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We can replace NULL values with a specific value using the SQL Server ISNULL Function. The syntax for the SQL ISNULL function is as follow. The SQL Server ISNULL function returns the replacement value if the first parameter expression evaluates to NULL.
In PHP, the empty string equals to a NULL value, but in MySQL, the case is the different i.e. empty string is not equal to NULL value. To understand the above syntax, let us create a column with NOT NULL constraint while you can insert an empty string.
Use IFNULL or COALESCE() function in order to convert MySQL NULL to 0. Insert some records in the table using insert command. Display all records from the table using select statement.
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`
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