Question

I’d like to update my table so that rows that share values (in the same columns) get tagged as such.

Example table definition:

CREATE TABLE `MyTable` (
  `id`  int NOT NULL ,
  `a`  varchar(10) NOT NULL ,
  `b`  varchar(10) NOT NULL , 
  `state`  tinyint NOT NULL , 
  PRIMARY KEY (`id`) 
) ;

I’d like to update “state” of every row that share values in the same column. So if row one has “a=AAAA” and row two has the same value for “a”, both should get updated.

I tried this but it’s too slow (not faster then doing it in Java using JDBC):

declare mycursor cursor for select id, a, b from mytable;
open mycursor;
repeat
   fetch mycursor into idid, aa, bb;
   update mytable set state=1 where (a=aa, b=bb)
until done end repeat;
close mycursor;

Any ideas to make this way better? I haven’t done any decent SQL in years.

Answer 1

The answer to my question seems to be the following:

update mytable  as t1 inner join mytable as t2 on (t1.a=t2.a or t1.b = t2.b) and t1.id <> t2.id set t1.state=1;

Please say so if it is not (it seems to work, but it might do strange things) 😉