Mysql insert column with function

Question

I want to add column if the column exist on table. But i my code doesn't work i get error the error message is ;

[Err] 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'ColumnType DEFAULT ( CASE DefaultValue WHEN NULL THEN SELECT 'NULL' ' at line 11

My Code MySql code block

CREATE DEFINER = CURRENT_USER FUNCTION `AddTableColumn` (
    TableName VARCHAR (500),
    ColumnName VARCHAR (500),
    IsPrimary TINYINT (4),
    IsNull TINYINT (4),
    DefaultValue VARCHAR (500),
    ColumnType VARCHAR (500)
) RETURNS INTEGER
BEGIN
    ALTER TABLE TableName ADD COLUMN
IF NOT EXISTS ColumnName ColumnType DEFAULT (
    CASE DefaultValue
    WHEN NULL THEN
        SELECT
            'NULL'
        ELSE
            SELECT
                DefaultValue
            END
)(
    CASE IsNull
    WHEN 1 THEN
        (SELECT 'IS NULL')
    ELSE
        SELECT
            ('IS NOT NULL')
        END
)(
    CASE IsPrimary
    WHEN 1 THEN
        (SELECT 'PRIMARY KEY')
    ELSE
        SELECT
            ''
        END
) RETURN 0;


END;

;