php mysql datetime

I have two tables in MySQL, and each table has its own datetime field. I want to copy the datetime of table A to overwrite the datetime of table B. I use PHP.

$result = mysql_query("select * 
                         from A 
开发者_JS百科                        where id = $key");
$row = mysql_fetch_array($result);
print $row[2]."\n"; // $row[2] is the datetime field
mysql_query("update B 
                set date_cal = $row[2]  
              where id = $key") // try to overwrite datetime in table B

$row[2] has the string representation of datetime field.

But the overwrite operation does not take effect. How to do to overwrite datetime field of table B using php?

If I insist using $row[2] to assign the new datetime field rather running mysql_query again, how to do?

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I believe you'll need to wrap the date in quotes in your update query.

mysql_query("update B set date_cal=$row[2] where id=$key")
Should be
mysql_query("update B set date_cal='$row[2]' where id=$key")

On another note, I'd suggest that you don't access your fields by index when you're doing a SELECT * query. If you add another field to your database, $row[2] could end up referring to something else. If you instead call $row = mysql_fetch_assoc($result);, you can refer to it by field name, eg
mysql_query("update B set date_cal='{$row['date_field]}' where id=$key")

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If I understand that correctly, you can do the same from within your query:

update B 
   set date_cal = (select date_cal from A where id = {$key})
where id = $key