The syntax you used is from a SQL Server example, not from MySQL. It would be good to check the MySQL documentation about ALTER TABLE syntax.

In MySQL, the ALTER COLUMN subclause can only be used for setting or dropping the default value of the column (SET DEFAULT literal or DROP DEFAULT).

You need to use either CHANGE COLUMN (note that column name is doubled as you could use this to change its name):

ALTER TABLE MyTable  
    CHANGE COLUMN comment comment BIGINT NOT NULL;

or MODIFY COLUMN:

ALTER TABLE MyTable 
    MODIFY COLUMN comment BIGINT NOT NULL;

Now, there are 2 more problems, beside the syntax:

• Seeing the CREATE TABLE, converting a "comment" column from TEXT to BIGINT does not make much sense. I'll assume that you want to keep it TEXT and only make it NOT NULL.

• There might be NULL values already in the table. If that's the case, the statement will fail with errors. So, you have to first UPDATE those values to some non-null value (say the empty string) and then modify the column:

So our statements become:

UPDATE glpi_ticketsatisfactions 
SET comment = ''
WHERE Comment IS NULL ;

ALTER TABLE glpi_ticketsatisfactions 
    MODIFY COLUMN comment TEXT COLLATE utf8_unicode_ci NOT NULL ;

Yes, by using COALESCE.

SELECT COALESCE(null_column, 0) AS null_column FROM whatever;

COALESCE goes through the list of values you give it, and returns the first non-null value.

In mysql you can achieve this as,

ALTER TABLE room MODIFY property_id bigint(20) not null;

You want the following:

ALTER TABLE mytable MODIFY mycolumn VARCHAR(255);

Columns are nullable by default. As long as the column is not declared UNIQUE or NOT NULL, there shouldn't be any problems.

Just use an ALTER TABLE... MODIFY... query and add NOT NULL into your existing column definition. For example:

ALTER TABLE Person MODIFY P_Id INT(11) NOT NULL;

A word of caution: you need to specify the full column definition again when using a MODIFY query. If your column has, for example, a DEFAULT value, or a column comment, you need to specify it in the MODIFY statement along with the data type and the NOT NULL, or it will be lost. The safest practice to guard against such mishaps is to copy the column definition from the output of a SHOW CREATE TABLE YourTable query, modify it to include the NOT NULL constraint, and paste it into your ALTER TABLE... MODIFY... query.

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

select IFNULL(prereq,"") from test