MySQL SET DEFAULT返回错误：查询为空

Question

I have an existing MySQL (version 5.6.12) table called 'users' and am trying to set the column defaults via php (version 5.4.12), for example:

$query = "ALTER TABLE users ALTER username SET DEFAULT NULL";
$result = mysqli_query($query);

However this generates a SQL error (which results in the message 'query is empty'). 'username' is a VARCHAR(200) type column.

Using a quoted literal (such as 'John') for the default value results in the same error. (Just for kicks, though I am using MySQL I have also tried modifying the query according to SQL/MS Access or Oracle syntax but it still doesn't work).

What am I doing wrong?

EDIT: Problem solved. The above query is fine and nothing is wrong.

I made an incredibly dumb error and forgot $ sign in front of $query , ie my code was written $result = mysqli_query(query);

(Just so you know, I am a programmer with decades of experience.)

Answer 1

Change query like below

$query = "ALTER TABLE users MODIFY Column username VARCHAR(200) NULL DEFAULT NULL";

or

 $query = "ALTER TABLE users CHANGE Column username username VARCHAR(255) NULL DEFAULT NULL";

Answer 2

I use this one for my website

$query = "ALTER TABLE 'users' CHANGE 'username' 'username' VARCHAR( 100 ) NULL DEFAULT NULL";
$result = mysqli_query($query);

Answer 3

Problem solved. The query is actually fine and nothing is wrong.

I made an incredibly dumb error and forgot $ sign in front of $query , ie my code was written $result = mysqli_query(query);

(Just so you know, I am a programmer with decades of experience.)

Answer 4

In MySQL you use backticked [`] quoted literals, don't use the quote sign [']. On the other side, every DDBB engine uses its slightly different flavour of SQL. Most of them anyway are able to interpret common subsets as described by the various active standards SQL89, SQL92, SQL99... and so on.

In short, your SQL is wrong, in regards to what MySQL expects it to be. You'll have to build your SQL skills on top of trial and error by working with different SQL engines, until you can give each of these engines the SQL variant they expect. Just don't give up and insist on the query. Simplify it, running shorter strings until you find one that works, then refine it by adding smaller pieces until you reach your objective.