MySql Query Replace NULL with Empty String in Select

Question

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

select IFNULL(prereq,"") from test

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.

Try below ;

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

Some of these built-in functions should work:

COALESCE(value,...)

Returns the first non-NULL value in the list, or NULL if there are no non-NULL values.

IS NULL

Tests whether a value is NULL.

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

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

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;

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

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