

  select if(prereq IS NULL ," ",prereq ) from test

回答4:

Some of these built in functions should work:

Coalesce
Is Null
IfNull

回答5:

The original form is nearly perfect, you just have to omit prereq after CASE:

SELECT
  CASE
    WHEN prereq IS NULL THEN ' '
    ELSE prereq
  END AS prereq
FROM test;

回答6:

Try COALESCE. It returns the first non-NULL value.

SELECT COALESCE(`prereq`, ' ') FROM `test`

回答7:

Try this, this should also get rid of those empty lines also:

SELECT prereq FROM test WHERE prereq IS NOT NULL;

回答8:

select IFNULL(`prereq`,'') as ColumnName FROM test

this query is selecting "prereq" values and if any one of the values are null it show an empty string as you like So, it shows all values but the NULL ones are showns in blank

回答9:

I have nulls (NULL as default value) in a non-required field in a database. They do not cause the value "null" to show up in a web page. However, the value "null" is put in place when creating data via a web page input form. This was due to the JavaScript taking null and transcribing it to the string "null" when submitting the data via AJAX and jQuery. Make sure that this is not the base issue as simply doing the above is only a band-aid to the actual issue. I also implemented the above solution IFNULL(...) as a double measure. Thanks.

回答10:

UPDATE your_table set your_field="" where your_field is null