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set primary key in existing table and field

hi, no matter how much i google search this, i cant find the answer.

how do i set the primary key and auto_increment on an already existing field in a table.

For the history, im copying 35 tables in a database with different table names using:

$table = "CREATE TABLE IF NOT EXISTS ".$username."_banner SELECT * FROM default_banner";
mysql_query($table);

but this does not copy over the primary key and auto_increment settings.
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Erron
Asked:
Erron
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1 Solution
 
Raynard7Commented:
alter table `test`.`tablename1` change `a` `a` int (10)   NOT NULL AUTO_INCREMENT , add primary key (`a` )

Will change a table (tablename1) in the test schema -
it will change field `a` to an autoincrement field and will be the primary key.
0
 
Raynard7Commented:
If there are null values it will not work
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